Investigation of the mechanism for abnormal renal 25-hydroxyvitamin D3-1-hydroxylase activity in the X-linked Hyp mouse.
Renal mitochondria from mutant hypophosphatemic male mice (Hyp/Y) fed a vitamin D-deficient, low calcium diet synthesize significantly less 1,25-dihydroxyvitamin D3 than mitochondria from normal male (+/Y) littermates on the same diet. Kinetic studies reveal that maximum velocity (Vmax) for 25-hydroxyvitamin D3-1-hydroxylase (1-hydroxylase) is lower in Hyp/Y relative to +/Y mice (0.21 +/- 0.02 vs. 1.06 +/- 0.12 pmol/mg protein X min) whereas the apparent Michaelis-Menten constant (Km) for the reaction is not different in both genotypes (0.55 +/- 0.05 vs. 0.50 +/- 0.08 microM). The presence of an inhibitor for 1-hydroxylase activity in renal mitochondria of Hyp/Y mice was ruled out by estimating enzyme activity in mixtures of renal mitochondria from +/Y and Hyp/Y mice. Phosphate in the incubation medium stimulated 1-hydroxylase activity in +/Y mitochondria. In Hyp/Y mice, the stimulation achieved was smaller in magnitude and the added phosphate did not restore mutant 1-hydroxylase activity to normal. The vitamin D-deficient, low calcium diet led to a significant and comparable increase in serum PTH and urinary excretion of cAMP in +/Y and Hyp/Y, suggesting that the mutant strain had an appropriate PTH response to the diet-induced fall in serum calcium. Furthermore, the fractional excretion index of phosphate which is significantly greater in Hyp/Y than +/Y mice fed the control diet increased 3-fold in both genotypes fed the vitamin D-deficient, low calcium diet. These results suggest that the abnormal renal 1-hydroxylase response in Hyp mice is not the result of generalized renal resistance to PTH in the mutant strain and suggest that the defect in Hyp/Y mice may reside at a regulatory step subsequent to cAMP production.
D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase / Q000378:metabolism*; D000818:Animals; D001696:Biomechanical Phenomena; D002118:Calcium / Q000172:deficiency; D000242:Cyclic AMP / Q000652:urine; D005260:Female; D007015:Hypophosphatemia, Familial / Q000201:enzymology; D007668:Kidney / Q000201:enzymology*; D007700:Kinetics; D008297:Male; D051379:Mice; D008817:Mice, Mutant Strains / Q000235:genetics / Q000378:metabolism*; D008928:Mitochondria / Q000201:enzymology; D010710:Phosphates / Q000378:metabolism / Q000494:pharmacology; D012729:Sex Chromosome Aberrations / Q000378:metabolism; D013250:Steroid Hydroxylases / Q000378:metabolism*; D014808:Vitamin D Deficiency / Q000097:blood; D014960:X Chromosome
D010710:Phosphates; D000242:Cyclic AMP; D013250:Steroid Hydroxylases; D015090:25-Hydroxyvitamin D3 1-alpha-Hydroxylase; D002118:Calcium