THE VISUAL PIGMENTS 



exposed by bleaching could be measured. In the example shown 

 (Fig. 4.6) the — SH groups were equivalent to 0-2 ml of 0-001 M 

 solution, i.e. to 2 x 10-^ moles of — SH groups. Since the rhodopsin 

 solution used was equivalent to 1-0 x 10"^ moles of retinenci, 

 bleaching had released — SH groups and retinenci in the ratio of 2: 1. 



Several experiments of this type were performed with rhodopsin 

 from cattle, frog and squid. In all cases the ratio of — SH to retinenej 

 was about 2, the variation being from 1-8 to 2-4 and the mean, 2-1. 



When these observations were made the stereoisometic aspects 

 (see Chap. 5) of the formation and bleaching of rhodopsin were 

 not yet appreciated, and an accurate value for the molecular extinc- 

 tion of rhodopsin was not available. In a subsequent paper, wald 

 and BROWN (1953) recalculated the results in the light of later 

 knowledge, and obtained somewhat higher values for the mean 

 values of the sulph-hydryl : retinene ratio, viz. 2-2 for cattle (mean 

 of 6), 2-7 for frog (mean of 5) and 2-9 for squid (1 result). 



STRUCTURE OF RHODOPSIN 



As a result of these experiments wald and brown (1952) sug- 

 gested possible formulae for rhodopsin. They considered that — SH 

 groups may take part in rhodopsin synthesis in two ways. They 

 could be engaged directly in binding the prosthetic group to opsin; 

 or alternatively they could yield hydrogen atoms for some reductive 

 transformation of retinenci into the prosthetic group, simultaneously 

 forming a disulphide ( — S — S — ) linkage. One could imagine also a 

 combination of both types of reaction.' 



The simplest reaction between retinenci and the — SH group of 

 opsin is 



^I9'^27^^0+ HS-opsin >■ C,9H2yC—S — opsin 



OH 



i.e. the formation of a hemi-thioacetal. If two — SH groups are 

 involved, the following reaction is possible, 



^19^27^^-^ + HS/°^^'" ^ C|9^^27C'^\ /°PS'" + HjG 



As wald and brown were aware, however, something more than 

 this is required to yield rhodopsin. The reactions given above 

 *involve, without any compensatory change, the loss of the conju- 

 gated carbonyl group of retinene^' and so would yield products whose 



120 



