title: The protein phosphatases involved in cellular regulation. Glycolysis, gluconeogenesis and aromatic amino acid breakdown in rat liver.
abstract: The identities of the protein phosphatases involved in the regulation of hepatic glycolysis, gluconeogenesis and aromatic amino acid breakdown were investigated using 6-phosphofructo-1-kinase, fructose-1,6-bisphosphatase, L-pyruvate kinase, phenylalanine hydroxylase and the bifunctional enzyme 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase as substrates. Purified preparations of protein phosphatases-1, 2A, 2B and 2C exhibited activity towards all five substrates in vitro, although phosphatases-1 and 2B were only weakly active. Studies in liver extracts using inhibitor-2 and trifluoperazine, which inhibit protein phosphatase-1 and 2B, respectively, confirmed that these phosphatases are unlikely to be important in dephosphorylating these substrates in vivo. Sequential fractionation of rat liver extracts by anion-exchange chromatography and gel-filtration failed to resolve any protein phosphatases acting on each substrate, apart from protein phosphatases-2A and 2C. The present results, together with those described in the following paper (in this journal) indicate that under the assay conditions used, protein phosphatase-2A is the most powerful phosphatase acting on each substrate, although protein phosphatase-2C contributes a significant percentage of the activity towards 6-phosphofructo-1-kinase. No clear evidence was obtained for a role of metabolites in the regulation of dephosphorylation of the five substrates. This study reinforces our contention that only a few serine-specific and threonine-specific protein phosphatase catalytic subunits participate in cellular regulation.
mesh_terms: D000494:Allosteric Regulation; D000596:Amino Acids / Q000378:metabolism*; D000818:Animals; D002848:Chromatography, DEAE-Cellulose; D003600:Cytosol / Q000201:enzymology; D005260:Female; D005943:Gluconeogenesis; D006019:Glycolysis; D008099:Liver / Q000378:metabolism*; D010749:Phosphoprotein Phosphatases / Q000378:metabolism*; D010766:Phosphorylation; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2; D051381:Rats; D011919:Rats, Inbred Strains; D013379:Substrate Specificity
chemical_list: D000596:Amino Acids; D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2