Uptake and isomerization of all-trans retinol by isolated bovine retinal pigment epithelial cells: further clues to the visual cycle.
The site and substrate for all-trans to 11-cis isomerization in the visual cycle have remained obscure for several decades. Only recently studies on a subcellular level have begun to shed some light on these phenomena. We have addressed this system on a cellular level by utilizing intact isolated bovine retinal pigment epithelial cells, maintained during short-term incubation in vitro. Supplementation with labeled all-trans retinol incorporated in a lipid vesicle carrier, in a range of 1-6 nmol per 10(6) cells, resulted in a rapid uptake of retinol. The majority of the internalized retinol was processed prior to mixing with endogenous retinoid pools and most of it was converted into all-trans retinylester. Up to 10% of the incorporated label was isomerized yielding 11-cis retinol, 11-cis retinaldehyde and 11-cis retinylester. The kinetics of the 11-cis retinoid formation indicated that 11-cis retinol is the first isomerization product. The level of 11-cis retinol apparently 'triggered' further processing into other 11-cis retinoids. An updated model with discussion topics is presented for the retinoid pathway relevant to the visual cycle.
Timmers
A M
AM
Department of Biochemistry, University of Nijmegen, The Netherlands.
van Groningen-Luyben
D A
DA
de Grip
W J
WJ
D000818:Animals; D002417:Cattle; D066298:In Vitro Techniques; D007536:Isomerism; D007700:Kinetics; D010857:Pigment Epithelium of Eye / Q000378:metabolism*; D013997:Time Factors; D014785:Vision, Ocular / Q000502:physiology; D014801:Vitamin A / Q000378:metabolism / Q000493:pharmacokinetics*
D014801:Vitamin A