Molecular mechanism of recombinant liver fatty acid binding protein's antioxidant activity

Hepatocytes expressing liver fatty acid binding protein (L-FABP) are known to be more resistant to oxidative stress than those devoid of this protein. The mechanism for the observed antioxidant activity is not known. We examined the antioxidant mechanism of a recombinant rat L-FABP in the presence of a hydrophilic (AAPH) or lipophilic (AMVN) free radical generator. Recombinant L-FABP amino acid sequence and its amino acid oxidative products following oxidation were identified by MALDI quadrupole time-of-flight MS after being digested by endoproteinase Glu-C. L-FABP was observed to have better antioxidative activity when free radicals were generated by the hydrophilic generator than by the lipophilic generator. Oxidative modification of L-FABP included up to five methionine oxidative peptide products with a total of ∼80 Da mass shift compared with native L-FABP. Protection against lipid peroxidation of L-FABP after binding with palmitate or α-bromo-palmitate by the AAPH or AMVN free radical generators indicated that ligand binding can partially block antioxidant activity. We conclude that the mechanism of L-FABP's antioxidant activity is through inactivation of the free radicals by L-FABP's methionine and cysteine amino acids. Moreover, exposure of the L-FABP binding site further promotes its antioxidant activity. In this manner, L-FABP serves as a hepatocellular antioxidant. Hepatocytes expressing liver fatty acid binding protein (L-FABP) are known to be more resistant to oxidative stress than those devoid of this protein. The mechanism for the observed antioxidant activity is not known. We examined the antioxidant mechanism of a recombinant rat L-FABP in the presence of a hydrophilic (AAPH) or lipophilic (AMVN) free radical generator. Recombinant L-FABP amino acid sequence and its amino acid oxidative products following oxidation were identified by MALDI quadrupole time-of-flight MS after being digested by endoproteinase Glu-C. L-FABP was observed to have better antioxidative activity when free radicals were generated by the hydrophilic generator than by the lipophilic generator. Oxidative modification of L-FABP included up to five methionine oxidative peptide products with a total of ∼80 Da mass shift compared with native L-FABP. Protection against lipid peroxidation of L-FABP after binding with palmitate or α-bromo-palmitate by the AAPH or AMVN free radical generators indicated that ligand binding can partially block antioxidant activity. We conclude that the mechanism of L-FABP's antioxidant activity is through inactivation of the free radicals by L-FABP's methionine and cysteine amino acids. Moreover, exposure of the L-FABP binding site further promotes its antioxidant activity. In this manner, L-FABP serves as a hepatocellular antioxidant. Liver fatty acid binding protein (L-FABP) is a low molecular weight protein (14–15 kDa) that belongs to a family of lipid binding proteins (1Glatz J.F. van der Vusse G.J. Cellular fatty acid-binding proteins: their function and physiological significance.Prog. Lipid Res. 1996; 35: 243-282Crossref PubMed Scopus (480) Google Scholar). The protein was first discovered by Ockner et al. in 1972 (2Ockner R.K. Manning J.A. Poppenhausen R.B. Ho W.K. A binding protein for fatty acids in cytosol of intestinal mucosa, liver, myocardium, and other tissues.Science. 1972; 177: 56-58Crossref PubMed Scopus (518) Google Scholar) and was originally named Z-protein. Since that time, there has been an explosive growth in information regarding the role of L-FABP in cellular homeostasis. While FABPs are present in many tissues, such as heart, brain, intestinal, skin, adipose, muscle, epidermal, ileal, myelin, and testis, L-FABP is found in abundance in hepatocytes where it accounts for ∼2% of the total cellular protein. Although L-FABP is abundant in the liver, it also is present in tissues such as murine alveolar macrophages (3Schachtrup C. Scholzen T.E. Grau V. Luger T.A. Sorg C. Spener F. Kerkhoff C. L-FABP is exclusively expressed in alveolar macrophages within the myeloid lineage: evidence for a PPARalpha-independent expression.Int. J. Biochem. Cell Biol. 2004; 36: 2042-2053Crossref PubMed Scopus (30) Google Scholar), kidney (4Yamamoto T. Noiri E. Ono Y. Doi K. Negishi K. Kamijo A. Kimura K. Fujita T. Kinukawa T. Taniguchi H. et al.Renal L-type fatty acid–binding protein in acute ischemic injury.J. Am. Soc. Nephrol. 2007; 18: 2894-2902Crossref PubMed Scopus (277) Google Scholar), and intestine (5Neeli I. Siddiqi S.A. Siddiqi S. Mahan J. Lagakos W.S. Binas B. Gheyi T. Storch J. Mansbach 2nd, C.M. Liver fatty acid-binding protein initiates budding of pre-chylomicron transport vesicles from intestinal endoplasmic reticulum.J. Biol. Chem. 2007; 282: 17974-17984Abstract Full Text Full Text PDF PubMed Scopus (93) Google Scholar). L-FABP is made up of 127 amino acids that compose 10 antiparallel β-strands. These stands are organized into two five-stranded β-sheets and two α-helices. The two α-helices are hypothesized to function as a portal that controls entry and release of ligands from the binding pocket created by the β-strands (6Thompson J. Winter N. Terwey D. Bratt J. Banaszak L. The crystal structure of the liver fatty acid-binding protein. A complex with two bound oleates.J. Biol. Chem. 1997; 272: 7140-7150Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar). A large interior water-filled cavity forms the confines of the β-strands, which serve as the hydrophobic ligand-binding site. L-FABP is able to bind and translocate many lipophilic substrates throughout the cytosol. Some of these substrates include long-chain fatty acids (7Norris A.W. Spector A.A. Very long chain n-3 and n-6 polyunsaturated fatty acids bind strongly to liver fatty acid-binding protein.J. Lipid Res. 2002; 43: 646-653Abstract Full Text Full Text PDF PubMed Google Scholar, 8Martin G.G. Atshaves B.P. McIntosh A.L. Mackie J.T. Kier A.B. Schroeder F. Liver fatty acid-binding protein gene-ablated female mice exhibit increased age-dependent obesity.J. Nutr. 2008; 138: 1859-1865Crossref PubMed Scopus (35) Google Scholar, 9Thumser A.E. Storch J. Liver and intestinal fatty acid-binding proteins obtain fatty acids from phospholipid membranes by different mechanisms.J. Lipid Res. 2000; 41: 647-656Abstract Full Text Full Text PDF PubMed Google Scholar), bile acids (10Martin G.G. Atshaves B.P. McIntosh A.L. Mackie J.T. Kier A.B. Schroeder F. Liver fatty-acid-binding protein (L-FABP) gene ablation alters liver bile acid metabolism in male mice.Biochem. J. 2005; 391: 549-560Crossref PubMed Scopus (56) Google Scholar), eicosanoids (11Raza H. Pongubala J.R. Sorof S. Specific high affinity binding of lipoxygenase metabolites of arachidonic acid by liver fatty acid binding protein.Biochem. Biophys. Res. Commun. 1989; 161: 448-455Crossref PubMed Scopus (72) Google Scholar), and hypolipidemic drugs (12Jefferson J.R. Slotte J.P. Nemecz G. Pastuszyn A. Scallen T.J. Schroeder F. Intracellular sterol distribution in transfected mouse L-cell fibroblasts expressing rat liver fatty acid-binding protein.J. Biol. Chem. 1991; 266: Full Text PDF PubMed Google Scholar). of these ligands from L-FABP to membranes is to by a the ligand first from the binding pocket and to its site of of ligands from other such as is to by a Storch J. acid from liver and intestinal fatty acid-binding proteins to membranes by different mechanisms.J. Biol. 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Specific high affinity binding of lipoxygenase metabolites of arachidonic acid by liver fatty acid binding protein.Biochem. Biophys. Res. Commun. 1989; 161: 448-455Crossref PubMed Scopus (72) Google Scholar). binding polyunsaturated fatty L-FABP the of these fatty acids to oxidative J.A. Spector A.A. acid binding proteins of acid and arachidonic Biophys. 1997; PubMed Scopus Google Scholar) and in this controls the of within the from this In to these have that L-FABP a further role in the cellular antioxidant mechanism G. Y. J. D. G. function of L-FABP in L-FABP transfected liver 2005; PubMed Scopus Google Scholar, G. H. G. Y. F. and antioxidant function of liver fatty acid binding protein in and J. 2007; PubMed Scopus Google Scholar, G. J. Y. of liver fatty acid binding protein in hepatocellular oxidative of and Biochem. 2007; PubMed Scopus Google Scholar). an L-FABP et al. G. Y. J. D. G. function of L-FABP in L-FABP transfected liver 2005; PubMed Scopus Google Scholar) that hepatocytes L-FABP were with of compared with hepatocytes devoid of L-FABP. a of this further that increased L-FABP were with function and lipid peroxidation products G. H. G. Y. F. and antioxidant function of liver fatty acid binding protein in and J. 2007; PubMed Scopus Google Scholar). The antioxidative function of L-FABP is to be to its amino acid L-FABP cysteine and methionine that are known to in cellular the mechanism for the L-FABP antioxidant is not it is to these amino acids. is L-FABP in lipophilic such as or in the hydrophilic In this the mechanism and of the recombinant L-FABP antioxidant when free radicals were into the or lipid was from and were from The and were from other were from The was digested with and for the of a rat L-FABP which was by a of by the to the sequence The and and were by of rat L-FABP was to G. Y. J. D. G. function of L-FABP in L-FABP transfected liver 2005; PubMed Scopus Google Scholar). The with L-FABP was to the and of L-FABP with was by and L-FABP was expressed by into and by of to in L-FABP was to the of The protein was first with the and the was by of or L-FABP digested from by of 10 and and L-FABP were by and time-of-flight The proteins were with for or to membranes for as by G. Y. of fatty acid binding protein and fatty acid liver in Biochem. 2004; PubMed Scopus Google Scholar). the a was proteins were were binding was by of membranes in for were with a against rat L-FABP as G. J. Y. of liver fatty acid binding protein in hepatocellular oxidative of and Biochem. 2007; PubMed Scopus Google Scholar) in being membranes were with for were an L-FABP proteins were digested with or in and the for of the were mass by MALDI The was with a MALDI and a were the of of the peptide to in and a MALDI MALDI MS the peptide were identified by as the The mass of L-FABP was by MALDI and also by mass the In the of the protein was in acid were by against the for the site the mass was through the These of up to and the of methionine and to acid and and and observed for was the of for MS and Da for was by the was the the L-FABP was for free radical by was to the oxidation by of a in with of for in the The was with of to a of of the The be by free radicals to a be were in with and different of L-FABP. of were from by in the presence of were a affinity were to were in with and in the to of were by a protein peroxidation was by two free radical a hydrophilic radical and a lipophilic radical the free radical generators were and with for AAPH was in AMVN was in The lipid peroxidation was by the J.A. lipid PubMed Scopus Google Scholar). the was to from lipid which has been as a of free radical for the was by of in a acid and The was to for and were in the of a was in a was as a acid were expressed as are expressed as included where two were was for was The to the of in The the rat L-FABP sequence was into the of the to for the and of rat in E. and of the the complex was from was by the of protein and of cellular of the a of of total E. and was to a of was to by which of the the protein and for of was from a were not present as observed by be from the complex by recombinant L-FABP was after the complex with and that recombinant L-FABP the molecular weight kDa) as L-FABP from The was as a for the of the antioxidative of recombinant L-FABP. Since L-FABP has been to a role in oxidative stress G. Y. J. D. G. function of L-FABP in L-FABP transfected liver 2005; PubMed Scopus Google Scholar, G. J. Y. of liver fatty acid binding protein in hepatocellular oxidative of and Biochem. 2007; PubMed Scopus Google Scholar), a in by L-FABP be observed in an in oxidation in recombinant L-FABP free radical as observed by a in Moreover, of the free radicals was increased with increased L-FABP of free radical release was which was observed to be of the that L-FABP the by binding or by other et al. G. J. Y. of liver fatty acid binding protein in hepatocellular oxidative of and Biochem. 2007; PubMed Scopus Google Scholar) the of L-FABP. that was not by L-FABP in this the that the in was in to L-FABP inactivation of free the of in lipophilic and hydrophilic free radical it was to the of AAPH and AMVN free radical generators in a of the lipid peroxidation were first examined as a function of of AMVN or AAPH The radical generator (AMVN) in than the generator for it was to AAPH and AMVN that of The of was by 10 AMVN and those were in in the of antioxidant activity L-FABP and acid or was with AAPH or 10 AMVN in the or the presence of different of acid or These two the oxidation of in a the AAPH acid was able to by against lipid peroxidation was observed in the in the AMVN a of of The of by in the lipid peroxidation that has a in the of oxidation by We were in L-FABP is more as a hydrophilic or lipophilic free radical Since L-FABP is it is that of the antioxidant be in the cytosol. as a protein it also lipophilic and as such of the antioxidant be the lipophilic or the oxidation by AAPH or 10 AMVN in the or presence of different L-FABP an in a in by L-FABP and AAPH to free a of L-FABP was able to by and a in was when the of L-FABP was increased to that 10 L-FABP a of as 10 acid more than 10 a of L-FABP against free radical more than or acid L-FABP was against when free radicals were by that L-FABP antioxidant against lipid peroxidation by AAPH than that of AMVN also that 10 L-FABP of and a to that of 10 in the lipophilic free radical the antioxidant activity of L-FABP was to and than acid the long-chain fatty acid binding to L-FABP its antioxidant was with AAPH or 10 AMVN in the or presence of 10 which was with α-bromo-palmitate is not or palmitate In the lipid peroxidation 10 L-FABP was able to by α-bromo-palmitate and palmitate partially the L-FABP antioxidative activity by and compared with L-FABP also that in the lipid peroxidation 10 L-FABP was able to by α-bromo-palmitate and palmitate L-FABP antioxidative activity by and with L-FABP These indicated that by the L-FABP binding cavity of the L-FABP antioxidant activity was Recombinant L-FABP was following by MALDI The amino acid sequence of recombinant rat L-FABP was with that of L-FABP from rat liver A. of rat liver proteins by time-of-flight mass following and Biochem. 2002; PubMed Scopus Google Scholar). presence of a mass of mass of L-FABP was than that the amino acid sequence The increased mass was to amino acids the and of the cysteine which was observed following peptide mass of L-FABP by with endoproteinase and and by of the peptide that the amino acid sequence was present in of the protein of the sequence an increased mass of observed the was from the by peptide has been to protein S. 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Ho A. of a recombinant of the of different 18: PubMed Scopus Google Scholar). sequence of the peptide was as after in included the of a an mass of is with the recombinant by the mass of Da with the mass of the total L-FABP mass of A more MALDI MS of the observed peptide of two by and is in protein sequence by MALDI mass to the of the amino acids the of that amino the L-FABP sequence and The cysteine is sequence is in a The to the of the amino acids the of that amino the L-FABP sequence and The cysteine is sequence is the modification of L-FABP by AAPH or L-FABP was with AAPH or 10 AMVN and following by the mass shift from of to or The mass shift of to ∼80 Da oxidation by The mass shift of to a Da oxidation by the L-FABP oxidative after with AAPH or L-FABP was with AAPH or 10 and the peptide mass of the was by with endoproteinase and by MS The in the of FABPs are in In the MALDI MS of L-FABP the observed and to five methionine and evidence for modification of methionine by free In the MALDI MS of L-FABP the observed to methionine which evidence for modification of methionine by free MS of the in the of the and of the observed peptide the peptide its peptide 10 the was the first site in this peptide as by of the observed peptide the peptide its peptide the was the first site in this peptide as by in a by free radicals lipophilic such as membranes or in forms of Lipid peroxidation products are of are and able to with many cellular lipophilic and acids H. and of by and lipid peroxidation Res. 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AMVN is a that to free of radicals is the lipophilic of where its activity is to phospholipid peroxidation E. radical as of or PubMed Scopus Google Scholar). the free radical generators AAPH and AMVN with of the controls acid and the of L-FABP as an antioxidant. and acid were in from free radicals generated by acid free not was L-FABP was in free radicals by AAPH and L-FABP is known to the of phospholipid membranes A.E. of recombinant rat liver fatty acid-binding protein to phospholipid vesicles in ligand a for binding and fatty acid PubMed Scopus Google Scholar) and can of phospholipid oxidation by with the in this this of is in the of cellular membranes of oxidative stress lipophilic have been to be in the of fatty liver J.A. C. and in and Liver 2008; Scholar), J. D. of and the in the J. 2004; PubMed Scopus Google Scholar), and C. E. C. L. C.M. in of and to oxidative Biochem. 1996; PubMed Scopus Google Scholar). was to the mechanism of L-FABP's antioxidant activity. acid sequence that recombinant protein cysteine and which was to the sequence by A. of rat liver proteins by time-of-flight mass following and Biochem. 2002; PubMed Scopus Google Scholar, S. Schroeder F. and of two forms of liver fatty acid binding protein from the Biophys. PubMed Scopus Google Scholar). in proteins are known to be to methionine by and can be to the by methionine N. H. and physiological role of methionine in Biochem. Biophys. PubMed Scopus Google Scholar). that in the AAPH hydrophilic five of the methionine amino acids were These included and these five amino was by and and were for with free is to the of these amino acids within the protein is the L-FABP binding cavity chain the is an of low it to be to free radicals (6Thompson J. Winter N. Terwey D. Bratt J. Banaszak L. The crystal structure of the liver fatty acid-binding protein. A complex with two bound oleates.J. Biol. Chem. 1997; 272: 7140-7150Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar). it is not to that was not in the AAPH or AMVN free radical of free radicals to amino acid also a in protein structure through G. modification and of amino acid as for protein Chem. 2005; PubMed Scopus Google Scholar), which other amino acids or for the radical generator of the in L-FABP were for While and were in was in the lipophilic was the methionine with the of to free radicals in the AMVN MS of the methionine in L-FABP with free the cysteine of L-FABP in the presence of free radicals can a acid that is by a through the of The of PubMed Scopus Google Scholar). of amino acids was by and G. modification and of amino acid as for protein Chem. 2005; PubMed Scopus Google Scholar). that cysteine and methionine were the of the amino acids. The cysteine in recombinant L-FABP was for with free radicals it was the by cysteine is known to be in free radicals A. T. F. the in with J. Nutr. 2002; PubMed Scopus Google Scholar) and also it a in L-FABP's antioxidant activity. the of L-FABP's binding pocket to its antioxidant activity in the hydrophilic and lipophilic free radical L-FABP with palmitate or binding to L-FABP is a with and A. acid binding protein is a of of palmitate and its J. Liver PubMed Scopus Google Scholar). not to its of metabolism it to in the it more binding to L-FABP compared with palmitate of binding to fatty acid binding protein the transport of fatty J. 1996; Google Scholar). of palmitate to L-FABP L-FABP's to free binding of long-chain fatty acids to L-FABP increased the of free radicals within the hydrophilic and lipophilic The of generated by AMVN was than that generated by that when palmitate is bound to the protein methionine be for with free is in the lipophilic of α-bromo-palmitate to L-FABP in a more in the AMVN generator. While in these were in the lipophilic is known to be in the binding cavity of L-FABP (6Thompson J. Winter N. Terwey D. Bratt J. Banaszak L. The crystal structure of the liver fatty acid-binding protein. A complex with two bound oleates.J. Biol. Chem. 1997; 272: 7140-7150Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar). Since α-bromo-palmitate is not it be to bind to a than palmitate and block the of to this amino binding of polyunsaturated long-chain fatty acids binding than that of palmitate be to have binding In such not be for with The methionine within the L-FABP binding cavity is of more when free radicals within a lipophilic and are the other two methionine that have the to with to a than is the binding of α-bromo-palmitate to L-FABP in the AMVN not the antioxidant of L-FABP. that as L-FABP in to the the protein such that the ligand binding pocket is to the by the and protein. have been to for and membranes J. Storch J. of portal in fatty acid binding protein and phospholipid 1996; 35: PubMed Scopus Google Scholar). In this manner, L-FABP with lipophilic The that of the free radicals were by L-FABP in the presence of α-bromo-palmitate indicated that in L-FABP binding cavity is the amino acid that is for L-FABP antioxidant activity in lipid In the a and to large of recombinant L-FABP. The that L-FABP from oxidation with a to than MS that the methionine of L-FABP were for the of its antioxidant activity in lipophilic and hydrophilic in the L-FABP binding cavity was to be the amino acid for with in the lipophilic free radical These that rat L-FABP can as a cellular antioxidant. liver fatty acid binding protein

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