Regulation of phosphofructokinase from muscle and liver of rainbow trout by protein phosphorylation.

Phosphofructokinase (PFK) from both white skeletal muscle and liver of trout is controlled by reversible phosphorylation. In vitro phosphorylation of purified muscle PFK with the catalytic subunit of cAMP-dependent protein kinase led to a 25% decrease in the S0.5 F6P and reduced inhibition by Mg.ATP and citrate. Phosphorylation of trout liver PFK lowered the I50 Mg.ATP (by 27%) but in vitro treatment with acid phosphatase reduced S0.5 F6P by 40% and increased I50 Mg.ATP by 50%. Thus, dephosphorylated trout liver PFK appears to be the more active enzyme form. Compared with mammalian PFK, the less rigorous effects of phosphorylation on trout liver PFK and relatively stronger phosphorylation control of skeletal muscle PFK may serve different patterns of carbohydrate metabolism in lower vertebrates, in particular the in situ processing of lactate in post-exercise muscle.