Phosphorylation of the alpha-chain of fibrinogen by a platelet kinase activity enhanced by interferon.

Treatment of patients with interferon or inducers of interferon results in an enhanced level of a protein kinase activity found in platelets (1,3). The kinase activity is responsible for the phosphorylation of a 70-72,000 molecular weight protein (72K protein) found in blood plasma. By the means of a technique based on the precipitation of this protein kinase system (the protein kinase and its substrate), we show here that the 72K protein is the alpha-chain of fibrinogen. During the coagulation process induced by thrombin, the 32P-labelled 72K protein is recovered in the clot. After incubation in the presence of thrombin, the 72K protein looses a small polypeptide of 2-3000 in molecular weight resulting a shift in its isoelectric point (pI) from 6.8-7.0 to 7.5. At the end of the coagulation process, the 32P-labelled 72K protein becomes undetectable since it gives rise to a covalently linked alpha-polymer of a high molecular weight. In accord with these results, the 72K protein could be precipitated by antibodies against human fibrinogen.