Biological control of primate macular pigment. Biochemical and densitometric studies.

The amounts of zeaxanthin (Z) and lutein (L), the carotenoids constituting the primate macular pigment, were measured in the central retinas of monkeys (Saimiri sciureus and Macaca fascicularis). Two independent methods--reverse-phase high-performance liquid chromatography (HPLC) and microdensitometry--were used for analysis of the same set of retinas. Most of the measurements were made on retinas that had been fixed by glutaraldehyde-paraformaldehyde perfusion of the animal. Control experiments showed that this fixation did not interfere with the quantitative extraction and analysis of the carotenoids. The amount of macular pigment calculated from microdensitometry of the foveal region was proportional to the amount of pigment assayed by HPLC of the same retinal area, demonstrating that either method can be used reliably to rank the carotenoid content of aldehyde-fixed foveas. The optical density of pigment in the axial direction through the retina was higher than would be predicted if the pigment were randomly oriented. This is consistent with the idea that the nonrandom orientation of the dichroic macular pigment molecules found in previous studies contributes to increased optical filtering of the retinal image. Comparisons of the amounts of Z and L between the left and right eyes of the same monkey, within 1 mm of the foveal center, always showed excellent agreement (averaging a 5% difference for Z and 11% difference for L), whereas differences among individual monkeys were very large (up to fourfold for Z). These results indicate that the uptake and assimilation of the macular carotenoids are biologically regulated by selective mechanisms in primate retinas.