Stimulation of muscle glycogen synthase phosphatase by polyamines

The naturally-occurring polyamines were found to stimulate glycogen synthase phosphatase from rat skeletal muscles. The sequence of effectiveness in the stimulation was spermine greater than spermidine greater than putrescine. It was shown that the spermine-sensitive phosphatase was present primarily in the soluble fraction of the muscle extract. In the presence of spermine, the phosphatase activity can be further stimulated by Mn2+; however, a lower Mn2+ concentration is required for the activation of the enzyme in comparison with that in the absence of spermine. Kinetic studies indicated that activation of glycogen synthase phosphatase by spermine was achieved by an increase in its V without significant alteration in the Km, suggesting that spermine directly stimulated the catalytic efficiencly of the phosphatase enzyme.