Comprehensive Vitamer Profiling of Folate Mono
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H OH metabolites OH Article Comprehensive Vitamer Profiling of Folate Mono- and Polyglutamates in Baker’s Yeast (Saccharomyces cerevisiae) as a Function of Different Sample Preparation Procedures Lena Gmelch 1, Daniela Wirtz 1, Michael Witting 1,2 , Nadine Weber 1 , Lisa Striegel 1 , Philippe Schmitt-Kopplin 1,2,* and Michael Rychlik 1,2,* 1 Chair of Analytical Food Chemistry, Technical University of Munich, 85354 Freising-Weihenstephan, Germany; [email protected] (L.G.); [email protected] (D.W.); [email protected] (M.W.); [email protected] (N.W.); [email protected] (L.S.) 2 Research Unit BioGeoChemistry, Helmholtz Zentrum Munich, 85764 Neuherberg, Germany * Correspondence: [email protected] (P.S.-K.); [email protected] (M.R.) Received: 14 June 2020; Accepted: 16 July 2020; Published: 23 July 2020 Abstract: Folates are a group of B9 vitamins playing an important role in many metabolic processes such as methylation reactions, nucleotide synthesis or oxidation and reduction processes. However, humans are not able to synthesize folates de novo and thus rely on external sources thereof. Baker’s yeast (Saccharomyces cerevisiae) has been shown to produce high amounts of this vitamin but extensive identification of its folate metabolism is still lacking. Therefore, we optimized and compared different sample preparation and purification procedures applying solid phase extraction (SPE). Strong anion exchange (SAX), C18 and hydrophilic–lipophilic-balanced (HLB) materials were tested for their applicability in future metabolomics studies. SAX turned out to be the preferred material for the quantitative purification of folates. Qualification of several folate vitamers was achieved by ultra-high pressure liquid chromatography quadrupole time of flight mass spectrometry (UHPLC-Q-ToF-MS) measurements and quantification was performed by liquid chromatography tandem mass spectrometry (LC-MS/MS) applying stable isotope dilution assays (SIDAs). The oxidation product s-pyrazino-triazine (MeFox) was included into the SIDA method for total folate determination and validation. Applying the best protocol (SAX) in regard to folate recovery, we analyzed 32 different vitamers in different polyglutamate states up to nonaglutamates, of which we could further identify 26 vitamers based on tandem-MS (MS2) spectra. Total folate quantification revealed differences in formyl folate contents depending on the cartridge chemistry used for purification. These are supposedly a result of interconversion reactions occurring during sample preparation due to variation in pH adjustments for the different purification protocols. The occurrence of interconversion and oxidation reactions should be taken into consideration in sample preparation procedures for metabolomics analyses with a focus on folates. Keywords: folate metabolism; folate polyglutamates; baker’s yeast; solid phase extraction; s-pyrazino-triazine (MeFox) 1. Introduction Metabolomics studies usually are performed to obtain a deeper insight into the metabolism of the organism investigated [1–3]. This information is then used to elucidate metabolic pathways [4,5], or to determine biomarkers for diseases [6–8], or influences of outer circumstances on metabolism [9–13]. To avoid artifacts generated by degradation reactions, typical sample preparation procedures are Metabolites 2020, 10, 301; doi:10.3390/metabo10080301 www.mdpi.com/journal/metabolites Metabolites 2020, 10, x FOR PEER REVIEW 2 of 20 Metabolites 2020, 10, 301 2 of 19 13]. To avoid artifacts generated by degradation reactions, typical sample preparation procedures are rather gentle [3,14–16]. Furthermore, ice-cold temperatures throughout the sample preparation procedurerather gentle or [boiling3,14–16 ].in Furthermore,aqueous ethanol ice-cold solutions temperatures are applied throughout to stop metabolic the sample processes preparation by denaturingprocedure orenzymes boiling [1,3,17,18]. in aqueous To date, ethanol mostly solutions metabolites are applied with the to highest stop metabolicabundanceprocesses are typically by investigateddenaturing enzymes [3,18,19],[ 1but,3,17 minor,18]. To compounds date, mostly such metabolites as vitamins with are the not highest analyzed abundance in detail. are However, typically vitaminsinvestigated such [ 3as,18 folates,19], but are minor known compounds to be inalienable such as for vitamins a proper are functioning not analyzed metabolism in detail. [20–23]. However, vitamins such as folates are known to be inalienable for a proper functioning metabolism [20–23]. Folates are a group of vitamers referring to the term vitamin B9, which is commonly known as Folates are a group of vitamers referring to the term vitamin B , which is commonly known as folic acid. Folate vitamers are known to be involved in C1 metabolism9 by providing C1-units for the purinefolic acid. and Folate pyrimidine vitamers synthesis are known as well to as be the involved synthesis in C of1 metabolismthymidylate by[24–27]. providing Furthermore, C1-units folates for the arepurine involved and pyrimidine in the regulation synthesis of asseveral well asneurotra the synthesisnsmitters of thymidylateand the re-methylation [24–27]. Furthermore, of homocysteine folates toare methionine involved in [20,28–30]. the regulation Thus, of intracellular several neurotransmitters accumulation of and homocysteine the re-methylation is prevented. of homocysteine Increased homocysteineto methionine [levels20,28 –are30]. linked Thus, intracellularto neurogenerative accumulation diseases of homocysteinesuch as Alzheimer’s is prevented. disease Increased [31,32]. Furthermore,homocysteine increased levels are risk linked for cardiovascular to neurogenerative diseases diseases [33–35], such or asprevalence Alzheimer’s of certain disease types [31,32 of]. cancerFurthermore, [29,30] are increased reported risk to be for linked cardiovascular to an insufficient diseases folate [33–35 supply.], or prevalence Most severe of is certain the incident types of neuralcancer tube [29,30 defects] are reported in newborns to be linkedafter a tomaternal an insu ffilackcient in folatefolates supply. during Mostpregnancy severe [27,36]. is the incident Humans, of however,neural tube are defects not able in to newborns synthesize after folates a maternal de novo lack and in thus folates are duringdependent pregnancy on an adequate [27,36]. Humans, external supplyhowever, with are folates not able [37]. to synthesizeDeficiencies folates are usually de novo overcome and thus by are supplementation dependent on an or adequatefortification external with folicsupply acid with [38–41]. folates However, [37]. Deficiencies in contrast are to synthe usuallytic overcome supplements, by supplementation natural folates have or fortification not been linked with tofolic increased acid [38 –risk41]. However,of colorectal in contrastcancer [29,38,42]. to synthetic Several supplements, biofortification natural folates approaches have not have been already linked shownto increased the potential risk of of colorectal promoting cancer folate [29 metabolism,38,42]. Several by the biofortification metabolic engineering approaches and upregulation have already ofshown enzymes the potential involved of promotingin folate folatemetabolism metabolism [43–48]. by theHowever, metabolic more engineering information and upregulation about folate of metabolismenzymes involved is needed in folate for targeted metabolism upregulation [43–48]. of However, several folate more vitamers information and about to increase folate the metabolism stability thereof.is needed for targeted upregulation of several folate vitamers and to increase the stability thereof. Figure 11 and Table1 1 give give anan overviewoverview of the core structure structure of of folates: folates: pteridine pteridine is is linked linked with with p- amino-benzoicp-amino-benzoic acid acid and and one one to to fourteen fourteen γγ-linked-linked L-glutamate L-glutamate moieties. moieties. Differences Differences in in the the vitamers vitamers 5 referrefer to thethe didifferentfferent oxidation oxidation states states of of the the pteridine pteridine ring, ring, diff differenterent C1-substitution C1-substitution at positions at positions C and C5 10 andC andC10 diandfferent different lengths lengths of the of polyglutamate the polyglutamate tail. Due tail. to Due those to variations, those variations, more than more 150 than different 150 differentvitamers vitamers are expected are expected to exist in to vivo exist [20 in,49 vivo,50]. [20,49,50]. FigureFigure 1. 1. MolecularMolecular structure structure of of main main folate folate vitamers vitamers known known so so far. far. Table 1. Substitution of main folate vitamers known so far. Table 1. Substitution of main folate vitamers known so far. Trivial Name Abbreviation R1 R2 R3 Trivial Name Abbreviation R1 R2 R3 5 8 Pteroylglutamic acid PteGlu PteGlu –N = –N5H= H –N = –N8= 7,8-dihydrofolate 7,8-H folate H H –N8= 7,8-dihydrofolate 2 7,8-H2folate H H –N8= 5,6,7,8-tetrahydrofolate 5,6,7,8-H4folate H H H 5,6,7,8-tetrahydrofolate 5,6,7,8-H4folate H H H 5-methyl-tetrahydrofolate 5-CH3-H4folate CH3 HH 5-methyl-tetrahydrofolate 5-CH3-H4folate CH3 H 8 H 5-methyl-dihydrofolate 5-CH3-H2folate CH3 H –N = 5-methyl-dihydrofolate 5-CH3-H2folate CH3 H –N8= 5-formyl-tetrahydrofolate 5-CHO-H4folate CHO H H 10-formyl-tetrahydrofolate5-formyl-tetrahydrofolate 10-CHO-H5-CHO-H4folate4folate H CHO CHO H H H 8 10-formyl-tetrahydrofolate10-formyl-dihydrofolate 10-CHO-H10-CHO-H2folate4folate