COMPARATIVE-STUDIES OF PYRUVATE-KINASE FROM PSE AND NORMAL PIG MUSCLES

A fast breakdown of glycogen is observed in muscles of stress-suscepti ble pigs leading to pale, soft and exudative (PSE) meat. We report a c omparative study of pyruvate kinase from muscles of normal and PSE-pro ne pigs. Compared with the enzyme from normal muscle; pyruvate kinase isolated from PSE muscle shows a five times lower Michaelis constant, K-m, for phosphoenol pyruvate and a more than ten times higher k(cat)/ K-m value. The pH dependency of the enzymatic activity is shift-ed to more acidic values for pyruvate kinase from PSE muscles. According to isoelectric focusing, pyruvate kinase from PSE muscle consist of three isoforms, while only two isoforms are detectable in pyruvate kinase p reparations from normal pigs. The various isoforms were isolated by pr eparative isoelectric focusing and their steady-stale properties were compared. Isoform 3, which, is found only in PSE muscle, shows a 10-fo ld higher specific activity, a 30-fold lower K-m value and a 100-fold increased k(cat)/K-m value for phosphoenol pyruvate as compared to iso form 1. The presence of isoform 3 in PSE muscle appears to be responsi ble for the high activity of this enzyme under the more acidic conditi ons prevailing in PSE muscle. In vitro phosphorylation and dephosphory lation experiments using total enzyme and purified isoenzyme 1 suggest that isoforms 2 and 3 arise from isoform I by phosphorylation. Thus p rotein phosphorylation seems to be responsible for the shift in activi ty of pyruvate kinase, a key enzyme of glycolysis, under the acidic co nditions of PSE muscles.