19:3/ Molecular Basis of Visior 



357 



Retinene 



vision and gives rise to an absorption curve different than the scotopic 

 relative luminosity curve. 



The nature of the bond between the carotenoid retinene and opsin 

 has been investigated. Certain pro- 

 teins are known to possess free f ^\ — s — h h 

 — S— Hgroupscalled^//%^/groups. ( 0psin ) S _ H + °=C- 

 These are very reactive with a variety ^- — ' 

 of compounds but are inhibited by the 

 compound para chloromercuriben- /- — -v 

 zoate(PCMB). The reaction of opsin '^ , Y ) S ^. 

 and retinene is also inhibited by \^ } — s "^ 15 

 PCMB; quantitative studies indicate 



that the reaction may involve two ° opsm ' 



sulfhydryl groups per retinene! mole- Figure 5. Pictorial representation of 

 cule. This is pictorially represented in , the reaction of retinene with the 

 Figure 5. This, however, may be an postulated sulfhydryl groups of opsin. 



oversimplification. During the reac- 

 tion, the pH changes in a manner which corresponds to blocking or 

 tying up a base such as histidine. The detailed reaction of opsin and 

 retinene is not known. 



3. Other Photopigments 



The cones of the human eye have yielded another pigment similar to 

 rhodopsin but having a different absorption maximum. This pigment, 

 called lodopsin, is of uncertain physiological action because its spectrum 

 does not correspond to any observed physiologically. In the scheme 

 diagrammed in Chapter 7, Talbot avoided this issue by adding a con- 

 tribution labeled "dz" from the rods and the hypothetical S cones. 

 Because the latter are only postulated, one certainly can account for 

 any physiological spectrum, but this theory lacks a conviction similar 

 to that presented by the similarity of the scotopic luminosity curve and 

 the rhodopsin absorption curve. 



An examination of iodopsin (from cones) shows that it is very similar 

 to rhodopsin. Iodopsin is a conjugated compound consisting of 

 retinene! and another protein, cone opsin or photopsin. (The rod 

 protein is referred to in various places as rod opsin and scotopsin.) The 

 rod opsin and the cone opsin extracted from a wide variety of animals, 

 including all the vertebrates, are essentially similar. 



Some animals have a different aldehyde called retinene 2 instead of 

 retinene!. Retinene 2 is the aldehyde of vitamin A 2 , which differs from 

 A x in having an extra double bond in the ring between 3-C and 4-C. 



