Phosphorylation of beta-glucuronidases from human normal liver and hepatoma by cAMP-dependent protein kinase

beta-Glucuronidases purified from human hepatoma and from normal liver could serve as a substrate for a cAMP-dependent protein kinase. The rate of phosphorylation reaction of the hepatoma beta-glucuronidase was rapid, whereas that of the normal liver beta-glucuronidase was slow and much lower. Stoichiometry of phosphorylation was 4.3 and 0.46 mol of phosphate/mol of the beta-glucuronidase from the hepatoma and normal liver, respectively. Tryptic peptide mapping of 32P-labeled beta-glucuronidase from hepatoma identified two distinct phosphopeptides (X and Y). The peptide from hepatoma hydrolase was phosphorylated predominantly at the X, while the peptide Y was the major phosphopeptide in the hydrolase of normal liver. Analysis of phosphoamino acids revealed two sites, phosphoserine and phosphothreonine. beta-Glucuronidase from hepatoma consisted of a major subunit with molecular mass of 64,000 (64 kDa) and a minor subunit with 76 kDa, whereas the hydrolase from normal liver had almost exclusively 64 kDa subunit. 32P-labeled beta-glucuronidase indicated that the 64 kDa subunit was phosphorylated both in hepatoma and normal liver beta-glucuronidases.