Phosphofructokinase from white skeletal muscle and liver of rainbow trout (Oncorhynchus mykiss): isolation, characterization and study of enzymatic regulation
Phosphofructokinase (PFK) isozymes from white skeletal muscle and liver of rainbow trout Oncorhynchus mykiss were purified to electrophoretic homogeneity. Muscle PFK was purified 175-fold using phosphocellulose, hydroxylapatite, and ATP-agarose affinity chromatography whereas liver PFK was purified 13,400-fold using acetone precipitation, heat treatment, ammonium sulfate fractionation, and ATP-agarose chromatography. Muscle PFK was a homohexamer having a native molecular mass of 478,000. The enzyme was regulated by the levels of fructose-6-phosphate (F6P), ATP, pH, and allosteric effectors including activators (NH4+, inorganic phosphate, AMP, ADP, and fructose-2,6-bisphosphate [F2,6P2]) and inhibitors (citrate, phosphoenolpyruvate [PEP], and ATP). Activators increased the enzyme affinity for F6P and released the inhibition by ATP or citrate. Citrate inhibited the enzyme synergistically with ATP. Arrhenius plots of the enzyme activity showed discontinuity at 15 to 16°C, presumably due to conformational alterations in the enzyme. The kinetic behavior of muscle PFK was significantly altered by protein kinase – mediated phosphorylation. The high-phosphate form of the enzyme showed higher activity with increased affinity for F6P and less inhibition by ATP. Protein concentration affected enzyme activity as assessed by two different methods. PFK showed a higher Vmax, lower S0.5 F6P and higher I50 values for ATP as enzyme concentration increased. The association of PFK with myofibrils of trout muscle was affected by pH, ionic strength, protein concentration, and the levels of metabolites or the effectors of the enzyme with binding favored by lower pH values and increased protein concentration. During exercise, muscle PFK is probably activated by increases in the levels of enzyme activators and enzyme phosphorylation state, and enhanced PFK association with myofibrils. Trout liver PFK was also regulated by the levels of F6P, ATP, NH4+, inorganic phosphate, AMP, and F2,6P2. However, the liver enzyme was not sensitive to citrate inhibition. Contrary to its muscle counterpart, liver PFK was inhibited by protein phosphorylation catalyzed by the catalytic subunit of cAMP-dependent protein kinase and activated by the removal of phosphate through acid phosphatase. The high-phosphate form of liver PFK exhibited a lower Vmax, an increased S0.5 F6P, and higher I50 values for ATP.