Research Progress in Enzymatic Synthesis of Vitamin E Ester Derivatives

catalysts

Review Research Progress in Enzymatic Synthesis of Vitamin E Ester Derivatives

Zhiqiang Zou 1, Lingmei Dai 1, Dehua Liu 1,2 and Wei Du 1,2,*

1 Key Laboratory for Industrial Biocatalysis, Department of Chemical Engineering, Tsinghua University, Ministry of Education, Beijing 100084, China; [email protected] (Z.Z.); [email protected] (L.D.); [email protected] (D.L.) 2 Tsinghua Innovation Center in Dongguan, Dongguan 523808, China * Correspondence: [email protected]

Abstract: Vitamin E is easily oxidized by light, air, oxidizing agents and heat, limiting its application in many ways. Compared to vitamin E, vitamin E ester derivatives exhibit improved stability and a stronger antioxidant capacity, and even gain new biological functions. In recent years, enzymatic synthesis of vitamin E ester derivatives has received increasing attention due to its environmental friendliness, high catalytic efﬁciency, and inherent selectivity. This paper reviews the related progress of lipase-mediated preparation of vitamin E ester derivatives. The function of different vitamin E ester derivatives, and the main factors inﬂuencing the enzymatic acylation process, including enzyme species, acyl donor and acceptor, reaction media and water activity, are summarized in this paper. Finally, the perspective of lipase-catalyzed synthesis of vitamin E ester derivatives is also discussed.

Keywords: antioxidants; derivatives; lipase; vitamin E; vitamin E ester

Citation: Zou, Z.; Dai, L.; Liu, D.; Du, W. Research Progress in Enzymatic Synthesis of Vitamin E Ester Derivatives. Catalysts 2021, 11, 739. 1. Introduction https://doi.org/10.3390/ Vitamin E (VE) is a group of eight liposoluble compounds, including four tocopherols catal11060739 and four tocotrienols. They are all composed of benzodihydropyran rings and 2-position phytogroups (Table1). Tocopherol has three chiral carbon atoms, and there are three Academic Editors: isolated double bonds on the phytogroups of tocotrienol [1]. Vegetable oil extracted from Rodica-Mihaela Dinică and oil-bearing seeds and nuts is the most abundant source of natural vitamin E. The total Lidia Favier vitamin E content in different oils varies from thousands ppm to tens ppm, and contains a variety of tocopherols and tocotrienols [2]. Chemically synthesized vitamin E is mainly a Received: 22 April 2021 racemic alpha-tocopherol by using trimethylhydroquinone and racemic isophytol [3]. Accepted: 15 June 2021 Natural antioxidant vitamin E is more suitable for food additives, health care, cosmet- Published: 16 June 2021 ics and other industries compared to synthetic antioxidants such as terbutyl hydroquinone (TBHQ), butyl hydroxytoluene (BHT), butyl hydroxyanisole (BHA), and propyl gallate, Publisher’s Note: MDPI stays neutral which may form toxic or carcinogenic compounds after degradation [4,5]. with regard to jurisdictional claims in Vitamin E is easily oxidized and is unstable to light and singlet oxygen. The antioxi- published maps and institutional affil- dant property of VE is reflected either in its termination of the free radical chain reaction iations. (Figure1a), or in the way that itself can be oxidized and lose its antioxidant property (Figure1b). Herein, when vitamin E is extracted from natural sources or is synthesized by chemical methods, it easily loses its original antioxidant function when added to products due to its own oxidation during the process of production, transportation, and storage. Copyright: © 2021 by the authors. Therefore, the modification on vitamin E to improve its stability is significant in promoting Licensee MDPI, Basel, Switzerland. its wider application [5]. This article is an open access article It is well recognized that vitamin E ester derivatives have a wide range of applica- distributed under the terms and tions in pharmaceuticals, cosmetics, daily chemicals, and other fields. At present, the conditions of the Creative Commons Attribution (CC BY) license (https:// most popular vitamin E ester products in the market are vitamin E acetate, vitamin E creativecommons.org/licenses/by/ succinate, vitamin E linoleic acid ester, and vitamin E niacin ester. Vitamin E acetate has 4.0/). been demonstrated to be effective in dealing with inflammation elimination, periodontal

Catalysts 2021, 11, 739. https://doi.org/10.3390/catal11060739 https://www.mdpi.com/journal/catalysts

ReviewReview

ResearchResearch Progress Progress in in Enzymatic Enzymatic Synthesis Synthesis of of Vitamin Vitamin E EEster Ester

Derivatives Review Derivatives

Research ProgressZhiqiangZhiqiang Zou in Zou 1, LingmeiEnzymatic 1, Lingmei Dai Dai 1, Dehua 1, DehuaSynthesis Liu Liu 1,2 and 1,2 and Wei of Wei Du Vitamin Du 1,2,* 1,2,* E Ester Derivatives 1 Key1 KeyLaboratory Laboratory for Industrial for Industrial Biocatalysis, Biocatalysis, Department Department of Chemical of Chemical Engineering, Engineering, Tsinghua Tsinghua University, University, MinistryMinistry of Education, of Education, Beijing Beijing 100084, 100084, China China; [email protected]; [email protected] (Z.Z.); (Z.Z.); Zhiqiang Zou 1, Lingmei Dai 1, Dehua Liu 1,2 and Wei Du 1,2,* [email protected]@tsinghua.edu.cn (L.D.); (L.D.); [email protected] [email protected] (D.L (D.L.) .) 2 Tsinghua2 Tsinghua Innovation Innovation Center Center in Dongguan, in Dongguan, Guangdong Guangdong 523808, 523808, China China 1 Key Laboratory for Industrial* CorrespondenceBiocatalysis,* Correspondence Department: duwei@tsinghua: duwei@tsinghua of Chemical .edu.cnEngineering,.edu.cn Tsinghua University, Ministry of Education, Beijing 100084, China; [email protected] (Z.Z.); [email protected] (L.D.); [email protected] (D.L.) Abstract: Vitamin E is easily oxidized by light, air, oxidizing agents and heat, limiting its application 2 Tsinghua Innovation Center in Dongguan,Abstract: GuangdongVitamin E 523808, is easily China oxidized by light, air, oxidizing agents and heat, limiting its application * Correspondence: duwei@tsinghuain manyin .edu.cnmany ways. ways. Compared Compared to vitamin to vitamin E, vitamin E, vitamin E ester E ester derivatives derivatives exhibit exhibit improved improved stability stability and and a a strongerstronger antioxidant antioxidant capacity capacity, and, and even even gain gain new new biological biological functions. functions. In recent In recent years, years, enzymatic enzymatic Abstract: Vitamin E is easilysynthesis oxidizedsynthesis ofby vitaminlight, of vitamin air, Eoxidizing ester E ester derivatives agents derivatives and heat,has hasreceived limiting received its increasin application increasing attention g attention due due to its to environmentalits environmental in many ways. Compared to vitamin E, vitamin E ester derivatives exhibit improved stability and a friendliness,friendliness, high high catalytic catalytic efficiency efficiency, and, and inherent inherent selectivity. selectivity. This This paper paper reviews reviews the therelated related pro- pro- stronger antioxidant capacity, and even gain new biological functions. In recent years, enzymatic gressgress of lipase of lipase-mediated-mediated preparation preparation of vitamin of vitamin E ester E ester derivatives. derivatives. The The function function of different of different vita- vita- synthesis of vitamin E ester derivatives has received increasing attention due to its environmental min E ester derivatives, and the main factors influencing the enzymatic acylation process, including friendliness, high catalytic efficiencymin E, esterand inherent derivat ives,selectivity. and the This main paper factors reviews influencing the related the pro- enzymatic acylation process, including gress of lipase-mediated preparationenzymeenzyme species, of species,vitamin acyl acylE donor ester donor derivatives. and and acceptor, acceptor, The reactionfunction reaction mediaof different media and and vita-water water activity activity, are, aresummarized summarized in this in this min E ester derivatives, andpaper. thepaper. main Finally, factors Finally, the influencing theperspective perspective the enzymatic of lipase of lipase acylation-catalyzed-catalyzed process, synthesis synthesis including of vitamin of vitamin E ester E ester derivatives derivatives is also is also enzyme species, acyl donordiscussed. anddiscussed. acceptor, reaction media and water activity, are summarized in this Citation:Citation: Zou,paper. Zou, Z.; Dai,Finally, Z.; Dai,L.; Liu, L.;the D.;Liu, perspective D.; of lipase-catalyzed synthesis of vitamin E ester derivatives is also Du, W. Research Progress in Du, W.discussed. Research Progress in Keywords:Keywords: antioxidants antioxidants; derivatives; derivatives; lipase; lipase; vitamin; vitamin E; vitamin E; vitamin E ester E ester Citation: Zou, Z.; Dai, L.; Liu,Enzymatic D.; Synthesis of Vitamin Enzymatic Synthesis of Vitamin Du, W. Research Progress in E EsterE Ester Derivatives.Keywords: Derivatives. Catalysts antioxidants Catalysts ; derivatives; lipase; vitamin E; vitamin E ester Enzymatic Synthesis of Vitamin 2021,2021 11, x., 11 https://doi.org/, x. https://doi.org/ E Ester Derivatives. Catalysts 10.3390/xxxxx10.3390/xxxxx 2021, 11, x. https://doi.org/ 1. Introduction1. Introduction 10.3390/xxxxx AcademicAcademicCatalysts1. Editor: Introduction Editor:2021 , 11 , 739 VitaminVitamin E (VE) E (VE) is a is group a group of eight of eight liposoluble liposoluble compounds, compounds, including including four four tocopher- tocopher-2 of 11 Rodica-Mihaela Dinicǎ and Lidia Academic Editor: Rodica-MihaelaVitamin Dinicǎ E and (VE) Lidia is aols group olsand ofand four eight four tocotrienols. liposoluble tocotrienols. compounds, They They are areall including composedall composed four of tocopher- benzodihydropyran of benzodihydropyran rings rings and and 2-posi- 2-posi- Rodica-Mihaela Dinicǎ and LidiaFavier Favier ols and four tocotrienols.tion Theytion phytogroups are phytogroups all composed (Table (Table of benzodihydropyran1). Tocopherol1). Tocopherol has has ringsthree three and chiral 2chiral-posi- carbon carbon atoms, atoms, and and there there are arethree three Favier tion phytogroups (Tableisolated 1). isolatedTocopherol double double has bonds three bonds onchiral theon carbon thephytogroups phytogroups atoms, and of tocotrienolthere of tocotrienol are three [1] . [1]Vegetable. Vegetable oil extractedoil extracted from from Catalysts 2021, 11, x FOR PEER REVIEWdisease prevention, ulcer healing, blood microcirculation enhancement of gingival, and 2 of 11 Received:Received: isolated22 April 22 April 2021 double 2021 bonds onoil the-oilbearing phytogroups-bearing seeds se edsof and tocotrienol and nuts nuts is [1] theis. Vegetable themost most abundant oilabundant extracted source source from of naturalof natural vitamin vitamin E. TheE. The total total Received: 22 April 2021 anti-aging [6]. Vitamin E succinate can effectively inhibit the growth of tumor cells without Accepted:Accepted: oil15 -Junebearing 15 June2021 202 se1 eds and nutsvitamin vitaminis the E most content E content abundant in different in sourcedifferent oilsof naturaloils varies varies vitamin from from thousands E. thousandsThe total ppm ppm to tens to tens ppm, ppm, and and contains contains Accepted: 15 June 2021 affecting the proliferation of normal cells [7]. Vitamin E linoleate has excellent skin mois- Published:Published:vitamin 16 June 16 June2021 E content 2021 in differenta varietya variety oils ofvaries tocopherols of tocopherolsfrom thousands and and tocotrienols ppm tocotrienols to tens [2]ppm, . [2]Chemically and. Chemically contains synthesized synthesized vitamin vitamin E is E mainly is mainly Published: 16 June 2021 a variety of tocopherols and tocotrienolsturizing properties [2]. Chemically and is synthesized mainly used vitamin in the E cosmetic is mainly industry as an additive in advanced a racemica racemic alpha alpha-tocopherol-tocopherol by usingby using trimethy trimethylhydroquinonelhydroquinone and and racemic racemic isophytol isophytol [3] . [3]. Publisher’sPublisher’sa racemic Note: Note: MDPI alpha MDPI stays-tocopherol stays bycosmetics usingVitamin trimethy [8]. lhydroquinone E is easily andoxidized racemic isophytoland is unstable[3]. to light and singlet oxygen. The antioxi- Publisher’s Note: MDPI stays neutralneutral with with regard regard to to neutral with regard to TableTabledant 1. Structure 1. property Structure of vitamin of ofvitamin VE E. isE. reflected either in its termination of the free radical chain reaction jurisdictionalTable claims 1. Structure in published of vitamin E. Table 1. Structure of vitamin E. jurisdictional claims in published jurisdictional claims in published (Figure 1a), or in the way that itself can be oxidized and lose its antioxidant property (Fig- maps and institutionalmaps maps Structural and and institutional Formula institutional TypeStructuralStructural R1 FormulaR Formula2 R3 Type Type R1 R1 RR24 R2 R3 R3 R4 R4 Structural Formula Type R1 R2 R3 R4 affiliations. affiliations.affiliations. ure 1b). Herein, when vitamin E is extracted from natural sources or is synthesized by α CH3 CH3 CH3 α α CHCH3 3CH CH3 3CH CH3 3 chemicalα methods,CH3 it CHeasily3 losesCH its3 original antioxidant function when added to products β CH3 H CH3 β β CHCH3 3 H H CHCH3 3 β CH3 H CH3

γ due Hto itsCH own3 CH 3oxidation during the process of production, transportation , and storage. γ γ H H CHCH3 3CH CH3 3 Copyright: © 2021 by the authors. γ HTocopherol CH3 CHTocotrienol3 Copyright:Copyright: © 2021 © 2021by the by authors. the authors. Therefore, the modification on vitamin ETocopherol TocopheroltoTocopherol improve its TocotrienolstabilityTocotrienolTocotrienol is significant in promot- Licensee MDPI, Basel, Switzer- Licensee MDPI, Basel, Switzer- δ δH H HCH3 H CH3 Licensee MDPI, Basel, Switzer- δ H H CH3 land. This article is an open access ing its wider application δ [5]H . H CH3 land.land. This Thisarticle article is an isopen an open access access article distributed under the article distributed under the terms and conditions of the Crea- article distributedNatural underantioxidant the vitamin E is more suitable for food additives, health care, cos- NaturalNatural antioxidant antioxidant vitamin vitamin E is E more is more suitable suitable for forfood food additives, additives, heal health care,th care, cos- cos- tive Commons Attribution terms (CC terms andmetics conditionsand conditions and of theother of Crea- the industriesCrea- compared to synthetic antioxidants such as terbutyl hydro- BY) license (http://creativecom-tive tive Commons Commonsquinone Attribution ( AttributionTBHQ (CC), butyl (CC meticshydroxytoluenemetics and and other other(BHT), industries industries butyl hydroxyanisole compared compared to ( syntheticBHAto synthetic), and antioxidantspropyl antioxidants such such as terbutylas terbutyl hydro- hydro- mons.org/licenses/by/4.0/). BY) BY) license licensegallate, (http://creativecom- (http://creativecom- which may formquinone toxicquinone or carcinogenic(TBHQ (TBHQ), butyl) ,compounds butyl hydroxytoluene hydroxytoluene after degradation (BHT), (BHT), [4,5]butyl butyl. hydroxyanisole hydroxyanisole (BHA (BHA), and), and propyl propyl mons.org/licenses/by/4.0/).mons.org/licenses/by/4.0/). gallate,gallate, which which may may form form toxic toxic or carcinogenicor carcinogenic compounds compounds after after degradation degradation [4,5] [4,5]. .

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CatalystsCatalysts 2021 2021, 11, ,x. 11 https://doi.org/10., x. https://doi.org/10.3390/xxxxx3390/xxxxx www.mdpi.com/journal/www.mdpi.com/journal/catalystscatalysts

Figure 1. (a) VE terminated free radical chain reaction, (b) VE quenched singlet oxygen itself oxidized. Figure 1. (a) VE terminated free radical chain reaction, (b) VE quenched singlet oxygen itself oxi- dizedVitamin. E ester derivatives are mainly synthesized by chemical and enzymatic methods (Figure2). The chemical method mainly adopts a Lewis acid and an organic base as the catalysts. Chen Dan used nano-silica and immobilized 4-dimethylaminopyridine to catalyzeIt is the well reaction recognized of α-tocopherol that withvitamin succinic E ester anhydride derivatives in n-hexane-acetone have a wide mixed range of applica- tionssolvent in to pharmaceuticals, produce alpha-tocopherol cosmetics, succinate daily [9]. Chen chemicals Xuebing, used and pyridine other fields. to catalyze At present, the most popularthe reaction vitamin of acetic E anhydrideester products and α-tocopherol in the market to produce are vitaminα-tocopherol E acetate, acetate invitamin a E succinate, vitaminsolvent-free E system, linoleic obtaining acid ester a 99.4%, and conversion vitamin rate E [10 niacin]. Hu Chuanrong ester. Vitamin used triethy- E acetate has been lamine to catalyze the reaction of α-tocopherol and succinic anhydride in n-hexane with demonstrateda 93.91% conversion to be rate effective [11]. Despite in thedealing fact that with chemical inflammation synthesis of vitaminelimination, E ester periodontal dis- easederivatives prevention, are extensively ulcer studied,healing, enzymatic blood micro catalysiscirculation offers a great enhancement perspective due toof its gingival, and anti- agingmilder [6] reaction. Vitamin conditions, E succinate simpler product can effectively separation, and inhibit environmental the growth friendliness. of tumor cells without affecting the proliferation of normal cells [7]. Vitamin E linoleate has excellent skin mois- turizing properties and is mainly used in the cosmetic industry as an additive in advanced cosmetics [8]. Vitamin E ester derivatives are mainly synthesized by chemical and enzymatic methods (Figure 2). The chemical method mainly adopts a Lewis acid and an organic base as the catalysts. Chen Dan used nano-silica and immobilized 4-dimethylaminopyridine to catalyze the reaction of α-tocopherol with succinic anhydride in n-hexane-acetone mixed solvent to produce alpha-tocopherol succinate [9]. Chen Xuebing used pyridine to catalyze the reaction of acetic anhydride and α-tocopherol to produce α-tocopherol acetate in a solvent-free system, obtaining a 99.4% conversion rate [10]. Hu Chuanrong used tri- ethylamine to catalyze the reaction of α-tocopherol and succinic anhydride in n-hexane with a 93.91% conversion rate [11]. Despite the fact that chemical synthesis of vitamin E ester derivatives are extensively studied, enzymatic catalysis offers a great perspective

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Catalysts 2021, 11, 739 3 of 11 due to its milder reaction conditions, simpler product separation, and environmental friendliness.

Figure 2. Various VE ester derivatives and their preparation methods. Figure 2. Various VE ester derivatives and their preparation methods. 2. Main Function of VE Ester Derivatives 2. Main Function of VE Ester Derivatives Generally speaking, vitamin E esters are more stable and have a longer shelf life than vitaminGenerally E in thespeaking, presence vitamin of light E and esters oxygen. are more Studies stable have and shown have a that longer theequimolar shelf life than vitaminquantity E ofin vitaminthe presence E ester of has light better and potency oxygen. than Studies corresponding have shown vitamin that E since the vitamin equimolar quantityE has serious of vitamin oxidative E ester destruction has better before potency being than absorbed corresponding by the small vitamin intestinal E since wall vitamin in E hmiceas serious [12–14]. oxidative Vitamin E destruction ester is hydrolyzed before bybeing esterase absorbed to free by vitamin the small E, and, intestinal with blood wall in micecirculation, [12–14]. freeVitamin vitamin E este E spreadsr is hydrolyzed to various by tissues esterase to perform to free this vitamin function E, and [15]., with blood circulation, free vitamin E spreads to various tissues to perform this function [15].

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Catalysts 2021, 11, 739 4 of 11 Vitamin E esters still retain free radical scavenging capacity. Free radicals generated by UV-induced melatonin interacted with vitamin E and vitamin E ester derivatives, in- cludingVitamin vitamin E estersE acetate, still retainvitamin free E radicallinoleic scavenging acid ester, capacity. and vitamin Free radicalsE succinate, generated and their freeby radical UV-induced scavenging melatonin abili interactedty was tested with [16 vitamin,17]. The E andresults vitamin showed E ester that derivatives,vitamin E lino- leicincluding acid ester vitamin had the E acetate, inhibitory vitamin effect E linoleicof 25.6% acid at ester,the mass and vitaminconcentration E succinate, of 1%, and while vitamintheir free E succinic radical scavenging acid ester abilityhad the was inhibitory tested [16 effect,17]. The of 37.6% results at showed the mass that concentration vitamin E of linoleic0.1% [17] acid. The ester DPPH had the free inhibitory radical scavenging effect of 25.6% experiment at the mass showed concentration that vitamin of 1%, while E acetate vitamin E succinic acid ester had the inhibitory effect of 37.6% at the mass concentration of still had the same free radical scavenging ability as vitamin E [18]. The above experiments 0.1% [17]. The DPPH free radical scavenging experiment showed that vitamin E acetate indicated that the esters of vitamin E did not lose their antioxidant capacity, but even had still had the same free radical scavenging ability as vitamin E [18]. The above experiments a strongerindicated anti thatoxidant the esters capacity of vitamin than E didvitamin not lose E. theirWhen antioxidant used as an capacity, effective but ingredient even had in cosmetics,a stronger vitamin antioxidant E ester capacity can effectively than vitamin prevent E. When the usedskin from as an oxidative effective ingredient damage [19] in . In thecosmetics, experiment vitamin of ultraviolet E ester can-induced effectively skin prevent tumors the in skin mice, from both oxidative vitamin damage E succinate [19]. In and vitaminthe experiment E can effectively of ultraviolet-induced inhibit tumor skin formation tumors in[20] mice,. both vitamin E succinate and vitaminSome E vitamin can effectively E ester inhibit derivatives tumor w formationere found [20 to]. have new biological functions. Vita- min E Somesuccinate vitamin was E esterfound derivatives to be capable were found of inhibit to haveing new the biological growth functions.of estrogen Vitamin receptor- negativeE succinate huma wasn breast found tocancer be capable cells ofand inhibiting inducing the their growth apoptosis of estrogen and receptor-negative death [21]. Vitamin E nichumanotinate breast had cancer important cells and applications inducing their in apoptosis lowering and blood death pressure, [21]. Vitamin lowering E nicotinate body fat, had important applications in lowering blood pressure, lowering body fat, and inhibiting and inhibiting virus proliferation [15]. Experiments in mice showed that the concentration virus proliferation [15]. Experiments in mice showed that the concentration of endogenous of endogenous vitamin E nicotinate was negatively correlated with heart failure [22]. vitamin E nicotinate was negatively correlated with heart failure [22].

3. 3.Research Research Status Status of of Enzymatic Enzymatic PreparationPreparation of of Vitamin Vitamin E EsterE Ester Derivatives Derivatives LipaseLipase-mediated-mediated synthesis synthesis of vitamin vitamin E E acetate, acetate, vitamin vitamin E succinate, E succinate, vitamin vitamin E E DHA/EPADHA/EPA ester ester,, and and vitamin vitamin E ferulic acidacid ester ester are are widely widely studied studied (Figure (Figure3). 3).

Figure 3. Enzymatic preparation of VE esters with various acyl donors. Figure 3. Enzymatic preparation of VE esters with various acyl donors.

3.1. VE Acetate Vitamin E acetate is one of the most extensively studied VE ester derivatives. Pamela Torres et al. reported the transesterification of vinyl acetate with α-tocopherol catalyzed by Novozym 435 in mixed solvent of n-hexane and 2-methyl-2-butanol, with a conversion

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3.1. VE Acetate Vitamin E acetate is one of the most extensively studied VE ester derivatives. Pamela Torres et al. reported the transesterification of vinyl acetate with α-tocopherol catalyzed by Novozym 435 in mixed solvent of n-hexane and 2-methyl-2-butanol, with a conversion rate of 60% achieved [23]. Hao Yuechuo et al. realized the high-efficiency expression of CRL1 in Pichia pastoris and catalyzed acetic anhydride and α-tocopherol esterification in a solvent-free system, with a conversion rate of 97% obtained [24]. Gong et al. carried out a comparative study on the esterification of α-tocopherol catalyzed by self-made immobilized lipase in organic solvent and solvent-free system, respectively [25]. The conversion rate was above 85% in petroleum ether solvent and 95% in the solvent-free system.

3.2. VE Succinate Vitamin E succinate is widely researched due to its medicinal value. Yin et al. used Novozym 435 to catalyze the synthesis of α-tocopherol succinate from succinic anhydride and α-tocopherol; the conversion rate was 94.4% [26]. Jiang et al. catalyzed the reaction of succinic anhydride and α-tocopherol to produce α-tocopherol succinate in DMSO with Candida rugosa lipase (CRL) as the catalyst and a yield of 46.95% was obtained [27]. Hu Y. et al. used sol-gel material to immobilize CRL for the synthesis of f α-tocopherol succinic acid ester [28]. They found that the enzymatic activity of the sol-gel immobilized enzyme was increased 6.7-fold compared to the free enzyme, while the enzymatic activity of the olive oil activated sol-gel immobilized enzyme was increased 1.43-fold compared to the pre-activation (the micromolar number of α-tocopherol converted into succinic acid ester per gram of enzyme per hour was deﬁned as the enzyme activity unit). Jiaojiao X. et al. immobilized CRL to catalyze the formation of vitamin E succinate; the conversion rate was 62.58% [29]. Zhao Junbo et al. used Lip400 to catalyze the reaction of α-tocopherol with succinic acid in dichloromethane; the yield of VE succinate was 80% [30].

3.3. Other VE Esters It was reported that introducing functional acyl donors could enable vitamin E esters novel functions. Zu et al. successfully prepared α-tocopherol DHA/EPA ester by introducing polyunsaturated fatty acids (PUFA) into α-tocopherol with the catalysis of Lipozyme RM IM [31]. Xin et al. used CRL to catalyze the transesteriﬁcation reaction between ethyl ferulate and α-tocopherol in toluene with a yield of 72.3% [32]. Xin et al. adopted Novozym 435 to catalyze the transesteriﬁcation of ethyl ferulate and α-tocopherol in the solvent-free system; the conversion rate was 35.4% [33].

4. Main Factors Influencing Enzymatic Synthesis of Vitamin E Ester Derivatives 4.1. Enzyme Candida antarctica lipase B (CALB), Rhizomucor miehei lipase (RML), C. rugosa lipase (CRL), and other lipases have been proven to catalyze the synthesis of vitamin E ester derivatives [23–31]. The properties of the enzyme itself, the immobilized carriers, and further modification on enzyme molecules have varied effects on VE ester synthesis. The catalytic performance of different enzymes is shown in Table2. Pamela Tor- res et al. studied the transesterification of vinyl acetate with α-tocopherol and found that only Novozym 435 could catalyze the reaction [23]. They thought that CALB had a deeper substrate binding site than other enzymes, which was key to the lipase-mediated transesterification. Studies also indicated that Lipozyme RM IM could catalyze the esterification of α-tocopherol with DHA/EPA [31], and that CRL could catalyze the esterification of α-tocopherol with succinic acid [27]. Jürgen Pleiss et al. compared the geometric structures and active sites of different lipases (Figure4)[ 34]. They found that the hydrophobic region in the active site of RML was larger than that of CALB. The geometric structure and charac- teristics of active sites was thought to play a key role in catalyzing the synthesis of different VE esters. Catalysts 2021, 11, x FOR PEER REVIEW 6 of 11

Catalysts 2021, 11, 739Table 2. Catalytic performance of different lipases in catalyzing the synthesis of VE esters6 of 11 . Enzyme Types of VE Esters Conversion (%) Ref.

Table 2. Catalytic performance of differentα-tocopherol lipases in catalyzing acetate the synthesis of VE esters. 60 [23] Novozym 435 Enzymeα Types-tocopheryl of VE Esters ferulate Conversion (%) Ref.35.4 [33] α-tocopherol acetate 60 [23] Novozym 435 α-tocopherol acetate 97 [24] α-tocopheryl ferulate 35.4 [33] CRL α-αtocopherol-tocopherol acetate succinate 97 [2446.95] [27] CRL α-tocopherol succinate 46.95 [27] αα--tocopheryltocopheryl ferulate ferulate 72.3 [3272.3] [32] succinic anhydride modiﬁed succinic anhydride modified Novozym 435 αα--tocopheroltocopherol succinate succinate 94.4 [2694.4] [26] Novozym 435 Nanogel immobilizedNanogel CRl immobilized CRl αα--tocopheroltocopherol succinate succinate 62.58 [2962.58] [29] Lipozyme RM IM Lipozyme RM IM α-αtocopherol-tocopherol DHA/EPA DHA/EPA ester ester 77.65 [3177.65] [31] Immobilized Candida sp. α-tocopherol acetate 39.37 [25] Immobilized Candida sp. 99–12599–125 α-tocopherol acetate 39.37 [25] Lip400 Lip400 αα--tocopheroltocopherol succinate succinate 86 [35]86 [35]

FigureFigure 4. Trigrams4. Trigrams of lipase of lipase active sites active in CRLB, sites RMLin CRLB and CRL, RML [34]. and Reprinted CRL [34] from. ChemistryReprinted from Chemistry andand Physics Physics of Lipids, of Lipids, Vol. 93, Vol. Jürgen 93, Pleiss,Jürgen Markus Pleiss, Fischer, Markus Rolf Fischer, D Schmid, Rolf Anatomy D Schmid, of lipase Anatomy of lipase bindingbinding sites: sites: the scissile the scissile fatty acid fatty binding acid site, binding Pages No. site, 67–80, Pages Copyright No. 67 1998,–80, with Copyright permission 1998, with permission fromfrom Elsevier. Elsevier. Immobilized carriers have varied inﬂuence on the enzymatic catalysis during VE ester’sImmobilized preparation. Pamela carriers Torres have et al. varied explored influence the effect on of the different enzym immobilizedatic catalysis during VE es- materialster’s preparation. and found that Pamela the catalytic Torres effect et al. of CALBexplored immobilized the effect by polypropyleneof different immobilized materials and found that the catalytic effect of CALB immobilized by polypropylene material with larger pore size and pore volume was more effective than other materials [23]. Jiaojiao X. used immobilized CRL to catalyze the formation of vitamin E succinate and found that CRL immobilized by polyacrylamide nanogels had a layer-by-layer stacking structure, a larger specific surface area and a larger pore volume [29]. Hu Y. immobilized CRL with sol-gel material; they found that the ratio of hydrophobic and hydrophilic silane precursors not only affected the micro-phase separation, but also influenced the immobilization efficiency as well as the specific enzyme activity [28].

Catalysts 2021, 11, 739 7 of 11

material with larger pore size and pore volume was more effective than other materials [23]. Jiaojiao X. used immobilized CRL to catalyze the formation of vitamin E succinate and found that CRL immobilized by polyacrylamide nanogels had a layer-by-layer stacking structure, a larger specific surface area and a larger pore volume [29]. Hu Y. immobilized CRL with sol-gel material; they found that the ratio of hydrophobic and hydrophilic silane precursors not only affected the micro-phase separation, but also influenced the immobilization efficiency as well as the specific enzyme activity [28]. Using specific reagents to modify the surface of enzyme molecules can enhance the catalytic effect of the enzyme. It was found that modification on the primary amino groups of enzyme molecules by using organic acids could greatly enhance the activity and thermal stability of the enzyme [36,37]. Yin Chunhua modified Novozym 435 with succinic anhydride; the enzyme’s thermal stability as well as the catalytic activity both improved. Hu Y. revealed that modification on enzyme molecules by using surfactants could open the hydrophobic lid of the enzyme’s active site with an increased activity achieved [28].

4.2. Reaction Medium Since vitamin E is highly viscous and water-insoluble, most studies related to enzymatic synthesis of VE esters are mainly carried in an organic solvent system. Pamela Torres explored the effect of solvents during Novozym 435-catalyzed transesterification of vinyl acetate with α-tocopherol [23]. A solvent mixture of 2-methyl-2-butanol and n-hexane was used. It was found that the conversion rate increased with the increase in the n-hexane addition ratio, but the conversion rate in pure n-hexane solvent was lower than that in pure 2-methyl-2-butanol solvent. They speculated that the addition of the n-hexane reduced the polarity of the solvent and increased the enzyme activity, but that the presence of a certain amount of the 2-methyl-2-butanol promoted the dissolution of vinyl acetate. In the reaction of succinic anhydride with α-tocopherol catalyzed by Novozym 435, Yin Chunhua found that, in a single solvent system, the yield was higher in DMF and DMSO [26]. DMF and DMSO could dissolve water-soluble and fat-soluble substrates well, but strong solvent polarity would deprive the necessary water on the surface of the enzyme molecule and affect the conformation of the enzyme molecule, resulting in lower enzyme activity. In order to find a balance between the solubility of the substrates and a relatively high enzyme activity, they used DMSO and tert-butanol to prepare a mixed solvent. The highest yield was obtained in the system when the volume ratio of DMSO and tert-butanol was 3:2. Zu Guoren used Lipozyme RM IM to catalyze the reaction of fish oil with α-tocopherol to prepare polyunsaturated fatty acid esters, and investigated the effects of different organic solvents on the reaction [31]. It was found that the conversion rate in the n-hexane solvent was the highest. Xin Jiaying explored the effect of different reaction mediums on the transesterification of ethyl ferulate and α-tocopherol, catalyzed by Novozym 435 [33]. They found that there was almost no reaction in tert-butanol, and that the conversion rate in toluene was 17.7%. The logP of solvent was thought to be the main parameter influencing the reaction during lipase-mediated preparation of VE esters. The related results are summarized in Table3. A few researchers have investigated the enzymatic synthesis of VE esters in the solvent-free system. When preparing α-tocopherol acetate, Gong et al. conducted a series of optimizations in organic solvent and solvent-free system. It was found that the highest yield of α-tocopherol in the organic solvent system was 85%, while in the solvent-free system the yield could reach 95% [25]. Xin Jiaying studied the synthesis of VE ferulic acid ester and found that the highest conversion rate of ethyl ferulate in the solvent-free system was 23.8%, while the highest conversion rate in the organic solvent was only 17.7% [33]. Despite of the fact that extensive research has been carried out to explore enzyme- catalyzed synthesis of VE esters in organic solvents, the use of organic solvents always causes problems of environmental pollution and increased difficulty in product separation. The solvent-free system or other green reaction systems needs to be explored further. Catalysts 2021, 11, 739 8 of 11

Table 3. VE esters synthesized by enzymatic method in different solvents.

α- α- α-tocopheryl α-tocopherol tocopherol tocopheryl Solvents logP Acetate Succinate DHA/EPA Ferulate Yield (%) Yield (%) EsterYield Yield (%) (%) DMSO −1.35 / / 78.12 28.37 31.01 / / DMF −1.0 / / 53.30 10.30 14.62 / / logP < 0 Acetonitrile −0.33 / / 37.54 1.35 1.87 / / Ethyl alcohol −0.24 / 1.82 / / / / / Acetone −0.23 / 3.77 4.43 / / / / Tetrahydrofuran 0.49 / 2.48 / / / / / Ethyl acetate 0.68 / / / 1.01 1.71 / / T-butanol 0.80 / / 23.71 / / / 0 0 < logP≤ 2 2-Methyl-2- 1.17 5 / / / / / / butanol T-amyl alcohol 1.3 / / / 0.97 1.40 / / Benzene 2.0 / 4.28 / / / / / Chloroform 2.0 / / / / / 47.00 / Toluene 2.5 / / 17.89 1.04 1.53 / 17.70 Cyclohexane 3.2 / 15.80 / / / / / Petroleum logP > 2 ~3.5 / 49.00 8.85 / / 49.68 / ether N-hexane 3.5 7 39.37 10.47 0 0 61.00 6.90 N-heptane 4.0 / 34.82 / / / / / Isooctane 4.5 / / / / / / 3.10 Ref. [23][25][26][27][29][31][33]

4.3. Acyl Donors and Acceptors As the most active vitamin E isomer, α-tocopherol is widely studied. In addition to α- tocopherol, oils also contain large numbers of other tocopherols and tocotrienols. Although other tocopherol isomers usually exhibit lower biological activity in organisms, some of them exhibit stronger antioxidant properties in vitro antioxidant studies [38]. Therefore, the acylation of other vitamin E isomers is of great significance for the full utilization of natural vitamin E. During the preparation of different VE esters, a variety of acyl donors can be selected, such as long-chain or short-chain fatty acids, anhydrides, fatty acid esters, ethyl ferulate, and so on. The selection of suitable acyl donors to acylate different VE isomers will contribute greatly to the full utilization of natural VE. Different isomers of vitamin E have different effects on the acylation reaction. Pamela Torres et al. used α-tocopherol and δ-tocopherol, two different acyl receptors, to react with vinyl acetate catalyzed by Novozym 435 [23]. It was found that δ-tocopherol was higher than α-tocopherol in terms of reaction rate and final conversion, which might be related to its less steric hindrance. Various acyl donors also bring different reaction results. For the preparation of VE acetate, the commonly used acyl donors are vinyl acetate and acetic anhydride. Pamela Torres reported the transesterification of vinyl acetate and α-tocopherol catalyzed by Novozym 435 [23]. α-Tocopherol reacted with vinyl acetate for 18 days to achieve a 60% conversion rate. Hao Yuechuo reported the reaction of acetic anhydride and α-tocopherol catalyzed by CRL1 with the conversion rate of 97% obtained [24]. For the preparation of VE succinate, Yin used Novozym 435 to catalyze succinic anhydride and α-tocopherol to synthesize α-tocopherol succinate [26]. After a 48 h reaction, the conversion rate reached 94.4%. Zhao Junbo used Lip400 to catalyze the reaction of α-tocopherol with succinic acid in dichloromethane solvent to prepare VE succinate with a 80% yield after a 6 h reaction [30]. Catalysts 2021, 11, 739 9 of 11

4.4. Water Activity Water activity has complicated the influence on the reaction of enzymatic synthesis of vitamin E ester derivatives. Water molecules directly or indirectly maintain the three- dimensional conformations necessary for the catalytic activity of enzyme molecules through hydrogen bonds, hydrophobic interactions, van der Waals forces, etc. Moreover, during the esterification for vitamin E ester preparation, by-product water is generated and the control of water content during the whole process also needs to be taking into consideration. The optimal water activity required by different enzymes often varies. Anna E.V. Petersson constructed an accurate water activity control system for the kettle reactor, and used Novozym 435 and immobilized CRL to catalyze the esterification of hexadecanol with palmitic acid [39]. It was found that, for Novozym 435, a faster catalytic rate and a higher conversion rate could be achieved at lower water activity, while the opposite was observed for the case of CRL. The optimal water activity of Novozym 435 and CRL was found to be 0.02 and 0.5, respectively. Xin Jiaying et al. explored the effect of water activity during the synthesis of α-tocopherol ferulic acid ester which was close to 25%, while, when the water activity was over 0.23, the average conversion rate was only 5%.

5. Purification of VE Ester Derivatives Solvent crystallization, urea complexation and molecular distillation are widely used for the purification of VE esters [25]. C.D. Robenson proposed a solvent crystallization method for the separation of VE acetate [40]. The experimental system was pyridine-catalyzed esterification of α-tocopherol with acetic anhydride. At the end of the reaction, pyridine, anhydride, and acetic acid were washed with 5% hydrochloric acid. The organic phase was distilled and the fractions at 130–180 ◦C were collected as crystallized raw materials. The residual acetic acid was removed by crystallization in the methanol solution at −30 ◦C, and then the needle-like crystallization of α-tocopherol acetate was obtained by crystallization in the methyl formate solution at −30 ◦C. For urea complexation, VE esters dissolved in organic solvents and crystallized with the added urea, and then other components such as fatty acids and fatty acid esters were removed, and VE esters with high purity could be obtained [41]. Isolation of VE esters generally require multi-stage molecular distillation. On the basis of primary molecular distillation, the temperature of the heating wall increased and the heavy phase obtained after primary molecular distillation was subjected to a second molecular distillation to separate it from vitamin E. Similarly, tertiary molecular distillation further increased the heating wall temperature to separate vitamin E esters [42].

6. Conclusions and Foresight As a natural antioxidant, vitamin E is widely used in food, feed, and cosmetics because of its no potential toxic effect. The conversion of vitamin E into vitamin E ester derivatives can not only improve the stability of vitamin E itself, but also enhance its antioxidant activity. Even the introduction of different acyl donors will obtain some new biological functions. Acylation of vitamin E plays an important role in promoting the sustainable development of the vitamin E industry. There are many kinds of vitamin E esters prepared by this chemical method. At present, the industrial preparation of VE esters adopts a chemical catalytic process. Chemi- cal synthesis of VE ester mainly uses a Lewis acid, an organic base, and other catalysts. With the increasing awareness of global environmental protection, the enzymatic preparation of VE esters has great potential.

Author Contributions: Z.Z.: Literature collection & manuscript writing; L.D.: Literature collection and analysis; D.L.: Content design; W.D.: Content design & revision. All authors have read and agreed to the published version of the manuscript. Funding: This research received the fund from from Dongguan Science & Technology Bureau (Innovative R&D Team Leadership of Dongguan City, 201536000100033). Catalysts 2021, 11, 739 10 of 11

Conﬂicts of Interest: The authors declare no conﬂict of interest.

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