Studies on the Stimulation by Ca2+ and the Inhibition by ADP of Steroid 11beta-Hydroxylation in Adrenal Mitochondria

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Abstract
The Ca2+ stimulation and ADP inhibition of the steroid 11.beta.-hydroxylation enzyme system in adrenal [beef] mitochondria were investigated. The Ca2+-stimulation is competitively inhibited by Sr2+. Nucleoside monophosphates were not inhibitory and among the nucleoside diphosphates and triphosphates only ADP, CDP and ATP were inhibitory with ADP being more inhibitory than the other nucleotides. None of the non-inhibitory nucleotides could reverse the ADP inhibition. ADP appears to inhibit the Ca2+-stimulation since the activity in the absence of Ca2+ was not inhibited by ADP. Ca2+ and ADP did not affect the binding of deoxycorticosterone to the mitochondria preparation. Ca2+ stimulated the reduction of cytochrome P-450 by NADPH and this was inhibited by ADP. These effects were independent of the presence of deoxycorticosterone. Neither Ca2+ nor ADP affected the reduction of adrenodoxin by NADPH. The inhibition by ADP with respect to Ca2+ or to deoxycorticosterone is non-competitive. The Ca2+ stimulation with respect to deoxycorticosterone is uncompetitive. Preincubation of the mitochondrial preparation with Ca2+ decreases the inhibition by ADP and preincubation with ADP increases its inhibitory effects. These data were interpreted to indicate that Ca2+ and ADP influence the relationship between adrenodoxin and cytochrome P-450 so that a complex of increased activity in the reduction of cytochrome P-450 is formed in the presence of Ca2+ and a less active complex is formed in the presence of Ca2+ plus ADP.

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