Inhibition of 25-hydroxyvitamin D 1 alpha-hydroxylase by renal mitochondrial protein kinase-catalyzed phosphorylation.

Partially purified chick kidney mitochondrial Type II protein kinase catalyzes the phosphorylation of 1 alpha-hydroxylase cytochrome P-450 without affecting the rate of product formation in vitro when 1 alpha-hydroxylase activity is reconstituted by the addition of [ferredoxin] and [ferredoxin reductase] to the phosphorylated cytochrome. The cytochrome's effective concentration, or its general spectral properties did not change upon phosphorylation. However, when the cytochrome and its ferredoxin were present simultaneously during the phosphorylation reaction, reconstitution of 1 alpha-hydroxylase activity by the addition of ferredoxin reductase failed to catalyze product formation. Although a several fold increase in the kinase activity could be demonstrated in the presence of cAMP, the above phosphorylation effects appear to be cAMP-independent.