Human fat cell adenylate cyclase: Enzyme characterization and guanine nucleotide effects on epinephrine responsiveness in cell membranes

Abstract

Human adenylate cyclase (ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1) has been studied in preparations of fat cell membranes (“ghosts”). As reported earlier, under ordinary assay conditions (1.0 mM ATP, 5 mM Mg 2+, 30°C, 10 min incubation) the enzyme was activated 6-fold by epinephrine in the presence of the GTP analog, 5′-guanylyl-imidodiphosphate [GMP-P(NH)P] (Cooper, B. et al. (1975) J. Clin. Invest. 56, 1350–1353). Basal activity was highest during the first 2 min of incubation then slowed and was linear for at least the next 18 min. Epinephrine, added alone, was often without effect, but sometimes maintained the initial high rate of basal activity. GMP-P(NH)P alone produced inhibition (“lag”) of basal enzyme early in the incubation periods. Augmentation of epinephrine effect by GMP-P(NH)P, which also proceeded after a brief (2 min) lag period, was noted over a wide range of substrate (ATP) concentrations. GTP inhibited basal levels of the enzyme by about 50%. GTP also allowed expression of an epinephrine effect, but only in the sense that the hormone abolished the inhibition by GTP. Occasionally a slight stimulatory effect on epinephrine action was seen with GTP. At high Mg 2+ concentration (>10 mM) or elevated temperatures (>30°C) GMP-P(NH)P alone activated the enzyme. Maximal activity of human fat cell adenylate cyclase was seen at 50 mM Mg 2+, 1.0 mM ATP, pH 8.2, and 37°C in the presence of 10 −4 M GMP-P(NH)P; under these conditions addition of epinephrine did not further enhance activity. Human fat cell adenylate cyclase of adults was insensitive to ACTH and glucagon even in the presence of GMP-P(NH)P.