454 IX. CAROTENOIDS AND VITAMINS A 



when /3-carotene was oxidized with hydrogen peroxide-osmium tetroxide.^^^ 

 Meunier and associates-^^-'^^ recorded a 60% yield of vitamin A aldehyde 

 on oxidizing /5-carotene on solid manganese dioxide. Ball and co-workers-^^ 

 and Wald-^^ likewise employed the latter method for transforming vitamin 

 A to retinene. This type of oxidation is referred to as a "chromatographic 

 oxidation," in which a solid acts both as the adsorbent and as the reagent. 

 It is suggested that this may mimic the specificity and the directness of an 

 enzymatic reaction. All of these results obtained in vitro tend to support 

 the hypothesis that vitamin A aldehyde is an intermediate in the conver- 

 sion of carotene to vitamin A. 



Since /S-carotene is insoluble in aqueous media, while the enzyme systems 

 are water-soluble, a question arises as to how a reaction can be effected 

 between the enzyme and the substrate. It is suggested by Wald-^^ that 

 the carotenoid becomes water-soluble by combining with a plasma protein, 

 as is evident from the pioneer study of Palmer and Eckles,'-''^ who prepared 

 "carotoalbumin" from cattle serum; this was shown to be a protein-)S- 

 carotene complex. The relationship between species and the ability of a 

 carotenoid to be absorbed may be related to the possibility that such a 

 combination exists. ^^^'-^^ This has also been suggested more recently by 

 Ganguly et al}'" Ball et al^^^ reported that retinene combines spon- 

 taneously with a variety of proteins, while Wald-^^ noted that retinene in 

 the retina and in tissues is combined with alcohol dehydrogenase and with 

 opsin. Finally, in the further metabolism of /3-carotene in the intestinal 

 wall, the vitamin A alcohol formed may be esteriiied in the gut wall as 

 described in the previous section. 



Thus, it is suggested by Wald-^^ that /S-carotene, probably in a water- 

 soluble complex with protein, is oxidized by carotene-oxidase to retinene. 

 This chromogen is reduced by the alcohol-dehydrogenase system to vita- 

 min A. 



"2 N. L. Wendler, C. Rosenblum, and M. Tishler, /. Am. Chem. Soc, 72, 234-239 

 (1950). 



2^3 P. Meunier, J. Jouanneteau, and G. Zwingelstein, Compt. rend., 231, 1170-1171 

 (1950). 



244 p_ Meunier, Rev. intern, vitaminol. {Intern. Z. Vitaminforsch.), 23, 21-3G (1951). 



2« G. Wald, J. Gen. PhijsioL, 31, 489-504 (1947-1948). 



2« L. S. Palmer and C. H. Eckles, /. Biol. Chem., 17, 223-236 (1914). 



'^" J. Ganguly, N. I. Krinsky, J. W. Mehl, and H. J. Deuel, Jr., Arch. Biochem. Bio- 

 phys., 38, 275-282 (1952). 



2« S. Ball, F. D. Collins, R. A. Morton, and A. L. Stubbs, Nature, 161, 424-426 

 (1948). 



