Tissue distribution of the AMP‐activated protein kinase, and lack of activation by cyclic‐AMP‐dependent protein kinase, studied using a specific and sensitive peptide assay

Abstract

1 We have synthesized two peptides, one based on the exact sequence around the unique site (Ser79) for the AMP-activated protein kinase on rat acetyl-CoA carboxylase (SSMS peptide) and another in which the serine residue corresponding to the site for cyclic-AMP-dependent protein kinase (Ser77) was replaced by alanine (SAMS peptide). 2 Both peptides were phosphorylated with similar kinetics by the AMP-activated protein kinase, but only the SSMS peptide was a substrate for cyclic-AMP-dependent protein kinase. The SAMS peptide was not phosphorylated by any of five other purified protein kinases tested. 3 The K_m of AMP-activated protein kinase for the SAMS peptide is higher than that for acetyl-CoA carboxylase, but the V_max for peptide phosphorylation is 2.5 times higher than that of its parent protein. This peptide therefore gives a convenient and sensitive assay for the AMP-activated protein kinase. 4 Acetyl-CoA-carboxylase kinase and peptide kinase activities copurify through six steps from a post-mitochondrial supernatant of rat liver, showing that the SAMS peptide is a specific substrate for the AMP-activated protein kinase in this tissue. We could not demonstrate AMP-dependence of the kinase activity in crude preparations, apparently due to endogenous AMP remaining bound to the enzyme. However, 8-bromoadenosine 5-monophosphate (Br⁸AMP) is a partial agonist at the allosteric (AMP) site, and inhibition by 2 mM Br⁸AMP can be used to test that one is measuring the AMP-stimulated form of the kinase. 5 Using this approach, we have examined the kinase activity in nine different rat tissues, plus a mouse macrophage cell line, and find that there is a correlation between tissues expressing significant levels of peptide kinase activity and those active in the synthesis or storage of lipids. 6 We also use the peptide assay to show that cyclic AMP-dependent protein kinase does not activate purified AMP-activated protein kinase, and does not affect the activation of partially purified AMP-activated protein kinase by endogenous kinase kinase.