GUANYLATE CYCLASE - SUBCELLULAR-DISTRIBUTION IN CARDIAC-MUSCLE, SKELETAL-MUSCLE, CEREBRAL-CORTEX AND LIVER

Guanylate cyclase (EC 4.6.1.2) of every fraction studied showed an absolute requirement for Mn2+ for optimal activity; with Mg2+ or Ca2+ reaction was barely detectable. Triton X-100 stimulated the particulate enzyme much more than the supernatant enzyme and solubilized the particulate-enzyme activity. Substantial amounts of guanylate cyclase were recovered with the washed particulate fractions of cardiac muscle (63-98%), skeletal muscle (77-93%), cerebral cortex (62-88%) and liver (60-75%) of various species [rat, hamster, mouse, guinea pig, rabbit, frog, clam, fish [Astronotus ocellatus], parakeet]. The supernatants of these tissues contained 7-38% of total activities. In frog heart, the bulk of guanylate cyclase was present in the supernatant fluid. Plasma-membrane fractions contained 26, 21, 22 and 40%, respectively, of the total homogenate guanylate cyclase activities present in skeletal muscle (rabbit), cardiac muscle (guinea pig), liver (rat) and cerebral cortex (rat). In each case, the specific activity of this enzyme in plasma membranes showed a 5- to 10-fold enrichment when compared with homogenate specific activity. These results suggested that guanylate cyclase, like adenylate cyclase, and ouabain-sensitive Na+ + K+-dependent ATPase, was associated with the surface membranes of cardiac muscle, skeletal muscle, liver and cerebral cortex; considerable activities were also present in the supernatant fractions of these tissues which contained very little adenylate cyclase or ouabain-sensitive Na+ + K+-dependent ATPase activities.