Plasma protein kinase activity enhanced by interferon is found in platelets

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• Interferon treatment in HeLa cells induces specific protein kinase activity.
• Human plasma, particularly platelet-rich plasma, exhibits analogous kinase activity.
• This activity involves the phosphorylation of an endogenous Mr 72,000 protein.

Purpose of the Study:

• To characterize the protein kinase system present in human plasma.
• To identify the components and localization of this plasma-based kinase activity.
• To investigate the interaction between the kinase and its substrate in plasma.

Main Methods:

• Assay of protein kinase activity in human plasma.
• Partial purification of the Mr 72,000 protein substrate using poly(G)-Sepharose.
• Localization studies of the kinase and substrate components within plasma and platelets.

Main Results:

• A protein kinase system analogous to that in interferon-treated cells was detected in human plasma.
• The kinase activity phosphorylates an endogenous Mr 72,000 protein.
• The protein kinase is localized to platelets, while the Mr 72,000 protein substrate is primarily free in plasma, with a fraction associated with platelet surfaces.

Conclusions:

• Human plasma contains a distinct, at least two-component, protein kinase system.
• Platelets harbor the protein kinase, and the substrate is largely soluble in plasma.
• This compartmentalization suggests specific regulatory mechanisms for kinase activity in the plasma environment.