Metabolic activation of 1alpha-hydroxyvitamin D3 in human liver microsomes.

1. 1Alpha-hydroxyvitamin D3 (1alpha-OH-D3) is a synthetic prodrug of the active form of vitamin D3, and requires the hydroxylation at the C-25 position before eliciting its biological activity. 2. 25-Hydroxylation activities for 1alpha-OH-D3 were present in both microsomal and mitochondrial fractions of human liver. 3. To determine the P450 enzyme(s) involved in microsomal 25-hydroxylation, 14 P450s (CYP1A1, 1A2, 1B1, 2A6, 2B6, 2C8, 2C9-Arg, 2C9-Cys, 2C19, 2D6-Val, 2D6-Met, 2E1, 3A4, 4A11) were tested for their 25-hydroxylation activity of 1alpha-OH-D3. None catalysed the 25-hydroxylation reaction. 4. 1Alpha-OH-D3 in a high concentration (2.5 ng ml(-1)) showed small but significant inhibition of the catalytic activities of CYP2C8, 2C9-Cys, 2C19, 2D6-Val and 2E1 for their typical substrates. However, 1alpha-OH-D3 in a clinically used low concentration will not significantly affect drug metabolism catalysed by the 14 P450s tested. 5. In summary, the 25-hydroxylation activity of 1alpha-OH-D3 that localizes in the microsomal fraction appears to be attributable to a cytochrome P450 other than the microsomal forms tested in this study.