The capacity of parathyroid hormone (PTH) to increase serum 1,25(OH)(2)D levels declines with age in both rats and humans. In young rats, PTH stimulates renal 1,25(OH)(2)D production and increases mRNA levels for the terminal mitochondrial P450 of the 1alpha-hydroxylase complex (CYP27B1 or CYP1alpha). However, in older rats PTH increases mRNA levels but not 1,25(OH)(2)D production. This suggests that in old animals there is either decreased CYP1alpha protein levels in response to PTH or that the protein produced lacks functionality. The CYP1alpha protein is located on the inner mitochondrial membrane, the site of increased free radical production with age. To study these possibilities, we examined the effect of PTH and free radicals on CYP1alpha expression in a model system-AOK-B50 renal tubular cells. PTH increased CYP1alpha mRNA and protein in a similar time-dependent manner, suggesting that CYP1alpha protein levels were largely regulated by mRNA levels. The effect of free radicals was determined by preincubation with hydrogen peroxide (H(2)O(2)), a standard model for studying free radical damage. H(2)O(2) inhibited PTH-stimulated CYP1alpha protein levels and 1,25(OH)(2)D production in a dose dependent manner. However, 1,25(OH)(2)D production was more sensitive to H(2)O(2) than was CYP1alpha protein levels. This suggests that the catalytic activity of the CYP1alpha protein may be reduced by free radical damage in these cells. Future studies will focus on detecting oxidative damage in this model system and in vivo.