Characteristics of adenylate cyclase activity in pig coronary arteries

Characteristics of adenylate cyclase activity of the intima plus media layer of pig coronary arteries have been studied. Enzyme activity in the 10,000 xg particulate fraction was linear with respect to time and protein during short incubations. Linear kinetics were observed for MgATP as a substrate and free Mg2+ as an activator. Mn2+ added in excess of ATP concentrations was only slightly stimulatory or inhibitory. Adenosine, GTP and GPP(N)P inhibited enzyme activity. Adenosine decreased the Vmax and the Ka for Mg2+ but had no effect on the Km for MgATP. The inhibition by adenosine was reversible, while that by GPP(N)P appeared to be irreversible. Although preincubation of the enzyme with GPP(N)P for short times caused irreversible inhibition, preincubation for more than 15 min caused up to 3-fold activation. Isoproterenol added to the incubation produced no or very slight stimulation with or without GPP(N)P or GTP, and preincubation with isoproterenol alone caused irreversible inhibition. However, preincubation with isoproterenol plus GPP(N)P for 30 min caused irreversible activation exceeding that observed with GPP(N)P alone. These observations suggest that vascular adenylate cyclase, like the enzyme from the other tissues, can exist in multiple states of activity and responsiveness.