PHOTOOXIDATION OF HISTIDINE AS A PROBE FOR AMINOTERMINAL CONFORMATIONAL-CHANGES DURING FIBRINOGEN-FIBRIN CONVERSION

Fibrinogen is known to become unclottable when irradiated with light i n the presence of methylene blue, the loss of clottability being due t o photo-oxidation of the histidine at position 16 of the B beta chain. In the present investigation it could be demonstrated that not only t his histidine but also the one at position 24 of the Aa chain was modi fied and that the rates of modification could be modulated by fibrinop eptide release, polymerization inhibition and denaturation. Accordingl y, the histidine modifications can be used as probes for surface acces sibility of and conformational differences among the various forms of the protein. Both histidines are shielded by the fibrin polymer struct ure. Fibrinopeptide A release alone leads to maximal protection of the one in the Aa chain, but only partial protection of the one in the B beta chain. Subsequent fibrinopeptide B release leads to maximal prote ction of the one in the B beta chain. The differential effects indicat e that two conformational changes have occurred. Polymerization inhibi tion reverses the protective effect. Denaturation leads to maximal and equal modification in all samples as a consequence of the loss of nat ive conformation.