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

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Abstract
Rat liver fructose-bisphosphatase (EC 3.1.3.11) and 3 other gluconeogenic fructose-bisphosphatases were tested as substrates for the catalytic subunit of cAMP-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 4 above fructose-bisphosphatases revealed that the subunit MW of the isolated rat liver enzyme (.apprx. 40,000-42,000) was greater than that of mouse liver, rabbit liver and pig kidney fructose-bisphosphatases (.apprx. 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 MW identical to that of the 3 other enzymes, with complete loss of the 32P-labeled site. Identical trypsin treatment of pig kidney fructose-bisphosphatase caused no change in the MW of the enzyme. The purified mouse liver, rabbit liver and pig kidney fructose-bisphosphatases are probably not substrates for the cAMP-dependent protein kinase in vitro because they lack the phosphorylation-site peptide.