Regulation of platelet phosphorylase

Area of Science:

• Biochemistry
• Cell Biology
• Enzymology

Background:

• Glycogen phosphorylase plays a key role in glycogen metabolism.
• Platelet activation involves complex signaling pathways.
• Understanding enzyme activation in platelets is crucial for studying hemostasis and thrombosis.

Purpose of the Study:

• To develop a sensitive fluorimetric enzyme assay for glycogen phosphorylase activation in platelets.
• To investigate the regulation of glycogen phosphorylase activity in intact platelets and platelet extracts.
• To elucidate the roles of calcium and cyclic AMP in platelet phosphorylase activation.

Main Methods:

• Development of a fluorimetric enzyme assay to measure glycogen phosphorylase activity.
• Stimulation of intact rat and human platelets with thrombin, ionophore A23187, cyanide, and dibutyryl cyclic AMP.
• Analysis of platelet extracts with varying incubation conditions including Mg2+-ATP, Ca2+, EGTA, cyclic AMP, and phosphorylase kinase.

Main Results:

• Thrombin stimulation of intact platelets caused a rapid (within 10 s) 7-fold increase in total glycogen phosphorylase activity.
• Divalent cation ionophore A23187 and cyanide also activated phosphorylase, suggesting calcium-dependent and metabolic regulation.
• Dibutyryl cyclic AMP partially activated phosphorylase and modulated responses to other stimuli.
• Platelet extracts showed Ca2+-dependent activation by a kinase, with cyclic AMP involvement, and conversion of AMP-dependent to AMP-independent forms.

Conclusions:

• Platelet glycogen phosphorylase exists in inactive and two active forms.
• A kinase requiring Ca2+ and activated via a cyclic AMP-mediated process catalyzes the conversion between these forms.
• Physiological stimulation leads to increased levels of both active forms of platelet phosphorylase.