ISOMERISM AND THE VISUAL PIGMENTS 

 REACTION BETWEEN THE RETINENESj AND CATTLE OPSIN 



HUBBARD and WALD (1952) Studied the reaction in digitonin 

 solution between the five isomers of retinene^ and opsin (prepared 

 from cattle retinae). The mixtures of opsin and each of the retinenci 

 isomers (in excess) were allowed to stand in darkness for 1|-2| hr. 

 After adding hydroxylamine to prevent further reaction, the differ- 

 ence spectrum of the product (if any) was then obtained by measuring 

 the absorption spectra of the solutions before and after bleaching. 

 The results obtained for the all-trans, neo-a, neo-b and iso-a isomers 

 of retinenci are given in Fig. 5.6. This shows that the 2i\\-trans and 

 neo-a isomers yielded no photosensitive pigment; the neo-b isomer 

 produced a rhodopsin apparently identical with the naturally- 

 occurring visual pigment ; while /^o-retinenci a yielded a photosensi- 

 tive pigment having Amax at about 487 m//, about 13 m/^ lower than 

 that for the visual pigment. The remaining isomer, /^-o-retinenej b 

 was found to be inactive, but, because it isomerized fairly readily 

 even in darkness to form /^o-retinenej a, some photosensitive pigment 

 with Amax at 487 m.fji was obtained. 



Thus of the five known isomers of retinenci only two {neo-b and 

 iso-a) could be induced to react with cattle opsin to form photo- 

 sensitive pigments. The pigment with Amax = 487 m/x obtained from 

 wo-retinene a was called wo-rhodopsin to distinguish it from the 

 naturally-occurring pigment cattle rhodopsin obtained from neo- 

 retinenci b. 



The term /^o-rhodopsin has also been used to describe regenerated 

 rhodopsin. In this context (collins and morton, 1950) it has 

 ^max = 487-488 when derived from rat or cattle rhodopsin and 

 ^►max = 493 mjjL when derived from frog rhodopsin. 



Reaction kinetics. The reaction between opsin and /leo-retinenci b 

 the precursor of rhodopsin, follows the course of a bimolecular 

 reaction (Fig. 5.7). This would be expected if the formation of 

 rhodopsin involves a reaction between one molecule of opsin and 

 one of retinene (cf. p. 94). chase and smith (1939), on the other 

 hand, found that the small amount of pigment regenerated in bleached 

 solutions of frog rhodopsin was formed according to the kinetics of 

 a monomolecular reaction, hubbard and v^ald (1952) reconciled 

 this result with their own by pointing out that the bleaching of 

 rhodopsin results mainly in the production of non-active isomers of 

 retinene (thus accounting for the small degree of regeneration usually 



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