ENZYMOLOGICAL INVESTIGATIONS ON THE CAUSES FOR THE PSE-SYNDROME .1. COMPARATIVE-STUDIES ON PYRUVATE-KINASE FROM PSE-MUSCLES AND NORMAL PIG MUSCLES

zA fast breakdown of glycogen L 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 enzyme from normal muscle, pyruvate kinase isol ated from PSE-muscle shows a five times lower K-m for phosphoenol pyru vate and a more than ten times higher k(cat)/K-m value. The pH-depencl ency of the enzymatic activity is shifted to more acidic values for py ruvate kinase from PSE muscles. According to Isoelectric focusing, pyr uvate kinase from PSE muscle consists of three isoforms, while only tw o isoforms are detectable in pyruvate kinase preparations from normal pigs. The various isoforms were isolated by preparative isoelectric fo cusing and their steady-state properties were compared. Isoform 3, whi ch is found only in PSE muscle, shows a 10-fold higher specific activi ty, a 30-fold lower K-m value and a 100-fold increased k(cat)/K-m valu e for phosphoenol pyruvate compared to isoform 1. The presence of isof orm 3 in PSE-muscle appears to be responsible for the high activity of this enzyme under the more acidic conditions prevailing in PSE-muscle . In vitro phosphorylation and dephosphorylation experiments using tot al enzyme and purified isoenzyme I suggest that isoforms 2 and 3 arise from isoform I by phosphorylation. Thus protein phosphorylation seems to be responsible for the shift in activity of pyruvate kinase, a key enzyme of glycolysis, under the acidic conditions of PSE-muscles. Cop yright (C) 1996 Elsevier Science Ltd.