III. niOCHEMICAL SYSTEMS 85 



aiul Ihi.s is then esterilied. The parallelism between both types of system is 

 apparent in such a diagram as follows: 



Rhodopsin /3-Carotene-protein 



\light carotene oxidase/ 

 \ / 



Retinene + protein 



alcohol dehydrogenase 

 cozymase 



Vitamin A 



esterifying system 



Vitamin A esters 



At present the principal usefulness of such a diagram is to pose a series 

 of problems. What is the nature of the combination of /S-carotene — a hydro- 

 carbon bearing no ob\'iously active groups — with protein? Is it true that 

 such protein complexes represent the metabolic form of carotene, and 

 perhaps also of vitamin A? Does there exist in the tissues a "carotene oxi- 

 dase" that acts as postulated here? What is the enzymatic mechanism 

 which esterifies vitamin A? And so on. 



If all these matters were settled, they would dispose only of the vegeta- 

 tive aspects of vitamin A metabolism. They would leave untouched the 

 much more interesting questions involving the utilization of vitamin A in 

 the tissues. Perhaps here also something can be learned from the utilization 

 of vitamin A in the retina. The widespread distribution of the alcohol 

 dehydrogenase system makes it possible for many tissues to convert vitamin 

 A to retinene, particularly in situations in which some receptor molecule 

 is available to condense with and remove retinene from the system. The 

 particular gain in oxidizing vitamin A to retinene as a first step in its 

 utilization is that retinene is an enormously more active molecule. It con- 

 denses spontaneously with amino and sulfhydryl groups on proteins and 

 other types of molecules, undergoes addition reactions of various kinds, 

 and exhibits in general the wide variety of reactions that go with the 

 carbonyl group. It is possible that in the synthesis of rhodopsin we have a 

 model for a general class of reactions in which vitamin A, through inter- 

 mediate conversion to retinene, is attached to other molecules to form the 

 complexes upon which its general cellular activities depend. 



In all these reactions the stereoisomerism of vitamin A, its derivatives, 

 and its carotenoid precursors must play a part. As already noted, changes 

 in cis-trans configuration affect the shape of the molecule; and for reactions 

 with enzymes and other proteins shape is everything. In addition to the 



