726 LIGHT AND LIFE 



T^hodopsin 



//gM 



L um i-rho dop s in 



\ Meta-rhodops/n 



reiinene redijcia&e\. L ^ 



D-PN-Hj. 



Vitamin A, + Opsin -* =f Fetinene, -f- Opsin 



ViiamcnAt from 

 pigmeni epitheUum 

 and circulaiion 



DPN-Hz 

 Fig. 1. Diagram of the rhodopsin system. (From Hubbard and Wald, 27). 



retinene is then reduced by alcohol dehydrogenase and DPNH to 

 vitamin A. 



The regeneration of rhodopsin depends primarily upon two reac- 

 tions — the oxidation of vitamin A to retinene, and the combination 

 of retinene with opsin — which are firmly coupled to each other (27) . 

 The equilibrium between vitamin A and retinene — as between any 

 alcohol and aldehyde in the alcohol dehydrogenase system — lies far 

 over toward reduction, toward the alcohol (3) . An appreciable oxi- 

 dation occurs only in the presence of an aldehyde-trapping reagent, 

 which displaces the equilibrium in the oxidative direction by re- 

 moving the aldehyde as fast as formed. In the retina, opsin itself per- 

 forms this function. The combination of retinene with opsin is a 

 spontaneous, hence exergonic, reaction, which proceeds irreversibly 

 in darkness, and requires no enzyme. By removing retinene to form 

 rhodopsin, this reaction drives the endergonic oxidation of vitamin A. 

 For this reason also, when all the ojjsin has been converted to 

 rhodopsin, the oxidation of vitamin A to retinene automatically 

 ceases (27) . 



In the intact eye the regeneration of photopigment is aided further 

 by two accessory processes: the migration of vitamin A into the rods 

 and cones from the pigment epithelium, the adjoining tissue with 

 which they lie in intimate contact; and the continuous re-oxidation 

 of DPN, the oxidant of vitamin A, by the respiratory enzymes of the 

 retina and oxygen from the blood. Both processes help to drive 



