Effect of prostaglandins on phosphorylase b.

Prostaglandins at mM concentration are able to induce a small activation to rabbit muscle glycogen phosphorylase b (1,4-alpha-D-glucan: orthophosphate alpha-D-glucosyltransferase, EC 2.4.1.1) in the absence of AMP. The extent of this activation depends on the nature of the molecular structure of prostaglandins. Saturated and unsaturated higher fatty acids were unable to substitute for prostaglandins. The main findings of our studies can be summarized as follows: (1) Prostaglandins inhibit the AMP-induced activation of phosphorylase b. (2) Modification of the AMP binding site with 2,3-butanedione could not inhibit the activity induced by prostaglandins. (3) Enzyme activation by prostaglandins is stimulated by spermine. (4) Phosphorylase b activation by prostaglandins and the observed stimulation of this activation by spermine were found to be temperature dependent. (5) Prostaglandins affect the quaternary structure of phosphorylase b inducing a partial enzyme tetramerization which is enhanced in the presence of spermine. The extent of this tetramerization is temperature dependent.