III. BIOCHEMICAL SYSTEMS 



71 



at all can bo distinguished. In part this behavior may be explained by the 

 fact that the surviving;- tissue tends to become acid as metabolic products 

 accumulate, so inclining the e(iuilibrium toward reduction. The system 

 may be thrown out of balance further by another process which has not 

 yet been mentioned. This is the esterification of vitamin A, which proceeds 



10 



10 



ee 



7.0 



74 



78 8.2 



dH 



8.6 



9.0 



9.4 



Fig. 17. The vitamin A-retinene and the ethanol-acetaldehyde equilibria as a 

 function of pH. Ordinatcs show values of the equilibrium constant, K. The data on 

 the vitamin A-retinene cquililjrium are from Bliss^' and involve the action of horse 

 liver alcohol dehydrogenase. The data on ethjd alcohol are from Racker^^ and involve 

 the action of crystalline j'east alcohol dehydrogenase. (From A. F. Bliss.") 



very actively in the retina and, by removing vitamin A, may displace the 

 equilibrium continuously in the reductive direction. 



E. THE SYNTHESIS OF RHODOPSIN 



We have to this point described the degradation of rhodopsin to a mixture 

 of vitamin A and opsin. In the eye these products recombine to form the 

 visual pigment. 



Many years ago Kiihne recognized that rhodop.sin is synthesized in the 

 retina in two ways: a relatively rapid "anagenesis" from yellow precursors, 



