356 



Molecular Basis of Vision 719:2. 



instance, by changing the pW of an opsin solution from a neutral 7.0 

 to either 5.0 or 8.0, opsin is 50 per cent irreversibly altered (denatured) 

 in an hour. The cis retinene is relatively easy to "isomerize" to the 

 all-trans form and is primarily converted to the alcohol (vitamin) form. 

 In contrast to opsin and retinene, the compound rhodopsin presents 

 remarkable stability as indicated by the extraction procedure. It is 

 stable over the pH range 3.9 to 9.6. It is easy to imagine that in vitamin 



Retinene\ + Opsin 



ADH 



+ 



DPN + 



ADH 



+ 

 DPNH 



Rhodopsin 



Enzyme ? 



Enzyme i 



Neural Impulse 



Trans -retinene + Opsin 



ADH 



+ 



DPN + 



ADH 



+ 

 DPNH 



Mono Cis -vitamin A 



Trans Vitamin A, 



Body 

 Vitamin A 

 Reserves 



Figure 4. The visual cycle of rhodopsin in the retina. 



A deficiency, the opsin might quickly degenerate. (Indeed, the rods do 

 show very rapid degeneration.) On addition of vitamin A to the diet, 

 the opsin formed would be stabilized as rhodopsin. Hence, the rods 

 (and cones?) could rebuild. This suggests that opsin is a type of 

 adaptive enzyme. This mutual stabilizing may be typical of other 

 adaptive enzymes. 



Under certain conditions, the step from rhodopsin to trans retinene 

 and opsin may be stopped at two intermediate points. If a dried film 

 of rhodopsin is exposed at — 70°C, a new compound, lumirhodopsin, is 

 formed. On heating to 20°C, this spontaneously changes to meta- 

 rhodopsin. This compound is stable when formed from squid rhodop- 

 sin. Metarhodopsin from vertebrate rhodopsin changes spontaneously 

 in the presence of water to trans retinene and opsin. 



No evidence exists concerning the possible appearance of lumi- 

 rhodopsin and metarhodopsin in the normal visual process. It is 

 possible that one of these is stable in the cones, giving rise to the S cone 

 postulated in Chapter 7. Perhaps partially bleached rhodopsin is 

 converted to one of these intermediates under the conditions of color 



