Vitamin D 1alpha-hydroxylase (1alpha(OH)ase), which converts the circulating prohormone 25-hydroxyvitamin-D(3) (25(OH)D(3)) to the active 1alpha-25-dihydroxyvitamin-D(3) (1,25(OH)(2)D(3)), is present in normal prostatic epithelium. However, prostate cancer cells, both primary cultured cells and cell lines, have greatly decreased activity of 1alpha(OH)ase and are therefore resistant to the tumor suppressor activity of circulating 25(OH)D(3). We quantitated 1alpha(OH)ase mRNA and protein levels to investigate mechanism(s) responsible for decreased 1alpha(OH)ase enzymatic activity in prostate cancer. Prostate cancer cell lines had low 1alpha(OH)ase mRNA levels. Primary prostate cell cultures derived from normal and cancer tissues had equivalent levels of 1alpha(OH)ase RNA and protein. Equivalent 1alpha(OH)ase protein levels were observed in prostate tissue sections containing normal and malignant cells. The protein levels of hsc70, whose homolog intracellular Vitamin D binding protein (IDBP-1) facilitates delivery of 25(OH)D(3) to 1alpha(OH)ase in monkey cells, were equivalent in the normal and cancer cells. Equivalent activity in normal and cancer cells of Vitamin D 24-hydroxylase, a mitochondrial enzyme that also uses 25(OH)D(3) as a substrate, further ruled out lack of access to substrate as a basis for low activity of 1alpha(OH)ase in cancer cells. We conclude that diminished 1alpha(OH)ase activity in prostate cancer cell lines is through decreased gene expression, whereas decreased activity in primary cultures and tissues is post-translational.