Properties of particulate and detergent-solubilized adenylate cyclase of rat testis. Effects of follitropin stimulation.

Basal, fluoride and follitropin stimulated activities of adenylate cyclase have been studied in the testes of immature rats. The enzyme was maximally activated (about twice the basal activity) by low concentrations of follitropin, with an apparent Km of about 9 . 10(-10) M. Both Mg2+ and Mn2+ activate the enzyme but the apparent Ka for Mg2+ is about 10 times that for Mn2+. However, the apparent Km values for MgATP2- and MnATP2- are nearly the same (1.4 . 10(-4) M) and the cation activation of the enzyme is mainly through an increase in V. Ca2+ inhibited all expressions of testicular adenylate cyclase activity. Follitropin and fluoride stimulated adenylate cyclase activity at all Mg2+ concentrations but did not significantly affect the apparent Ka for Mg2+ or the Km for the substrate (MgATP2-). The stimulatory effect of the hormone or fluoride is mainly through an increase in V. Testicular adenylate cyclase could be solubilized with Triton X-100 or Lubrol-PX. The detergent-solubilized enzyme exhibited Km for substrate and Ka values for divalent cations similar to those of the membrane-bound enzyme and remained responsive to stimulation by fluoride. The stimulatory effect of follitropin, however, was lost. Responsiveness to follitropin was also lost by membrane-bound adenylate cyclase after treatment with phospholipase, although the fluoride effect was unchanged. These results reflect the essential role of lipids in the regulation of the follitropin-specific response.