Identification and Characterization of a Pi Isoform of Glutathione S-Transferase (GSTP1) as a Zeaxanthin-binding Protein in the Macula of the Human Eye

Abstract

Uptake, metabolism, and stabilization of xanthophyll carotenoids in the retina are thought to be mediated by specific xanthophyll-binding proteins (XBPs). A membrane-associated XBP was purified from human macula using ion-exchange chromatography followed by gel-exclusion chromatography. Two-dimensional gel electrophoresis showed a prominent spot of 23 kDa and an isoelectric point of 5.7. Using mass spectral sequencing methods and the public NCBI database, it was identified as a Pi isoform of human glutathione S-transferase (GSTP1). Dietary (3R,3′R)-zeaxanthin displayed the highest affinity with an apparent Kd of 0.33 μm, followed by (3R,3′S-meso)-zeaxanthin with an apparent Kd of 0.52 μm. (3R,3′R,6′R)-Lutein did not display any high-affinity binding to GSTP1. Other human recombinant glutathione S-transferase (GST) proteins, GSTA1 and GSTM1, exhibited only low affinity binding of xanthophylls. (3R,3′S-meso)-Zeaxanthin, an optically inactive nondietary xanthophyll carotenoid present in the human macula, exhibited a strong induced CD spectrum in association with human macular XBP that was nearly identical to the CD spectrum induced by GSTP1. Like-wise, dietary (3R,3′R)-zeaxanthin displayed alterations in its CD spectrum in association with GSTP1 and XBP. Other mammalian xanthophyll carrier proteins such as tubulin, high-density lipoprotein, low-density lipoprotein, albumin, and β-lactoglobulin did not bind zeaxanthins with high affinity, and they failed to induce or alter xanthophyll CD spectra to any significant extent. Immunocytochemistry with an antibody to GSTP1 on human macula sections showed highest labeling in the outer and inner plexiform layers. These results indicate that GSTP1 is a specific XBP in human macula that interacts with (3R,3′S-meso)-zeaxanthin and dietary (3R,3′R)-zeaxanthin in contrast to apparently weaker interactions with (3R,3′R,6′R)-lutein. Uptake, metabolism, and stabilization of xanthophyll carotenoids in the retina are thought to be mediated by specific xanthophyll-binding proteins (XBPs). A membrane-associated XBP was purified from human macula using ion-exchange chromatography followed by gel-exclusion chromatography. Two-dimensional gel electrophoresis showed a prominent spot of 23 kDa and an isoelectric point of 5.7. Using mass spectral sequencing methods and the public NCBI database, it was identified as a Pi isoform of human glutathione S-transferase (GSTP1). Dietary (3R,3′R)-zeaxanthin displayed the highest affinity with an apparent Kd of 0.33 μm, followed by (3R,3′S-meso)-zeaxanthin with an apparent Kd of 0.52 μm. (3R,3′R,6′R)-Lutein did not display any high-affinity binding to GSTP1. Other human recombinant glutathione S-transferase (GST) proteins, GSTA1 and GSTM1, exhibited only low affinity binding of xanthophylls. (3R,3′S-meso)-Zeaxanthin, an optically inactive nondietary xanthophyll carotenoid present in the human macula, exhibited a strong induced CD spectrum in association with human macular XBP that was nearly identical to the CD spectrum induced by GSTP1. Like-wise, dietary (3R,3′R)-zeaxanthin displayed alterations in its CD spectrum in association with GSTP1 and XBP. Other mammalian xanthophyll carrier proteins such as tubulin, high-density lipoprotein, low-density lipoprotein, albumin, and β-lactoglobulin did not bind zeaxanthins with high affinity, and they failed to induce or alter xanthophyll CD spectra to any significant extent. Immunocytochemistry with an antibody to GSTP1 on human macula sections showed highest labeling in the outer and inner plexiform layers. These results indicate that GSTP1 is a specific XBP in human macula that interacts with (3R,3′S-meso)-zeaxanthin and dietary (3R,3′R)-zeaxanthin in contrast to apparently weaker interactions with (3R,3′R,6′R)-lutein. Carotenoids are ubiquitous highly colored pigments synthesized exclusively by plants and microorganisms. Over 600 carotenoids have been isolated from natural sources, and ∼370 of these carotenoids are chiral, bearing one to five asymmetric carbon atoms (1Pfander H. Methods Enzymol. 1992; 213: 3-13Crossref Scopus (85) Google Scholar). Many carotenoids are classified as xanthophylls because they contain at least one oxygen atom along their C40H56 isoprenoid core structure. Higher animals cannot synthesize carotenoids, but they can ingest them in their diet and subsequently utilize them for important physiological functions. Whereas >15 different dietary carotenoids are detectable in human serum, only lutein ((3R,3′R,6′R)-β,ϵ-carotene-3,3′-diol) and zeaxanthin (a mixture of (3R,3′R)-β,β-carotene-3,3′diol and (3R,3′S-meso)-β,β-carotene-3,3′-diol) (see Fig. 1) and their metabolites are found to any substantial extent in the retina (2Bernstein P.S. Khachik F. Carvalho L.S. Muir G.J. Zhao D.Y. Katz N.B. Exp. Eye Res. 2001; 72: 215-223Crossref PubMed Scopus (324) Google Scholar, 3Bone R.A. Landrum J.T. Hime G.W. Cains A. Zamor J. Investig. Ophthalmol. Vis. Sci. 1993; 34: 2033-2040PubMed Google Scholar, 4Landrum J.T. Bone R.A. Arch. Biochem. Biophys. 2001; 385: 28-40Crossref PubMed Scopus (676) Google Scholar). High macular levels of these antioxidant and blue light-screening xanthophyll carotenoids are associated with decreased risk of age-related macular degeneration (5Moeller S.M. Jacques P.F. Blumberg J.B. J. Am. Coll. Nutr. 2000; 19: 522S-527SCrossref PubMed Scopus (203) Google Scholar, 6Bernstein P.S. Zhao D.Y. Wintch S.W. Ermakov I.V. McClane R.W. Gellermann W. Ophthalmology. 2002; 109: 1780-1787Abstract Full Text Full Text PDF PubMed Scopus (226) Google Scholar), the leading cause of blindness in the developed world. In the foveal region of the macula of the primate retina, lutein and zeaxanthin concentrations are at their highest, and they are spatially localized to the outer plexiform layer (also known as the receptor axon or Henle fiber layer) and to the inner plexiform region (7Snodderly D.M. Auran J.D. Delori F.C. Investig. Ophthalmol. Vis. Sci. 1984; 25: 674-685PubMed Google Scholar). The concentration of the macular carotenoids falls precipitously outside of the foveal region, so that the concentration of lutein and zeaxanthin in the peripheral retina per unit area is 1% of the concentration at the fovea (8Snodderly D.M. Handelman G.J. Adler A.J. Investig. Ophthalmol. Vis. Sci. 1991; 32: 268-279PubMed Google Scholar), and a considerable portion of the extrafoveal carotenoids is associated with the rod outer segments (9Rapp L.M. Maple S.M. Choi J.H. Investig. Ophthalmol. Vis. Sci. 2000; 41: 1200-1209PubMed Google Scholar, 10Sommerberg O.G. Siems W.G. Hurst J.S. Lewis J.W. Kliger D.S. van Kuijk F.J. Curr. Eye Res. 1999; 19: 491-495Crossref PubMed Scopus (160) Google Scholar). Whenever a tissue exhibits highly selective uptake and deposition of biological molecules, it is likely that specific binding proteins are involved. These binding proteins may act as cell surface receptors, transmembrane transport proteins, metabolic enzymes, intracellular mediators of the biological actions of the ligand, or sites for the deposition and stabilization of the ligand. Carotenoid-binding proteins have been described in plants (11Ruban A.V. Young A.J. Horton P. Biochemistry. 1996; 35: 674-678Crossref PubMed Scopus (115) Google Scholar), microorganisms (12Bullerjahn G.S. Sherman L.A. J. Bacteriol. 1986; 167: 396-399Crossref PubMed Google Scholar, 13Reddy K.J. Masamoto K. Sherman D.M. Sherman L.A. J. Bacteriol. 1989; 171: 3486-3493Crossref PubMed Google Scholar), and invertebrates (14Tabunoki H. Sugiyama H. Tanaka Y. Fujii H. Banno Y. Jouni Z.E. Kobayashi M. Sato R. Maekawa H. Tsuchida K. J. Biol. Chem. 2002; 277: 32133-32140Abstract Full Text Full Text PDF PubMed Scopus (85) Google Scholar, 15Zagalsky P.F. Wright C.E. Parsons M. Adv. Space Res. 1995; 16: 91-94Crossref PubMed Scopus (13) Google Scholar), but relatively little information has been available about specific carotenoid-binding proteins in any vertebrate system. In 1997, Rao et al. (16Rao M.N. Ghosh P. Lakshman M.R. J. Biol. Chem. 1997; 272: 24455-24460Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar) reported the existence of a β-carotene-binding protein in ferret liver, although no sequence data have been provided yet. More recently, we have described the properties of a partially purified membrane-associated xanthophyll-binding protein (XBP) 1The abbreviations used are: XBP, xanthophyll-binding protein; CHAPS, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid; GST, glutathione S-transferase; GSTA, glutathione S-transferase Alpha; GSTM, glutathione S-transferase Mu; GSTP, glutathione S-transferase Pi; HDL, high-density lipoprotein; IEF, isoelectric focusing; LDL, low-density lipoprotein; MALDI, matrix-assisted laser desorption ionization; MES, 4-morpholineethanesulfonic acid; THF, tetrahydrofuran; HPLC, high-performance liquid chromatography; PBT, phosphate buffer containing 0.1% Triton X-100. found in human retina and macula (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar, 18Billsten H.H. Bhosale P. Yemelyanov A. Bernstein P.S. Polivka T. Photochem. Photobiol. 2003; 78: 138-145Crossref PubMed Scopus (55) Google Scholar). We have also reported that retinal tubulin exhibits carotenoid binding properties (19Bernstein P.S. Balashov N.A. Tsong E.D. Rando R.R. Investig. Ophthalmol. Vis. Sci. 1997; 38: 167-175PubMed Google Scholar, 20Crabtree D.V. Ojima I. Geng X. Adler A.J. Bioorg. Med. Chem. 2001; 9: 1967-1976Crossref PubMed Scopus (45) Google Scholar), but with less specificity and affinity than XBP (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar). Likewise, several other mammalian proteins such as high-density lipoprotein (HDL), low-density lipoprotein (LDL), albumin, and β-lactoglobulin can act as carrier proteins for carotenoids, although there is no evidence that they do so with high specificity or affinity (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar, 21Clevidence B.A. Bieri J.G. Methods Enzymol. 1993; 214: 33-46Crossref PubMed Scopus (166) Google Scholar, 22Zsila F. Bikadi Z. Simonyi M. Biochem. Pharmacol. 2002; 64: 1651-1660Crossref PubMed Scopus (56) Google Scholar, 23Van den Berg H. Nutr. Rev. 1999; 57: 1-10Crossref PubMed Scopus (145) Google Scholar). In this study, we further purify human macular XBP and identify the major protein spot on two-dimensional gels as a Pi isoform of glutathione S-transferase (GSTP1). Pharmacological and spectroscopic binding studies with human recombinant GSTP1 that its interactions with zeaxanthin of XBP purified from human of from of XBP was on methods (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar), with human from in of a cell containing in buffer with and at for at in a low-density containing XBP. The was with buffer to the concentration to A of the was at for at The was and the was with on of in buffer containing and followed by at for at The was buffer several followed by as described The and to using an mass and of chromatography was on a liquid chromatography human macular an buffer buffer containing at at at for followed by a from to chromatography was on an buffer buffer containing CHAPS, followed by a from to gel chromatography was using a gel to from the ion-exchange the gel and phosphate buffer containing was used as the at a of the by a with a high with an of and The of containing proteins associated with macular carotenoids exhibited strong protein and prominent at of xanthophyll and of purified at of chromatography with of to and they of was by the of of The by in a and in of was at a of on a inner The was at and the was at by spectra and by with as and with an Two-dimensional and using gel The in with the protein in the was in a cell unit at with a of The for the was The with containing and on a protein with protein and using a for mass spectral protein with to a reported A. M. M. Chem. 1996; PubMed Scopus Google Scholar). and to by using The matrix-assisted laser desorption was with of and of as the a with a by was on The spectrum for was to the for (3R,3′R)-zeaxanthin and (3R,3′S-meso)-zeaxanthin from and from was a from XBP to and and the also in the xanthophyll carotenoid carotenoid binding for XBP and other proteins in the of identical concentrations of In a binding carotenoids in to of buffer containing of protein μm, on a mass of 23 the mixture was at binding studies with and (3R,3′S-meso)-zeaxanthin that was carotenoids by of with of followed by for at in a and of the was from the a of and in the as the for carotenoid and because they do not to of carotenoid-binding proteins (16Rao M.N. Ghosh P. Lakshman M.R. J. Biol. Chem. 1997; 272: 24455-24460Abstract Full Text Full Text PDF PubMed Scopus (30) Google Scholar, A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar). the with no of In and CD spectra and carotenoid of binding using and using binding and for binding by in protein been in a at protein concentrations with human recombinant glutathione S-transferase and GSTM1, and with other mammalian carotenoid carrier proteins, tubulin, human HDL, human LDL, and human of these proteins from spectra at with an CD at a of and a spectral of with of used for CD and the spectra in the of spectrum was the of in of reported in of The on the using an at at a of with in with of Immunocytochemistry and from the Eye of the and the from a was using in phosphate buffer for and in in a of retina on the fovea was in and in PBT, and for using in to GSTP1 and from R. of at The antibody GSTP1 was in using protein as the in and and used at a for at to sites of The was using a to an of μm. in in was in and found to be for on and to using a at for binding was by the in 0.1% in 1% in for at on an The was with of buffer and with antibody for at on an of antibody was in The was in the antibody for at on an and developed using of have reported that partially purified from human macula have and specific binding sites for xanthophyll carotenoids (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar). described we have the XBP by the of a and by XBP is a membrane-associated protein; it is to to the protein reported for of XBP CHAPS, an for membrane-associated proteins (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar, L.M. A. Methods Enzymol. 1984; PubMed Scopus Google Scholar). CHAPS, in protein is a A. Vis. Res. PubMed Scopus Google Scholar). these can of studies for of proteins, we also the of such as Triton for XBP was in that we protein studies reported in the and in the to for the of a The XBP that from the gel chromatography exhibits a carotenoid a at with at and of the purified that zeaxanthin and lutein are present in a of XBP purified from human macula only carotenoids displayed a of of of lutein or zeaxanthin to XBP, followed by to carotenoids, the to that purified macular XBP is with of gels of purified human macular XBP have only a major at but two-dimensional that there are with isoelectric from to with These are as in Fig. it is the and the protein in XBP we mass spectral sequencing on spot A We strong at and on several two-dimensional gels from These further using to sequence information on Using a and the public NCBI database, we to identify spot A as GSTP1. The to the and of the sequence of GSTP1 at and spectra from these was to have an isoelectric point of and mass strong of and further of their it is that these are the apparent at is the with an as of the spectra from these any for in the public or and the spectra have not been for and with of kDa and isoelectric of and have not been and with GSTP1 and of dietary (3R,3′R)-zeaxanthin and nondietary (3R,3′S-meso)-zeaxanthin to recombinant human GSTP1 was with apparent of 0.33 and 0.52 than the of that we reported for the of partially purified human macular XBP with lutein and zeaxanthin (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar). The for of zeaxanthin to the of GSTP1 was to be at and binding provided no evidence for receptor binding studies did not have glutathione present in the and the of to glutathione did not alter zeaxanthin binding to GSTP1. We to the of xanthophyll binding on the of GSTP1 because the for xanthophyll binding its we other such as human recombinant and we binding evidence of we have that human macular XBP can bind lutein (17Yemelyanov A.Y. Katz N.B. Bernstein P.S. Exp. Eye Res. 2001; 72: 381-392Crossref PubMed Scopus (92) Google Scholar), we to high-affinity specific binding of lutein to any of the of recombinant human We also GSTP1 with a of mammalian proteins thought to with xanthophyll carotenoids albumin, HDL, LDL, and high-affinity zeaxanthin binding with GSTP1 was detectable although did high-affinity binding for lutein binding studies of tubulin, albumin, HDL, LDL, and β-lactoglobulin with macular xanthophylls. of (3R,3′R)-zeaxanthin (3R,3′S-meso)-zeaxanthin and in to of buffer containing of protein tubulin, human albumin, human HDL, human LDL, the at carotenoids by of with of data are The only high-affinity binding interactions for the zeaxanthins GSTP1 and (3R,3′R)-zeaxanthin and GSTP1 and (3R,3′S-meso)-zeaxanthin (see Fig. for GSTP1 binding exhibited high-affinity binding for lutein binding The binding of dietary (3R,3′R)-zeaxanthin and nondietary (3R,3′S-meso)-zeaxanthin to GSTP1 was further using and CD The spectrum of purified XBP its carotenoid displayed a of as with the spectrum of lutein and zeaxanthin in The spectrum of XBP is identical to that of the macular carotenoid in (8Snodderly D.M. Handelman G.J. Adler A.J. Investig. Ophthalmol. Vis. Sci. 1991; 32: 268-279PubMed Google Scholar, L.M. Maple S.M. Choi J.H. Investig. Ophthalmol. Vis. Sci. 2000; 41: 1200-1209PubMed Google Scholar, 10Sommerberg O.G. Siems W.G. Hurst J.S. Lewis J.W. Kliger D.S. van Kuijk F.J. Curr. Eye Res. 1999; 19: 491-495Crossref PubMed Scopus (160) Google Scholar). GSTP1 no The CD spectrum of XBP displayed a at is of a protein with A.J. Methods Enzymol. PubMed Scopus Google Scholar), the human recombinant GSTP1 a CD at The CD spectrum and of XBP bearing its was and with and at and the GSTP1 no detectable of of of zeaxanthin to XBP or GSTP1 in substantial alterations of their and CD spectra and The of XBP a of to the spectrum of zeaxanthin in or CHAPS, and the of GSTP1 The CD of GSTP1 to binding of that of human macular XBP its spectra of zeaxanthins to GSTP1 and human macular XBP. of (3R,3′R)-zeaxanthin and (3R,3′S-meso)-zeaxanthin with of human recombinant GSTP1 and with human macular XBP at and carotenoids by at concentrations in and in buffer used as spectra as described (3R,3′S-meso)-zeaxanthin has no detectable CD in THF, a In CHAPS, a there was no detectable that the (3R,3′S-meso)-zeaxanthin in the was and displayed a induced with a at and a at and with a point at to recombinant with the weaker CD of XBP with (3R,3′S-meso)-zeaxanthin The of the induced with the and was at a of in with the binding dietary (3R,3′R)-zeaxanthin has a strong CD spectrum in any In THF, its CD spectrum is in the and the in the are to be 2001; PubMed Scopus Google Scholar, R. K. Scholar). in CHAPS, the CD of (3R,3′R)-zeaxanthin is no a is present at and a is present at and The point is at the of one of the to XBP or these CD by The spectra and the of zeaxanthin to GSTP1 or XBP are of a association to with the F. Z. J. Simonyi M. 2001; PubMed Scopus Google Scholar, M. Bikadi Z. F. J. 2003; PubMed Scopus Google Scholar). We also and CD spectra of (3R,3′S-meso)-zeaxanthin and (3R,3′R)-zeaxanthin with GSTA1 and and a of other mammalian proteins thought to be relatively of xanthophyll carotenoids albumin, LDL, HDL, and In nearly the proteins induced no significant alterations in the or CD spectra to the The only was tubulin, induced that in than with XBP or GSTP1. of GSTP1 in with a antibody to GSTP1 on a human that GSTP1 is from but is present the human macula, and it is in the outer plexiform layer (also known as the Henle fiber or receptor axon layer) and the inner plexiform layer sections of peripheral of the retina did not such GSTP1 labeling of the inner and outer plexiform layers. The labeling with GSTP1 in the was with that of H. T. J. W. Investig. Ophthalmol. Vis. Sci. 2003; PubMed Scopus Google Scholar) to the and of The of high levels of GSTP1 in the plexiform of the human macula with the of the macular carotenoids reported by et al. (7Snodderly D.M. Auran J.D. Delori F.C. Investig. Ophthalmol. Vis. Sci. 1984; 25: 674-685PubMed Google Scholar). than carotenoids in human serum, only lutein and are present in the macula of the human and their concentrations at the fovea are on the of selective uptake of these xanthophyll carotenoids the existence of specific but no and specific vertebrate XBP has been identify a human macular XBP, we have of the high levels of colored lutein and zeaxanthin in the human macula as a for proteins that with xanthophyll carotenoids In purified XBP we have only one major at 23 kDa that can be on two-dimensional The prominent spot has a of and we to identify it as GSTP1 by and mass spectral and spectroscopic binding studies with human recombinant protein that GSTP1 physiological of zeaxanthin with high affinity and are of a of that of identical are known to be in the of glutathione to metabolites by in the on their and can be at least different and Biochem. J. 2001; PubMed Scopus Google Scholar), and these can one Pi isoform of GST, is known to be in human and its is localized to GSTP1 is in human and it is known to be present in the retina, although the of GSTP1 the retina has not been proteins have other physiological their may act as in other and binding is thought to be the in a in J. Biol. Chem. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar), and GSTP1 can act as a in a H. M.R. Biochem. J. 1997; PubMed Scopus Google Scholar). Other are in and used as in such as and J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). oxygen the of known as an of containing an to the of that the from that and oxygen J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). is on the Pi of because the is the of and it is the in human F. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). has been known that as binding proteins for In as as the of to their to bind and other metabolites with high affinity Z. Sci. A. Scopus Google Scholar). More recently, a isoform in was reported to bind and from P. Biophys. 2003; PubMed Scopus Google Scholar). In are reported to be proteins Rev. Biol. 1996; PubMed Scopus Google Scholar). this not known to have any physiological interactions with Using recombinant human we to that zeaxanthin present in the human macula bind and with and CD spectral studies with dietary (3R,3′R)-zeaxanthin and nondietary (3R,3′S-meso)-zeaxanthin further for a strong and specific with GSTP1. studies with other available of human and and other mammalian xanthophyll carrier proteins albumin, HDL, LDL, and showed little or no evidence of specific interactions with of GSTP1 are known to alter the binding properties and glutathione S-transferase of GSTP1 the protein is in F. J. J. Biol. Chem. 1997; 272: Full Text Full Text PDF PubMed Scopus Google Scholar). of these have been associated with risk of in the human a tissue known to contain GSTP1 and of zeaxanthin (2Bernstein P.S. Khachik F. Carvalho L.S. Muir G.J. Zhao D.Y. Katz N.B. Exp. Eye Res. 2001; 72: 215-223Crossref PubMed Scopus (324) Google Scholar, A. R. A. K. A.V. Investig. Ophthalmol. Vis. Sci. 2000; 41: Google Scholar). Whereas the association of risk of age-related macular degeneration with of glutathione S-transferase and has been K. Y. T. Am. J. Ophthalmol. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar), no studies of GSTP1 have been or or in GSTP1 the of GSTP1 to bind zeaxanthin and other carotenoids, risk of age-related macular degeneration be to low levels of macular carotenoid is from the studies reported that GSTP1 is a but the human recombinant protein from human macular XBP in several XBP purified from human macula with lutein and GSTP1 only dietary zeaxanthin and human macular XBP is membrane-associated and for human recombinant GSTP1 is a In only a have been reported to be membrane-associated J. Biol. Chem. 1993; Full Text PDF PubMed Google Scholar). these but it is that human macular XBP is a of GSTP1 that is in such a as to it also alter its binding affinity and one of the as proteins that with GSTP1 in macular XBP may be a The of GSTP1 in the macula to be it is as a zeaxanthin uptake and stabilization its retinal be to the of zeaxanthin in the retina, with high concentrations in the inner and outer plexiform of the macula the and levels in the studies of GSTP1 in human macula this is also that GSTP1 may an xanthophyll the mediated by is to the for the of to (3R,3′S-meso)-zeaxanthin J. Biol. Chem. 2001; Full Text Full Text PDF PubMed Scopus Google Scholar, F. Zhao D.Y. Bernstein P.S. Investig. Ophthalmol. Vis. Sci. 2002; Google Scholar). it is that interactions with the GSTP1 protein may the antioxidant of zeaxanthin the macula and that GSTP1 is a protein has the as because proteins and carotenoids to important in and associated with studies of the physiological interactions of proteins and xanthophyll carotenoids in the and in the to be of We and of the Eye identified and human that for the of this We of the of of for of