Chicken retinas contain a retinoid isomerase activity that catalyzes the direct conversion of all-trans-retinol to 11-cis-retinol.
Vertebrate retinas contain two types of light-detecting cells. Rods subserve vision in dim light, while cones provide color vision in bright light. Both contain light-sensitive proteins called opsins. The light-absorbing chromophore in most opsins is 11-cis-retinaldehyde, which is isomerized to all-trans-retinaldehyde by absorption of a photon. Restoration of light sensitivity requires chemical re-isomerization of retinaldehyde by an enzymatic pathway called the visual cycle in the retinal pigment epithelium. The isomerase in this pathway uses all-trans-retinyl esters synthesized by lecithin retinol acyl transferase (LRAT) as the substrate. Several lines of evidence suggest that cone opsins regenerate by a different mechanism. Here we demonstrate the existence of two catalytic activities in chicken retinas. The first is an isomerase activity that effects interconversion of all-trans-retinol and 11-cis-retinol. The second is an ester synthase that effects palmitoyl coenzyme A-dependent synthesis of all-trans- and 11-cis-retinyl esters. Kinetic analysis of these two activities suggests that they act in concert to drive the formation of 11-cis-retinoids in chicken retinas. These activities may be part of a new visual cycle for the regeneration of chromophores in cones.
Mata
Nathan L
NL
Sytera, Inc., La Jolla, California 92037, USA.
Ruiz
Alberto
A
Radu
Roxana A
RA
Bui
Tam V
TV
Travis
Gabriel H
GH
D000818:Animals; D002417:Cattle; D002645:Chickens; D002851:Chromatography, High Pressure Liquid; D004952:Esters / Q000378:metabolism; D007536:Isomerism; D007700:Kinetics; D008861:Microsomes / Q000201:enzymology; D008956:Models, Chemical; D010857:Pigment Epithelium of Eye / Q000201:enzymology; D012160:Retina / Q000201:enzymology*; D014801:Vitamin A / Q000096:biosynthesis* / Q000378:metabolism*; D019745:cis-trans-Isomerases / Q000302:isolation & purification* / Q000378:metabolism*; D000099178:Retinoid Isomerohydrolase
D004952:Esters; D014801:Vitamin A; D019745:cis-trans-Isomerases; D000099178:Retinoid Isomerohydrolase