Selective inactivation of the D or I forms of glycogen synthetase from rat skeletal muscle

Abstract

The inactivation of the D or I forms of glycogen synthetase (dependent or indepen­ dent of glucose-6-phosphate) was demonstrated in this laboratory for the enzyme systems from frog skeletal muscle, heart and kidney from rat and also for human polimorpho- nuclear leucocytes and lymphocytes. A scheme of metabolic interconversions between different molecular forms of glycogen synthetase was suggested (10) in order to explain such phenomena. In the scheme coexisted enzyme forms completely active (I-forms) or less active (D-forms) with other completely inactive (X-forms).

In this paper experimental data are shown which demonstrate the possibility of selec­ tively inactivating either the I or the D forms of glycogen synthetase from rat skeletal muscle, by reactions promoted by the addition of ATP and Mg2+ to the enzyme extracts. Neither of these inactivating reactions observed could be attributed to the MgI+ or ATP independently, therefore they must be attributed to phosphorylating reactions brought about by protein kinases.

These inactive molecular species fit adequately in the scheme of interconversions proposed for other enzyme sources (8, 16) and can be logically explained with the recent knowledge obtained on these enzyme systems.