356 



ANIMAL BIOCHEMISTRY 



tical importance. The plant sterol ergosterol (page 94) is changed 

 on irradiation into vitamin D2, and an animal counterpart, 7-dehydro- 

 cholesterol derived from cholesterol (page 93), is converted to vita- 

 min D3. These substances differ only in their side chains and in 

 common with the other active compounds have the same structures 

 otherwise. Names approved internationally for these substances but 



CH3 



CH. 



CH3 



CH3 GH3 



-CHCH=CHCH— CHGH3 



HO 



vitamin D2 (calciferol) 

 CH3 CH3 



CH. 



CH3 



1 I 



CHGH2CH2CH2CHCH3 



HO 



vitamin D3 



not yet in common usage are ergocalciferol and cholecalciferol. The 

 latter compound, vitamin D3, is the form stored by fish in their livers 

 and is natural to mammals exposed to sunlight. Both compounds are 

 available commercially in large quantities at 2.5^/million units. 



Function 



For most animals D^ and D3 can be used interchangeably, but birds 

 cannot utilize calciferol and must have vitamin D3. The reasons for 

 this difference are not known. 



Vitamin D plays a role in the calcification of bones and teeth. This 

 hardening process may be retarded by deficiencies of calcium, phos- 

 phorus, or the vitamin. Unusual proportions of the two elements in 

 the diet may also be detrimental. The mechanism by which vitamin D 

 promotes normal deposition of calcium phosphate is unknown. It has 

 been suggested that the vitamin participates in the intestinal absorp- 

 tion of the minerals. Very recently it has been observed that Do 

 reduces the oxidation of citrate and isocitrate; as a result of this func- 

 tion, the vitamin may control the concentration of these two metabo- 



