Adenylate cyclase from Phycomyces sporangiophore

Abstract

Transient changes in cyclic AMP levels accompany the light-growth response of the sporangiophore of Phycomyces blakesleeanus. Furthermore growth is regulated by endogenous hormones. Since adenylate cyclase may perform a role in these events, some properties of the enzyme from the sporangiophores of Phycomyces blakesleeanus are reported here. The enzyme is mostly particulate and activity is dependent on a divalent cation possibly Mg 2+; Mn 2+ and Ca 2+ are inhibitory. Its K m is 0.5 mM and the pH optimum is 7.8. Low levels of GTP markedly enhance activity. Nueleoside triphosphates, including ATP at high concentrations, are inhibitory while AMP and ADP and to a lesser extent IMP increase activity. Ouabain, NaF, and alloxan also inhibit Phycomyces cyclase. Pyruvate, imidazole, nucleoside monophosphates other than AMP and IMP, histamine, glucagon, octopamine, γ-aminobutyric acid and norepinephrine have little or no effect. However, high concentrations of epinephrine and dopamine tripled activity. The effect of dopamine was shown to be saturable. Adenylate cyclase extracted in the dark was significantly activated upon simultaneous exposure to light and substrate. An inference is made that sensory transduction in Phycomyces may involve adenylate cyclase, although the interaction may or may not be a direct one.