Evidence for Two Different Forms of Guanylate Cyclase in Rat Heart

Abstract

Abstract We assayed guanylate cyclase activity in preparations of heart and other rat tissues. Enzymatic activity was found in both particulate and supernatant fractions of homogenates from a number of different tissues, and the distribution of activity varied with the tissue examined. All tissues studied, however, had a significant amount of activity associated with the particulate fraction. In heart the activity was about equally divided between the 1,000 x g particulate and the 100,000 x g supernatant fractions with small amounts of activity in other fractions. The two apparently different enzymatic activities were separable with gel filtration due to their dissimilar sizes. For both enzymes plots of activity versus Mn2+ concentration were sigmoidal, and the slopes of Hill plots were about 2. This indicates that there are two or more interactive sites for Mn2+ on each enzyme. The ratio of Mn2+ to GTP for apparent maximal activity was 4 for the soluble enzyme and 2 for the particulate enzyme. With 1 mm GTP, concentrations of Mn2+ greater than 2 to 4 mm markedly inhibited the particulate enzyme but had little effect on the supernatant enzyme. The apparent Km for GTP of the latter was 0.065 mm. With the particulate enzyme plots of activity versus GTP concentration were sigmoidal, double reciprocal plots were not linear, and the slope of Hill plots was 1.74 which indicates that there are two or more interactive sites for GTP. Calcium ion increased the activity of the supernatant enzyme and inhibited the activity of the particulate enzyme. Both enzymes were also inhibited with ATP. Half-maximal inhibition of the supernatant enzyme occurred with 0.4 mm ATP while greater than 1 mm ATP was required for the particulate enzyme. Triton X-100 increased the activity of the particulate enzyme 4- to 5-fold with its solubilization. Triton X-100 increased the activity of the 100,000 x g supernatant enzyme about 50 to 60%. In the presence of Triton X-100, 80% of the homogenate activity was derived from particles and 20% from the supernatant fraction. The properties of heart guanylate cyclase were clearly different from those of adenylate cyclase. From these studies it is apparent that two forms of guanylate cyclase activity are present in cell-free preparations of rat heart.