抄録
Muscle, heart (brain) and liver type isoenzymes of glycogen phosphorylase α [EC 2. 4. 1. 1] were purified to homogeneity from the pig skeletal muscle, heart and liver. Dephosphorylations of these isoenzymes by phosphoprotein phosphatases [EC 3. 1. 3. 16] with a molecular weight of 224, 000 purified from these pig tissues were studied. Apparent Km values of phosphoprotein phosphatases from the skeletal muscle, heart, and liver for the homologous tissue type isoenzymes were 8.8, 9.0, and 8.8 μM, respectively. Apparent Vmax values of dephosphorylation were 2.5, 6.5, and 1.2 nmol/min per unit, respectively (the unit was defined with rabbit skeletal muscle phosphorylase α as a substrate). Inhibition of dephosphorylation of the heart type isoenzyme by glucose 6-phosphate was partially competitive with a K1 value of 0.05mM. Though dephosphorylation of the liver type isoenzyme was inhibited competitively by glucose 6-phosphate with a K1 value of 3.5mmM, dephosphorylation of the muscle type isoenzyme was not affected by 0.1-1mM glucose 6-phosphate. AMP was a strong competitive inhibitor of the dephosphorylation reactions with respect to substrate isoenzymes and also served as an allosteric effector of the heart type isoenzyme. AMP concentrations required for 50% inhibition were 2 μM with the muscle type isoenzyme, 1 μM with the heart type isoenzyme, and 38 μM with the liver type isoenzyme. Differences in Vmax values and in degrees of inhibition by the metabolites of dephosphorylation of the isoenzymes were observed with phosphoprotein phosphatases from either homologous or heterologous tissues indicating that these differences were mainly attributed to phosphorylase α isoenzymes. A method for the purification of phosphorylase b from pig liver containing a low level of glycogen was developed.
