Guanylate cyclase from Dictyosteliumdiscoideum

Guanylate cyclase from crude homogenates of vegetative $\text{Dictyostelium}$$\text{discoideum}$ has been characterized. It has a pH optimum of 8.0, temperature optimum of 25°C and requires 1 mM dithiothreitol for optimal activity. It strongly prefers Mn⁺⁺ to Mg⁺⁺ as divalent cation, requires Mn⁺⁺ in excess of GTP for detectable activity, and is inhibited by high Mn⁺⁺ concentrations. It has an apparent Km for GTP of approximately 517 μM at 1 mM excess Mn⁺⁺.The specific activity of guanylate cyclase in vegetative homogenates is 50–80 pmoles cGMP formed/min/mg protein. Most of the vegetative activity is found in the supernatant of a 100,000 x g spin (S100). The enzyme is relatively unstable. It loses 40% of its activity after 3 hours storage on ice. Enzyme activity was measured from cells that had been shaken in phosphate buffer for various times. It was found that the specific activity changed little for at least 8 hours. Cyclic AMP at 10^?4 M did not affect the guanylate cyclase activity from crude homogenates of vegetative or 6 hour phosphate-shaken cells.