Fructose-bisphosphatase as a substrate of cyclic AMP-dependent protein kinase

We have tested rat liver fructose-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) and three other gluconeogenic fructose-bisphosphatases as substrates for the catalytic subunit of cyclic AMP-dependent protein kinase. In contrast to the rat liver enzyme, homogeneous preparations of mouse liver, rabbit liver, and pig kidney fructose-bisphosphatase could not be phosphorylated by the kinase. Comparative sodium dodecyl sulfate/polyacrylamide gel electrophoresis of the four above fructose-bisphosphatases revealed that the subunit molecular weight of the isolated rat liver enzyme (ca. 40,000-42,000) was greater than that of mouse liver, rabbit liver, and pig kidney fructose-bisphosphatases (ca. 36,000-37,000). Treatment of 32P-labeled rat liver fructose-bisphosphatase with trypsin resulted in the conversion of the rat liver enzyme to an active species with a subunit molecular weight identical to that of the three other enzymes, with complete loss of the 32P-labeled site. Identical trypsin treatment of pig kidney fructose-bisphosphatase caused no change in the molecular weight of the enzyme. The results suggest that the purified mouse liver, rabbit liver, and pig kidney fructose-bisphosphatases are not substrates for the cyclic AMP-dependent protein kinase in vitro because they lack the phosphorylation-site peptide.