STEREOCHEMISTRY OF THE HUMAN MACULAR CAROTENOIDS

Purpose. To complete identification of the major components of the hum an macular pigment. Methods. Chemical ionization mass spectra of the m acular pigment components were obtained and compared with those of zea xanthin and lutein standards. A comparison was also made using chiral column high-performance liquid chromatography, which is capable of res olving individual stereoisomers of these carotenoids. Zeaxanthin and l utein from human blood plasma were similarly analyzed. Results. The ma ss spectrometry data supported earlier work in which high-performance liquid chromatography, UV-visible spectrometry and chemical modificati on showed that the macular pigment comprises two carotenoids with iden tical properties to those of zeaxanthin and lutein. Chiral column chro matography showed that the ''zeaxanthin'' fraction is a mixture of two stereoisomers, zeaxanthin itself [(3R,3'R)-beta,beta-Carotene-3,3'-di ol] and meso-zeaxanthin [(3R,3'S)-beta,beta-Carotene-3,3'-diol]. The o ther fraction is the single stereoisomer, lutein [(3R,3'R,6'R)-beta,ep silon-Carotene-3,3'-diol]. In human blood plasma, only zeaxanthin and lutein were found. Conclusions. The results strongly suggest that meso -zeaxanthin results from chemical processes within the retina. Noting that lutein exceeds zeaxanthin in plasma but that the combined zeaxant hin stereoisomers exceed lutein in the retina, the possibility was con sidered that meso-zeaxanthin is a conversion product derived from reti nal lutein. Under nonphysiologic conditions, the authors demonstrate t hat a base-catalyzed conversion of lutein to zeaxanthin yields only th e meso-(3R,3'S) stereoisomer.