658 X. VITAMINS D 



these compounds from the intestinal tract. Activated 7-dehydrocholesterol 

 was the exception. ^^ There seems to be Httle doubt that the provitamins 

 D can be activated in the skin, and that this explains the action of sunlight 

 or of ultra\dolet light in preventing rickets as effectiA^ely as do the vitamins 

 D themselves. 



One of the facts which offers presumpti\'e proof of the in vivo conversion 

 of provitamins D to vitamins D is that the former are found in higher con- 

 centrations in the skin than elsewhere. This is the logical site for the 

 transformation to take place. 7-Dehydrocholesterol (provitamin D3) is a 

 constant contaminant of cholesterol,-^" but Rosenl)erg^"^ points out that 

 it usually occurs in higher proportion in the skin lipids than in those of the 

 inner tissues. Windaus and Bock'*^ separated 7-dehydrocholesterol in 3 to 

 6% of the total sterols of pigskin. Rosenberg'"^ recorded the following 

 percentages of total sterols which are present as provitamins D in the skin 

 of several species: man, 0.15 to 0.43; cattle, 0.18; calf, 0.68; mouse, 

 0.87; and chicken (feet) 1.0 to 4.0. Rosenberg-^^ has recently confirmed 

 the presence of 7-dehydrocholesterol (pro\'itamin D3) in the feet of chickens. 

 Moore and Baumann^*^ noted the presence of 22 to 36% of a sterol, which 

 reacts rapidly with the Schoenheimer-Sperry reagents, in the sterols of rat 

 skin. Since cholesterol itself does not possess the ability to react promptly 

 with the above reagent, while 7-dehydrocholesterol and 7-hydroxycholes- 

 terol are known to have this propertj^ it is presumed that this represents 

 provitamin D in the rat skin. Idler and Baumann-''^ demonstrated that 

 the major component of the fast-reacting sterol fraction, as determined by 

 chromatographic separation of its azoyl ester on silica, is A^-cholestene-3j5- 

 ol. Glover, Glover, and Morton -^^ reported that cholesterol is dehydro- 

 genated to 7-dehydrocholesterol in the mucous membrane of the small 

 intestine. 



5. The Effect of the Vitamins D on Various Physiologic Processes 



{!) Inirochiction 



Clues as to the nature of the several physiologic processes which are 

 under the control of the vitamins D were first obtained by investigation of 

 the abnormal conditions which result in avitaminosis D. Thus, in all 



^* A. G. Boer, E. H. Reerink, A. van Wijk, and J. van Niekerk, Proc. Acad. Sci., Ams- 

 terdam, 39, 622-632 (1936). 



2" H. R. Rosenberg, Arch. Biochem. Biophys., 42, 7-11 (1953). 

 "2 p. R. Moore and G. A. Baumann, J. Biol. Chem., 195, 615-621 (1952). 

 2" D. R. Idler and G. A. Baumann, ./. Biol. Chem., 195, 623-628 (1952). 

 2^* M. Glover, J. Glover, and R. A. Morton. Biochem. J., 51, 1-9 (1952). 



