The human renal 25-hydroxyvitamin D3-1α-hydroxylase: properties studied by isotope-dilution mass spectrometry

. The renal 25-hydroxyvitamin D3-α-hy-droxylase activity has been measured in normal human kidney cortex, using a highly specific assay based on isotope-dilution mass spectrometry. The cortex was obtained from kidneys removed due to renal tumours. The subcellular distribution of 25-hydroxyvitamin D3-1α-hydroxylase activity was studied. Enzyme activity was only observed in the mitochondrial fraction. Mitochondria from non-tumourous kidney cortex had a Vmaxof 0.17 ±0.02 pmol min-1 mg-1 protein and the apparent Km was in the range 14 μmol l-1. There was a tendency to a higher 25-hydroxyvitamin D3-1α-hydroxylase activity in preparations from male kidney (0.21±0.03 pmol min-1 mg-1 protein) than female (0.12±0.02, P<0.05). A significant inverse correlation between serum phosphate and 25-hydroxyvitamin D3-1α-hydroxylase activity was found. No correlation was observed between enzyme activity and serum levels of 1,25-dihydroxyvitamin D (total and free index), PTH, total calcium or ionized calcium. The results indicate that there is a sex difference in human 25-hydroxyvitamin D3-1α-hydroxylase activity similar to the one observed in laboratory animals. Furthermore, the data support the hypothesis that serum phosphate is a major regulator of 1,25-dihy-droxyvitamin D3 production in man.