Soluble Mn2+-dependent adenylate cyclase activity in the testis of the blue fox (Alopex lagopus).

Soluble Mn2+-dependent adenylate cyclase (MnAC) activity was found in testicular cytosol from blue foxes castrated during the breeding season. The rate of MnAC activity was approximately constant for 30 min at 35 degrees C and for 2 hr after storage at 25 degrees C. Activity was directly proportional to cytosol protein concentration and was optimal in the physiological pH range. Enzyme activity declined in the presence of an alkylating agent (N-ethyl maleimide, NEM) and was eliminated at a concentration of 1 mM NEM. Low concentrations (0.1-10 mM) of a reducing agent (beta-mercapto ethanol, beta ME) did not increase MnAC activity, whereas a high concentration (100 mM) led to a significant reduction (p less than 0.01) in activity. Substitution of Mn2+ in the assay medium with Mg2+ led to a total loss of enzyme activity, which could not be regained by adding hormones or by preincubation of cytosol for 60 min. The Km for Mn2+ was estimated to be 3.5 mM. The affinity of the enzyme for Mn2+ was not altered by varying the concentration of ATP. In contrast, increasing concentrations of Mn2+ appeared to increase the affinity of the enzyme for MnATP2-. The Km for MnATP2- thus varied from 6 to 18 mM.