Regulation of 25-Hydroxyvitamin D3-1-hydroxylase in Vivo

Abstract

Abstract The elucidation of the factors which govern the production by the kidney of 1,25-dihydroxycholecalciferol (1,25-(OH)2D3) is essential to an understanding of calcium homeostasis. Changes in the specific activity of the renal 25-OH-cholecalciferol-1-hydroxylase (1-hydroxylase) were followed after alterations in dietary vitamin D, calcium, and phosphorus or parathyroidectomy using an enzyme assay which measures initial reaction rates. 1-Hydroxylase specific activity varied over a 5- to 20-fold range with changes in dietary vitamin D; it was decreased to a specific activity of 0.1 to 0.6 pmole per min per mg of protein by administration of vitamin D3 (1.6 nmoles per day) or equally effectively by administration of 1,25-dihydroxyvitamin D3 (0.52 nmole per day), and it increased markedly to a level of 0.7 to 1.8 pmoles per min per mg of protein upon withdrawal of the steroids. These changes were not dependent upon measurable changes in serum calcium and occurred regardless of dietary calcium and phosphorous levels. The time period required for the enzyme activity to fall to values characteristic of a vitamin D-repleted chick was inversely proportional to the magnitude of the dose of vitamin D3 given (7 days with 3.2 nmoles per day; 2 days with 32.5 nmoles per day). In the continued presence of vitamin D3, the 1-hydroxylase activity is modulated over a more narrow range (0.1 to 0.6 pmole per min per mg of protein) and enzyme levels reflect the dietary availability of calcium. Changes in dietary calcium were reflected by parallel changes in serum calcium. There was a highly significant (p l 0.001) inverse correlation between 1-hydroxylase activity and serum Ca2+ levels. No significant correlations were found between the enzyme activity and serum phosphorous or renal inorganic phosphorous levels. Parathyroidectomy of cholecalciferol-depleted chicks resulted in a decrease, in 24 hours, of 1-hydroxylase activity to levels characteristic of vitamin D-treated chicks (0.2 to 0.04 pmoles per min per mg of protein). This suggests that parathyroid hormone status is equally as important as calciferol status in determining 1-hydroxylase specific activity. The half-life and rates of synthesis and degradation of 1-hydroxylase in cholecalciferol-deficient and -repleted chicks were determined by measuring enzyme activity following cycloheximide treatment (Berlin, C. M., and Schimke, R. T. (1965) Mol. Pharmacol. 1, 149). Vitamin D status had no effect on the half-life or degradation rate (t1/2 = 4 to 5 hours); the rate of synthesis in deficient birds is 4.7 times that in vitamin D-repleted birds. In summary, the two major components which regulate the steady state level of 25-OH-D3-1-hydroxylase (vitamin D status and serum calcium-parathyroid hormone) appear to exert their effect by changes in the rate of the biosynthesis of this enzyme.