II. CHEMISTRY 159 



tion of traces of 7-dehydrocholesterol by some sort of oxidation-reduction 

 mechanism may also explain the reported presence of the unstable provi- 

 tamin D in roughly normal amount in the cholesterol from the brain of a 

 mummy 1400 years old.^^^ 



It may be that the biosynthesis of 7-dehydrocholesterol occurs in steps, 

 rather than as a dehydrogenation-hydrogenation reaction. Bergstrom and 

 Wintersteiner^^^ have shown that position 7 in cholesterol is extremely sus- 

 ceptible to attack by molecular oxygen. Haslewood^^* and others have de- 

 tected 7-hydroxycholesterol in the animal body, and although this was 

 possibly an artifact, the ease with which it is formed from cholesterol in 

 stabilized colloidal solutions points to a possible route of synthesis, since 

 only the dehydration step is needed to convert it to 7-dehydrocholesterol. 

 7-Ketocholesterol, which precedes 7-hydroxycholesterol in the laboratory 

 synthesis, has also been found in the animal body.'^^- ^^^ 



Whatever the reactions involved, the fact of conversion of cholesterol 

 to 7-dehydrocholesterol appears to be established by the experiments of 

 Scott et al}^^ These workers found provitamin D in the lining of the small 

 intestine of guinea pig, rat, and ox in amounts as large or larger than in 

 the skin, usually regarded as the site of the greatest concentration. It per- 

 sisted in the gut wall despite fasting for 24 hours or the feeding of a low- 

 sterol diet. When spectroscopically pure cholesterol was fed to fasted guinea 

 pigs the amount of provitamin in the small intestine increased during the 

 period of absorption but returned to normal when absorption was 

 completed. Since during the same cycle the amount of provitamin in the 

 liver increased progressively, it follows that the provitamin was formed in 

 the gut wall and stored in the liver. The concentration of provitamin was 

 greatest in the duodenum and was associated largely with the mucosa and 

 lamina propria. 



Partially supporting this work is a note by Rosenberg,i^i who found in 

 clams feeding on algae a much higher concentration of provitamin in the 

 viscera than in the body. The kind of provitamin was not detennined, but 

 the efficacy ratio was the same for the visceral as for the body material, and 

 it was much higher than would be expected of a vegetable source. It would 

 seem that in these clams provitamin D was being elaborated in the gut wall, 

 from nutrients in the process of absorption, but whether the synthesis was 

 a total one or merely a dehydrogenation is not clear. 



"6H. King, O. Rosenheim, and T. A. Webster, Biochem. J. 23, 166 (1929). 

 '" S. Bergstrom and O. Wintersteiner, /. Biol. Chem. 141, 597 (1941). 

 "8 G. A. D. Haslewood, Biochem. J. 33, 709 (1939); Nature 154, 29 (1944). 

 "9 V. Prelog, L. Ruzicka, and P. Stein, Helv. Chim. Acta 26, 2222 (1943). 

 "" M. Scott, J. Glover, and R. A. Morton, Nature 163, 530 (1949). 

 "1 H. R. Rosenberg, Nature 164, 795 (1949). 



