High doses of methylene blue/light treatment crosslink the A-alpha-subunit of fibrinogen: influence of this photooxidization on fibrinogen binding to platelets.

Irradiation in the presence of 1 microM methylene blue (MB) inactivates enveloped viruses in human plasma. This method is a useful tool to enhance the safety of human fresh-frozen plasma in blood donor banks. Via a photooxidative mechanism, viral structures as well as plasma proteins, especially fibrinogen, can be damaged. We investigated the effect of MB/light-induced photooxidative modification on the structure of fibrinogen and its function in the interaction with platelets in the concentration range of 150 microM MB. Densitometric scanning of reduced SDS-PAGE showed a decrease in the Aalpha-subunit of fibrinogen as well as an increase in high-molecular-weight (HMW) aggregates with increasing MB concentrations. The HMW aggregates may be due to covalent crosslinking of single Aalpha-subunits via photooxidation induced by MB/light treatment. Fibrinogen treated with high doses of MB (50 microM) showed a weaker binding to the fibrinogen receptor (GP IIb/IIIa) on the platelet surface and a decrease in platelet aggregation after stimulation with ADP and photooxidized fibrinogen. These effects were not detected in fibrinogen isolated from virus-inactivated plasma (1 microM MB). This was in agreement with normal binding of fibrinogen to GP IIb/IIIa on platelets.