Guanyl Cyclase, an Enzyme Catalyzing the Formation of Guanosine 3',5'-Monophosphate from Guanosine Triphosphate

Abstract Guanyl cyclase, an enzyme catalyzing the formation of guanosine 3',5'-monophosphate from guanosine triphosphate, has been identified in a variety of animal tissues. Conditions were established under which formation of cyclic GMP was proportional to the amount of enzyme added and to incubation time. The enzyme differs markedly from adenyl cyclase in a number of respects. It is largely soluble in most tissues studied whereas adenyl cyclase is particulate. The relative distribution of the two cyclase activities among several tissues is quite different. In broken cell preparations, guanyl cyclase activity is not stimulated by fluoride, glucagon, or epinephrine under conditions where adenyl cyclase activity is stimulated. The activity of both guanyl cyclase and adenyl cyclase is greater in the presence of Mn++ than Mg++, but the effect of Mn++ is much more striking with guanyl cyclase. The enzyme has a pH optimum between 7.4 and 8.0. The apparent Km for GTP is between 0.02 and 0.1 mm and that for Mn++ is about 0.5 mm. Guanyl cyclase is heat-labile and inhibited by a number of nucleotides including dGTP, ATP, ADP, and ITP, and by P-enolpyruvate and oxalacetate. Zn++, Cd++ and Hg++ inhibit guanyl cyclase in a manner that is prevented by SH reagents. Preliminary attempts to purify the enzyme by conventional techniques have yielded a purification of about 20-fold.