Modulation by substrate and cations of guanylate cyclase activity in detergent-dispersed plasma membranes from rat adipocytes.

Guanylate cyclase activity in Triton X-100-treated plasma membranes exhibits sigmoidal profiles as a function of MgGTP, irrespective of the excess Mg²⁺⁽¹⁾ concentration. In contrast, at low excess Mn²⁺ (0.2 mM) the activity vs substrate (MnGTP) concentration profile corresponds to a michaelian behaviour. In addition the enzyme does not require similar excess Mn²⁺ and Mg²⁺ for optimal activity at various substrate concentrations. Moreover, low concentrations of Ca²⁺ are capable of stimulating guanylate cyclase activity with Mg²⁺ as the major divalent cation.