Vitamin E: a Review of Its Application and Methods of Detection When Combined with Implant Biomaterials
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materials Review Vitamin E: A Review of Its Application and Methods of Detection When Combined with Implant Biomaterials Francesca Gamna * and Silvia Spriano Department of Applied Science and Technology, Politecnico di Torino, 10129 Torino, Italy; [email protected] * Correspondence: [email protected] Abstract: Vitamin E is a common compound used for tocopherols and tocotrienols (α, β, γ, δ); it is the component of many natural products of both plant and animal origin. Thanks to its powerful antioxidant capacity, vitamin E has been very successful in hip and knee arthroplasty, used to confer resistance to oxidation to irradiated UHMWPE. The positive results of these studies have made vitamin E an important object of research in the biomedical field, highlighting other important properties, such as anti-bacterial, -inflammatory, and -cancer activities. In fact, there is an extensive literature dealing with vitamin E in different kinds of material processing, drug delivery, and development of surface coatings. Vitamin E is widely discussed in the literature, and it is possible to find many reviews that discuss the biological role of vitamin E and its applications in food packaging and cosmetics. However, to date, there is not a review that discusses the biomedical applications of vitamin E and that points to the methods used to detect it within a solid. This review specifically aims to compile research about new biomedical applications of vitamin E carried out in the last 20 years, with the intention of providing an overview of the methodologies used to combine it with implantable biomaterials, as well as to detect and characterize it within these materials. Citation: Gamna, F.; Spriano, S. Keywords: vitamin E; biomaterials; biomedical applications Vitamin E: A Review of Its Application and Methods of Detection When Combined with Implant Biomaterials. Materials 2021, 14, 3691. https://doi.org/10.3390/ 1. Introduction ma14133691 1.1. Structure of Vitamin E Vitamin E was first discovered and described by Evans and Bishop in 1922; it includes Academic Editor: Silvie Rimpelová eight natural forms (α, β, γ, δ; tocopherols and tocotrienols), and it can be found in various products bearing fats of both vegetable and animal origin, such as in olive or almond oil, Received: 17 May 2021 hazelnuts, and egg yolk, and in the liver. Basically, tocopherols and tocotrienols have the Accepted: 29 June 2021 same chemical structure, characterized by a 16-carbon lateral chain attached to position Published: 1 July 2021 2 of a benzopyran ring. The two isoforms differ substantially from the saturation of the long radical chain: tocopherols have a fully saturated chain, while tocotrienols have an Publisher’s Note: MDPI stays neutral unsaturated chain. As Figure1 explains, the two homologs were named according to the with regard to jurisdictional claims in position and number of the methyl group bound to the phenolic ring [1–3]. published maps and institutional affil- Among all the isoforms of vitamin E, α-tocopherol is the most abundant in the blood, iations. because it is the only one that is absorbed within the body, while the other isoforms are excreted through the intestine. The liver, which takes up nutrients after they are absorbed by the small intestine, absorbs vitamin E thanks to the plasmatic lipoproteins that function as carriers. Among all the various forms of the vitamin E, alpha-tocopherol is re-secreted Copyright: © 2021 by the authors. through the liver protein of α-tocopherol transfer (α-TTP) and distributed to circulating Licensee MDPI, Basel, Switzerland. lipoproteins (LDL, IDL, VLDL). The other forms are metabolized and then expelled through This article is an open access article the intestine (Figure2)[4]. distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Materials 2021, 14, 3691. https://doi.org/10.3390/ma14133691 https://www.mdpi.com/journal/materials Materials 2021, 14, x FOR PEER REVIEW 2 of 19 Materials 2021, 14, x FOR PEER REVIEW 2 of 19 Materials 2021, 14, 3691 2 of 19 Figure 1. Structure of tocotrienol and tocopherol. Among all the isoforms of vitamin E, α-tocopherol is the most abundant in the blood, because it is the only one that is absorbed within the body, while the other isoforms are excreted through the intestine. The liver, which takes up nutrients after they are absorbed by the small intestine, absorbs vitamin E thanks to the plasmatic lipoproteins that function as carriers. Among all the various forms of the vitamin E, alpha-tocopherol is re-secreted through the liver protein of α-tocopherol transfer (α-TTP) and distributed to circulating lipoproteins (LDL, IDL, VLDL). The other forms are metabolized and then expelled through the intestine (Figure 2) [4]. Figure 1. Structure of tocotrienol and tocopherol. Figure 1. Structure of tocotrienol and tocopherol. Among all the isoforms of vitamin E, α-tocopherol is the most abundant in the blood, because it is the only one that is absorbed within the body, while the other isoforms are excreted through the intestine. The liver, which takes up nutrients after they are absorbed by the small intestine, absorbs vitamin E thanks to the plasmatic lipoproteins that function as carriers. Among all the various forms of the vitamin E, alpha-tocopherol is re-secreted through the liver protein of α-tocopherol transfer (α-TTP) and distributed to circulating lipoproteins (LDL, IDL, VLDL). The other forms are metabolized and then expelled through the intestine (Figure 2) [4]. FigureFigure 2. 2.Vitamin Vitamin E metabolism.E metabolism. 1.2.1.2. Biological Biological Role Role of of Vitamin Vitamin E E Alpha-tocopherolAlpha-tocopherol is aisphenolic a phenolic antioxidant. antioxidant. The The scavenging scavenging mechanism mechanism involves involves the the donationdonation of of hydrogen hydrogen from from the the hydroxyl hydroxyl group group (-OH) (-OH) of the of phenolicthe phenolic ring toring free to radicalsfree radicals (ROS).(ROS). In In this this way, way, the the free free radicals radicals become become unreactive unreactive and unable and unable to do any to do more any damage. more dam- After this reaction, also the phenolic compound itself becomes relatively unreactive with a age. After this reaction, also the phenolic compound itself becomes relatively unreactive higher stability. Its stability is guaranteed by the now unpaired electron which is on the with a higher stability. Its stability is guaranteed by the now unpaired electron which is oxygen atom and which is delocalized in the structure of the aromatic ring. α-tocopherol is on the oxygen atom and which is delocalized in the structure of the aromatic ring. α-to- located within the phospholipid membrane of the cell, and it occurs with the radical chain embeddedcopherol is in located the hydrophobic within the core phospholipid of the double membrane layer [5]. Its of concentration, the cell, and it compared occurs with to the Figure 2. Vitamin E metabolism. theradical lipids chain present embedded in the membrane, in the hydrophobic is very low, core but inof spitethe double of this, layer it plays [5]. an Its important concentration, rolecompared in preserving to the thelipids integrity present of thein the membrane membrane, by preventing is very low, lipid but peroxidation in spite of this which, it plays 1.2. 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Thatnow said,unpaired it is easy electron to think which of is onvitamin the oxygen E that, atom thanks and to its which important is delocalized qualities asin an the antioxidant, structure of may the have aromatic an important ring. α-to- copherolrole in the is integritylocated within of the brain. the phospholipid To confirm this, membrane a high level of the of αcell-TTP, and was it foundoccurs in with the the radicalbrain [9 chain]. embedded in the hydrophobic core of the double layer [5]. Its concentration, comparedVitamin to Ethe is lipids an important present anti-inflammatory in the membrane, molecule is very sincelow, but it acts in onspite many of this different, it plays factors that affect, directly or indirectly, the immune system. Vitamin E is able to modulate an important role in preserving the integrity of the membrane by preventing lipid perox- idation which causes damage of cellular membranes, lipoproteins, and other molecules that contain lipids, in conditions of oxidative stress [1,6,7]. Materials 2021, 14, x FOR PEER REVIEW 3 of 19 Oxidative stress is a pathological condition caused by the imbalance between the generation and elimination of chemical oxidant species (ROS), and it is involved in several neurodegenerative diseases such as Alzheimer’s and Parkinson's disease that are impli- cated in free radical processes and oxidative damage [8]. That said, it is easy to think of vitamin E that, thanks to its important qualities as an antioxidant, may have an important role in the integrity of the brain.