With isolated adipocytes from rat epididymal rat pads as a model system, we have examined various aspects of hormonal control of metabolic processes. A) Phosphorylation of the purified hormone- sensitive lipase by exogenous protein kinases has been compared with phosphorylation of the lipase by endogenous kinases in cells subjected to various hormones. The enyzme contains several phosphorylation sites, one of which is dephosphorylated by insulin in intact cells in a manner independent of changes in cAMP- dependent protein kinase (A-kinase) activity. Moreover, the phosphorylation of this site does not require elevation of A- kinase. B) Since our previous findings suggest that insulin activates a phosphatase which acts on the hormone-sensitive lipase, we have examined the effects of insulin on the phosphorylation state of other, more abundant cellular proteins, which are more easily detected by SDS-PAGE and autoradiography. Insulin reduces the phosphorylation of one prominent A-kinase substrate (65 KD), and this effect is not explained by a reduction in cellular A-kinase activity. Thus, the above findings indicate that insulin stimulates the dephosphorylation of target sites for A-kinase and other kinases. C) Adenylate cyclase linked receptors (R) and GTP regulatory components (G), both stimulatory (RsGs) and inhibitory (RiGi) are thought to reside exclusively in plasma membranes. However, we find one component of the inhibitory circuit, Gi alpha, in both cell surface and low density microsomal membranes. Insulin stimulates the translocation of a substantial fraction of this GTP- binding protein to the plasma membrane. D) Previously, we found purified protein kinase C stimulates adenylate cyclase in isolated membranes. A transferable ATP Tau-phosphate is required, indicating that phosphorylation of a component of the cyclase system occurs. However, the GTP-binding complexes are not C- kinase substrates, indicating that another component, perhaps the cyclase catalytic unit, is the C-kinase target.
