The biochemistry of the lead histochemical technique for demonstrating adenylate cyclase was studied. The enzyme activity of fat cell plasma membranes, using 5'-adenylyl-imidodiphosphate (AMP-PNP) as substrate, was completely inhibited at 1 times 10- minus 4 M Pb(NO3)2 and yet at 4 times 10- minus 3 M Pb(NO3)2 precipitate could be demonstrated by electron microscopy on both sides of plasma membrane vesicles. No lead-diphosphoimide or lead-phosphate precipitate could be visualized by electron microscopy when the lead was reduced to a level (2 times 10- minus 5 M) which caused only 50% inhibition of the enzyme. A solubility product coefficient of 1 times 10- minus 10 M was found necessary to allow precipitation of lead-phosphate complex in the adenylate cyclase medium. Varying the ratio of substrate or dextran relative to the lead failed to protect the inhibition of the enzyme. Increasing concentrations of beta-mercaptoethanol restored the basal and stimulated activity of adenylate cyclase but also prevented the precipitation reaction. Lead at 2 times 10- minus 3 M caused the nonenzymatic hydrolysis of AMP-PNP, resulting in the production of small but significant quantities of cyclic AMP and substantial amounts of AMP. This hydrolysis was inhibited by alloxan but unaffected by dextran of NaF. The adenylate cyclase activity of pancreatic islet homogenates and of fat pad capillaries was completely inhibited by lead concentrations equal to or less than those used in histochemical studies (Howell, S. L., and M. Whitfield. 1972. J. Histochem. Cytochem. 20:873-879. and Wagner, R. C., P. Kreiner, R. J. Barrnett, and M. W. Bitensky. 1972. Proc. Natl. Acad. Sci. U.S.A. 69:3175-3179.). The present study shows that the lead histochemical method cannot be used for localization of adenylate cyclase because of the inhibition of the enzyme and artifacts produced by high lead concentrations and the inability to produce a visible precipitate at low lead concentrations which only partially inhibit the enzyme.

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