HUMAN PLATELET PHOSPHORYLASE

Human platelets contain phosphorylase activity similar in some respects to that of skeletal muscle. Sucrose gradient analysis of partially purified phosphorylase revealed a phosphorylase b dimer with apparent molecular weight of 177 000, S20,w = 8.9 S; a phosphorylase a dimer with apparent molecular weight 177 000, s20,w = 8.9 S; and a phosphorylase a tetramer with apparent molecular weight of 326 000, s20,w = 13.5 S. AMP-independent phosphorylase activity (phosphorylase a) in vitro could be varied from 2.3% to 90% of total activity by suitable incubation or dialysis with MgATP, NaF or EDTA. Of interest is the observation of an active phosphorylase a dimer (AMP-independent). Phosphorylase a dimer was the predominant form of phosphorylase a with the experimental condition employed. A kinetic analysis of phosphorylase b revealed a sigmoidal AMP-dependence curve with 1 2νmax = 6·10-5 M. Phosphorylase a had an apparent Km for glycogen and orthophosphate of 0.2 and 0.7 mM, respectively. ATP and ADP served as noncompetitive inhibitors with respect to glycogen and competitive inhibitors with respect to orthophosphate. The apparent Ki for ATP and ADP with respect to orthophosphate was 0.82 and 2.9 mM, respectively. The apparent Ki for ATP and ADP with respect to glycogen was 2.5 and 3.5 mM, respectively. © 1969.