668 X. VITAMINS D 



high excretion of this phosphate in the urine. ^"^ The data presented by 

 Harrison and Harrison^"" appear to demonstrate that reabsorption of 

 phosphate by the renal tubules is increased in the kidneys of rachitic 

 puppies when vitamin D is given. However, it is possible that the in- 

 creased reabsorption of phosphate under these conditions is an indirect 

 result following the primary effect of the vitamin D in increasing the ab- 

 sorption of calcium from the intestine.^ This would increase plasma cal- 

 cium, which itself might decrease phosphate excretion via the kidney by 

 causing an interaction of calcium and phosphate in the tubules. In fact, 

 Wolf and Ball''"^ demonstrated that, when a calcium salt was given to 

 dogs intravenously as a slow infusion, only small changes in the blood 

 calcium were noted, while no alteration obtained in the blood inorganic 

 phosphate. However, a striking depression in phosphate excretion in the 

 urine was observed. The suggestion that the effect of vitamin D in in- 

 creasing phosphate retention by increasing kidney reabsorption would 

 seem to be a valid one, but, in all probability, its occurrence is secondary 

 to the effect of the vitamin in augmenting the intestinal absorption of 

 calcium. 



b. Phytic Acid and the Action of Vitamin D. Phytic acid, which is 

 hydrolyzed to inositol and six orthophosphate molecules by the enzyme, 

 phytase (see The Lipids, Vol. H, pp. 67-69), may likewise be related to the 

 action of vitamin D. Mellanby^^'^"- and Bruce and Callow^**^ were the 

 first to identify phytic acid as the rickets-producing substance known to be 

 present in cereals. Even in the absence of calcium, phytic acid is not 

 completely available to the body as a source of phosphate. When cal- 

 cium is present, phytic acid is even less effective as a source of phosphate, 

 as its digestion is depressed under these conditions. ^"^ On the other hand, 

 Harrison and Mellanby^*^^ noted that the calcification of the bones of puppies 

 was reduced when phytic acid was included in the diet. The explanation 

 offered for the latter phenomenon was that phytic acid prevents calcium 

 absorption; this suggestion was confirmed later experimentally by Mel- 

 lanby.2^^ According to Harris,^^^ phytic acid interferes with the absorption 



«»» H. E. Harrison and H. C. Harrison, /. Clin. Invest., 20, 47-55 (1941). 



301 A. V. Wolf and S. M. Ball, Am. J. Phxjsiol., 158, 205-217 (1949). 



^^ E. Mellanby, Experimental Rickets. The Effect of Cereals and Their Interaction with 

 Other Factors of Diet and Environment in Producing Rickets. Med. Research Council 

 (London), Spec. Rept. Ser. No. 93, (1925); cited by R. Nicolaysen and N. Eeg-Larsen, 

 Vitamins and Hormones, 11, 29-60 (1953), p. 42. 



303 H. M. Bruce and R. K. Callow, Biochem., J. 28, 517-528 (1934). 



30" J. T. Lowe and H. Steenbock, Biochem. J., 30, 1991-1995 (1936). 



30B D. C. Harrison and E. Mellanby, Biochem. J., 33, 1660-1680 (1939). 



