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ARS Home » Midwest Area » Ames, Iowa » National Animal Disease Center » Ruminant Diseases and Immunology Research » Research » Publications at this Location » Publication #103295
Title: PHYSIOLOGICAL SIGNIFICANCE OF C-28 HYDROXYLATION IN THE METABOLISM OF 1ALPHA,25-DIHYDROXYVITAMIN D2

Author
item RAO, D - WOMEN&INFANTS HOSPITAL,RI
item SIU-CALDERA, MEI-LING - WOMEN&INFANTS HOSPITAL,RI
item USKOKOVIC, MILAN - HOFFMAN-LAROCHE, NJ
item HORST, RONALD
item REDDY, G - WOMEN&INFANTS HOSPITAL,RI

Submitted to: Archives of Biochemistry and Biophysics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/1999
Publication Date: N/A
Citation: N/A

Interpretive Summary: Milk fever is a disease affecting 6-8% of all U.S. dairy cows each year, which means approximately 700,000 cows are affected each year. This disease has been estimated to cost approximately $300/episode ($210 million annually) or as a result of treatment and production losses. The major clinical symptom seen in cows developing this disorder is their inability to stand and eventual lapse into coma if not treated medically. The cows lose muscle and nerve function because blood calcium concentrations fall below the level required to maintain normal electrical activity of the tissues. All mammals have evolved a very intricate system designed to maintain normal blood calcium. Calcium leaves the body in large amounts, when cows begin making milk which is rich in calcium. This calcium must be replaced, either by absorbing more from the diet or by removing calcium from the skeleton. In cows that develop milk fever, this system has broken ndown. Vitamin D plays a major role in controlling the movement of calcium into and out of the body. In this paper we described an enzymatic pathway leading to the deactivation of vitamin D2. Our findings suggest that hydroxylation of vitamin D2 at carbon-28 results in the total loss of its ability to control calcium movements. This information will assist scientists, as well as nutritionists, and veterinarians alike in understanding how vitamin D is utilized by the body. The information may assist in developing various strategies for treating milk fever and other metabolic disorders in both human and veterinary medicine.

Technical Abstract: In our previous study, we indicated for the first time that C-28 hydroxylation plays a significant role in the metabolism of 1alpha,25-dihydroxyvitamin D2 [1a,25(OH)2D2] by identifying 1a,24(S),25,28-tetrahydroxyvitamin D2 [1a,24(S),25,28(OH)4D2] as a major renal metabolite of 1a,25(OH)2D2. The present study is performed to establish the physiological significance of C-28 hydroxylation in the metabolism of 1a,25(OH)2D2. We perfused rat kidneys in vitro with 1a,25(OH)2[26,27-**3H]D2 (5 x 10**-10M) and demonstrated that 1a,24(R),25-trihydroxyvitamin D2 [1a,24(R),25(OH)3D2] and 1a,24(S),25,28(OH)4D2 are the only two major physiological metabolites of 1a,25(OH)2D2. In the same perfusion experiments, we also noted that there is no conversion of 1a,25(OH)2D2 into 1a,25,28-trihydroxyvitamin D2 [1a,25,28(OH)3D2]. Moreover, 1a24(S),25,28(OH)4D2 is not formed in the perfused rat kidney when synthetic 1a,25,28(OH)3D2 is used as the starting substrate. This finding indicates that C-28 hydroxylation of 1a,25(OH)2D2 occurs only after 1a,25(OH)2D2 is hydroxylated at C-24 position. At present, the enzyme responsible for the C-28 hydroxylation of 1a,24(R),25(OH)3D2 in rat kidney is not known. The biological activity of 1a,24(S),25,28(OH)4D2, determined by its ability to induce intestinal calcium transport and bone calcium resorption in the rat, was found to be almost negligible. Also, 1a,24(S),25,28(OH)4D2 exhibited very low binding affinity towards bovine thymus vitamin D receptor. These studies firmly establish that C-28 hydroxylation is an important enzymatic reaction involved in the inactivation of 1a,25(OH)2D2.