ASSIGNMENT OF A SINGLE DISULFIDE BRIDGE IN HUMAN ALPHA(2)-ANTIPLASMIN- IMPLICATIONS FOR THE STRUCTURAL AND FUNCTIONAL-PROPERTIES

Human alpha(2)-antiplasmin (alpha(2)AP) is a serpin involved in the re gulation of blood coagulation. Most serpins, unlike smaller serine pro teinase inhibitors, do not contain disulphide bridges. alpha(2)AP is a n exception from this generalization and has previously been shown to contain four Cys residues organized into two disulphide bridges [Lijne n, Holmes, van Hoef, Wiman, Rodriguez and Collen (1987) fur. J. Bioche m. 166, 565-574]. However, we found that alpha(2)AP incorporates iodo[ C-14]acetic acid, suggesting that the protein contains reactive thiol groups. This observation prompted a re-examination of the state of the thiol groups, which revealed (i) a disulphide bridge between Cys(43) and Cys(116), (ii) that Cys(76) is bound to a cysteinyl-glycine dipept ide, and (iii) and Cys(125) exists as either a free thiol or in a mixe d disulphide with another Cys residue. The disulphide identified betwe en Cys(43) and Cys(116) appears to be conserved in orthologous protein s since the homologous Cys residues form disulphide bonds in bovine an d possibly mouse alpha(2)AP. The conservation of this disulphide bridg e suggests that it is important for functional aspects of alpha(2)AP. However, the structural and functional analysis described in this stud y does not support this conclusion.