III. BIOCHEMICAL SYSTEMS 83 



/3-carotene with hytlrogeu peroxide in chloroform-acetic acid. More recently 

 Wendler et a/." obtained yields of retinene as high as 30% on oxidizing 

 /3-carotene with hydrogen peroxide-osmium tetroxide. 



It is particularly interesting that jMeunier et al.^^ have reported obtaining 

 retinene in about 60% yield by oxidizing /3-carotene on solid manganese 

 dioxide. This is essentially the procedure described earlier for oxidizing 

 vitamin A to retinene.^' '^ I have pointed out that such "chromatographic 

 oxidations," in which a solid acts at once as adsorbent and reagent, may 

 mimic on occasion the specificity and directedness of an enzj^matic reaction. 

 Meunier's observations have not yet been confirmed; and it is already clear 

 from trials in a number of laboratories, including our own, that success in 

 this procedure depends upon conditions which are not yet wholly specified, 

 and which we have not yet been able to reproduce. 



The general trend of all these experiments, however, is to encourage the 

 hope that an enzyme system may be found in the tissues which oxidizes 

 /3-carotene to retinene. Should such an enzyme — a carotene oxidase — exist, 

 then together with alcohol dehydrogenase to reduce retinene this system 

 would accomplish the complete conversion of jS-carotene to vitamin A. 



This thought introduces a second consideration. How is such a substance 

 as /3-carotene, a hydrocarbon insoluble in aqueous solutions, brought to- 

 gether with enzymes? How indeed is it transported to the blood and lymph, 

 and so distributed to the tissues? The same problems of course exist for 

 retinene and vitamin A. 



The answer to the latter question is that, in the blood, carotenoids, prob- 

 ably including vitamin A, are bound to plasma proteins. Palmer and 

 Eckles" first showed that in cattle serum /3-carotene is bound in a complex 

 with protein which they called "caroto-albumin." In the blood serum of 

 the chicken we found xanthophyll in a similar state. ^^ 



It seems possible that the existence in the plasma of proteins capable of 

 forming such complexes with carotenoids may be the condition which 

 determines whether or not they are absorbed from the intestine. Zech- 

 meister^^ has stressed the distinction between such "carotene animals" as 

 the horse and cow, which absorb and store almost exclusively the carotenoid 

 hydrocarbons, and "xanthophyll animals," such as the chicken, which 

 store almost exclusively hydroxycarotenoids. Still other forms, such as 

 man and the frog, seem to absorb all types of carotenoids; and such "white 

 fat" animals as the pig and rabbit store almost no plant carotenoids at all. 



" N. L. Wendler, C. Rosenblum, and M. Tishler. /. Am. Chem. Soc. 72, 234 (1950). 

 '•P. Meunier, J. Jouanneteau, and G. Zwingelstein, Compt. rend. 231, 1170 (1950); 



P. Meunier, Rev. Intern. Vitaminologie 23, 21 (1951). 

 " L. S. Palmer and C. H. Eckles, J. Biol. Chem. 17, 223 (1914). 

 " G. Wald and H. Zu.ssman, J. Biol. Chem. 122, 449 (1938). 

 *' L. Zechmeister, Ergeb. Physiol, biol. Chem. u. exptl. Pharmakul. 39, 117 (1937). 



