ISOMERISM AND THE VISUAL PIGMENTS 



presumption that frog rhodopsin behaves similarly. Unfortunately 

 little work to date has been carried out with visual pigments from 

 other species. It would be dangerous to assume that other visual 

 pigments, or, even, that 'rhodopsins' from other species, behave in a 

 strictly parallel fashion. 



CRESCITELLI and DARTNALL (1954) reported some comparable 

 phenomena in visual pigment solutions from the carp. They first 

 estabhshed, by the method of partial bleaching (Chap. 6), that their 

 solutions contained a single photosensitive pigment having its 

 absorption maximum at 523 m/u. 



The difference spectra obtained by bleaching alkaline solutions of 

 this pigment showed slight, but definite, variations in the negative 

 portions depending on the wavelength of the light used for bleaching. 

 Furthermore, solutions exhaustively bleached with either red 

 (630 mju) or green (530 m/0 light (see Fig. 5.8) suffered sHght addi- 

 tional density losses, maximal at about 420 m/i, when subsequently 

 exposed to short wavelength light. If, however, a solution was first 

 exhaustively bleached with short wavelength Hght (430 mju), then no 

 further changes occurred when it was afterwards exposed to light of 

 long wavelength. To this extent, the solutions behaved in a manner 

 strictly analogous to those of frog or cattle rhodopsin. Here the 

 resemblance ceased, however. For it was found that regeneration 

 occurred most readily after exposure to light of long wavelength — 

 the very reverse of what happens with frog and cattle rhodopsin. In 

 the case of the carp pigment, therefore, the product of bleaching is 

 already suitable for regeneration. Isomerization of the photoproduct 

 by Hght of short wavelength results in the production of less active 

 forms. The regenerated pigment is apparently identical with the 

 original one as is shown in Fig. 5.9 in which the difference spectra for 

 bleaching and for regeneration are compared. It will be noticed from 

 Fig. 5.9 that the negative portions of the two difference spectra are 

 quite different and reproduce in an exaggerated form the differences 

 found between bleachings with long and short wavelength hght 

 (Fig. 5.8). The negative portion of the regeneration difference spec- 

 trum shows that regeneration occurs from a photoproduct having a 

 greater extinction than the average isomerized product of bleaching. 

 This may indicate that the carp pigment — visual pigment 523 — 

 requires an edl-trans isomer (of retinenca) for its synthesis. However, 

 WALD (1953) states *the bleaching of porphyropsin yields what is 

 apparently the all- trans isomer of retinencg. This is inactive in 



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