Enzymological investigations on the causes for the PSE-syndrome, I. Comparative studies on pyruvate kinase from PSE-...

A fast breakdown of glycogen is observed in muscles of stress-susceptible pigs leading to pale, soft and exudative (PSE) meat. We report a comparative study of pyruvate kinase from muscles of normal and PSE-prone pigs. Compared with enzyme from normal muscle, pyruvate kinase isolated from PSE-muscle shows a five times lower 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 shifted to more acidic values for pyruvate kinase from PSE muscles. According to isoelectric focusing, pyruvate kinase from PSE muscle consists of three isoforms, while only two isoforms are detectable in pyruvate kinase preparations from normal pigs. The various isoforms were isolated by preparative isoelectric focusing and their steady-state properties were compared. Isoform 3, which is found only in PSE muscle, shows a 10-fold higher specific activity, a 30-fold lower K(m) value and a 100-fold increased k (cat)K (m) value for phosphoenol pyruvate compared to isoform 1. The presence of isoform 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 total enzyme and purified isoenzyme 1 suggest that isoforms 2 and 3 arise from isoform 1 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.