Enzymological investigations on the causes for the PSE-syndrome, II. Comparative studies on glycogen phosphorylase...

In order to investigate the cell biological causes for the fast breakdown of glycogen which is observed during the development of the PSE (pale, soft, exudative) syndrome in muscles of stress-susceptible pigs, muscle glycogen phosphorylase (GP) as a key enzyme in two isoforms, a and b, of the energy turnover was isolated from M. longissimus dorsi of normal and PSE-prone pigs of the German Landrace. GP b as well as GP a from normal and PSE-muscles exist in a dimeric form with a molecular weight of 97 000 D per subunit. The tendency for tetramerization of GP b increases in the presence of ATP, whereas the enzyme activity is simultaneously inhibited. The catalytic activities of GP a and GP b from both groups of animals show an optimum at pH 7.0. GP b can be activated to GP a by phosphorylation with the result of a 25% higher optimum specific activity in the case of normal and PSE-muscles. In interaction with glycogen and glucose-1-phosphate GP b follows the characteristics of a Michaelis-Menten kinetic, whereas the binding of AMP and phosphate proves to be allosteric. In comparison of the structural and kinetic characteristics of GP from normal as well as PSE-muscles no significant differences could be determined, indicating that GP does not belong to those factors which are triggering an accelerated energy turnover of ATP in muscles of stress-susceptible pigs.