Regulatory features of glycogen phosphorylase from frog brain (Rana temporaria)

Abstract

1. Glycogen content and the activity of glycogen phosphorylase (GPase) are much higher in brain tissue of the Common frog (Rana temporaria) than in brain tissue of mammals and birds (Table 1). 2. In phosphate buffer GPase is extracted from frog orain in a form completely active without addition of AMP and has therefore to be regarded as phosphorylase a. Several procedures to extract the b-form of the enzyme from the tissue have been unsuccessful. In resting skeletal muscle predominantly the AMP dependent b-form is present (Table 1). 3. In vitro, however, the existence of the complete interconverting system can be demonstrated. If NaF (a phosphatase inhibitor) was omitted from the homogenization medium, GPase activity decreased but concomitantly a significant activation by AMP was observed. The process is reverted under conditions favouring protein phosphorylation (Fig. 1). 4. Lowering the AMP concentration by dilution or dialysis of tissue extract also causes a loss of GPase activity, but a different mechanism is involved. This effect is quite specific and based on a dissociation of the dimeric enzyme into subunits (Fig. 4). Reactivation is specifically blocked byl-cysteine (Figs. 2 and 3). 5. Both substrates, glycogen andPi, stabilize the enzyme synergistically (Fig. 5). 6. At low levels ofPi and AMP, temperature markedly affects the dissociation of the enzyme with the consequence that its catalytic activity is nearly independent of temperature. This might be interpreted as a metabolic adaptation contributing to the ability of the CNS to function over a wide range of temperatures.