Effects of Human Erythrocyte Guanine Nucleotide-Binding Regulatory Protein on Parathyroid Hormone-Responsive Adenylate Cyclase from Canine Renal Cortex

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Abstract
The effects of the guanine nucleotide-binding regulatory protein (Gs) from human erythrocytes on PTH [parathyroid hormone]-responsive adenylate cyclase from partially purified membranes of canine renal cortex (CRC) were studied. Extracts of erythrocyte membranes, containing soluble Gs, was obtained by treatment with a detergent (Lubrol PX). Gs did not stimulate adenylate cyclase activity by itself, but amplified the response of adenylate cyclase in CRC membranes to both synthetic bovine PTH-(1-34) [bPTH-(1-34)] and to the hydrolysis-resistant GTP analog 5''-guanylimido-diphosphate [Gpp(NH)p]. Gs increased PTH stimulation of adenylate cyclase activity in both the presence and absence of Gpp(NH)p. In the absence of Gpp(NH)p, the potentiating effect of Gs occurred only when the concentration of bPTH-(1-34) was > 10 ng/ml. bPTH-(1-34), Gpp(NH)p, and Gs each enhanced the catalytic activity of adenylate cyclase when added separately or in combination by increasing the apparent maximum velocity (Vmax) of the enzyme without altering the apparent Km for MgATP. The effect of Gs on CRC membrane adenylate cyclase activity in the presence of NaF (10 mM) and forskolin (100 .mu.M) was also examined. NaF- and forskolin-stimulated enzyme activities were significantly increased by Gs in both the presence and absence of Gpp(NH)p (100 .mu.M). Analysis of double reciprocal plots of substrate concentration and enzyme activity revealed that NaF and forskolin increased the Vmax of the catalytic activity and did not alter the apparent Km of the enzyme for MgATP. The role of Gs as a regulator of the response of adenylate cyclase to hormones, guanyl nucleotides, NaF, and forskolin is supported. The relative functional stoichiometry between Gs and catalytic unit present in CRC membranes is considered. The CRC adenylate cyclase system must contain insufficient Gs to couple with all available catalytic units. Deficiency of Gs may impair hormonal stimulation by diminishing the apparent Vmax of the catalytic unit and does not alter the apparent affinity of the enzyme for MgATP.

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