SAMS peptide is a substrate for AMP-activated protein kinase (AMPK). Phosphorylated rapidly by AMPK, the Km of AMP-activated protein kinase for the SAMS peptide is higher than that for acetyl-CoA carboxylase, but the Vmax for peptide phosphorylation is 2.5 times higher than that of the parent protein. SAMS peptide therefore gives a convenient and sensitive assay for AMPK