Effect of Calinodulin on Aldosterone Synthesis by a Cytoclirome P-45011β-Reconstituted System from Bovine Adrenocortical Mitochondria1

Abstract

Bovine adrenocortical calmodulin was purified and its general properties were examined. The latter were similar to those of bovine brain calmodulin. When added to a cytochrome P.450_11β system in the presence of dilauroylphosphatidyicholine, calmodulin decreased the rate of aldosterone production from corticosterone from 0.8 to 0.1 nmol/(min-nmol P-450), while it increased the rate of 18-hydroxycorticosterone production from 1.8 to 4.6 nmol/(min-nmol P.450). This effect of calmodulin on steroid production was maximum at a concentration of 1 μM, when 1μM cytochrome P-450_11β was used. The effect was dependent on the presence of Ca²⁺ and maximal response was observed at less than 1 μM Ca²⁺. There was essentially no difference in the effect when bovine brain calmodulin was used. Calmodulin induced a change in the activity of cytochrome P-410_11β in the presence of a wide concentration range of corticosterone as a substrate. As for 18-hydroxycorticosterone production, calmodulin increased both the maximal activ ity and the apparent K_m for corticosterone, but it decreased the apparent K_m for adrenodoxin. Adrenodoxin at a concentration of less than 20 μM did not fully abolish the effect of calmodulin. A small type I difference spectrum appeared when calmodulin was added to cytochrome P-450_11β. The difference spectrum increased significantly in the presence of both Ca²⁺ and adrenodoxin. These results suggest that calmodulin interacts with cytochrome P-450_11β in the presence of adrenodoxin and then modulates the activity of aldosterone synthesis catalyzed by cytochrome P-450_11β.