INVESTIGATION OF MECHANISM-BASED INHIBITORS OF COMPLEMENT TARGETING THE ACTIVATED THIOESTER OF HUMAN C3

An intramolecular thioester bond in complement protein C3 is vital for covalent attachment of C3b (the proteolytically activated form of C3) to biological surfaces and for activation of the complement system. P roteolytic removal of C3a from C3 activates the thioester in the C3b f ragment. Activated C3b primarily forms ester bonds with hydroxyl group s of carbohydrates on complement activating surfaces, but it has also been shown to react with the hydroxyl group of tyrosine and with speci fic Ser and Thr residues on IgG and on complement protein C4b. To exam ine the reactivity of the thioester, several families of hydroxylated compounds were examined. Reactivity of a series of substituted phenols varied over two orders of magnitude and demonstrated a linear correla tion between reactivity and the Hammett substituent constants. Hydroxy lated drugs including members of the L-DOPA/epinephrine family and hyd roxamic acids also were examined. Compounds were identified that were 20,000 times more reactive than carbohydrates. These compounds were fo und to inhibit both the classical and alternative pathways of compleme nt activation. Although the specificity of the thioester for its natur al biological targets appears to be determined by many structural feat ures, the data presented here demonstrate that increasing the nucleoph ilic character of the target hydroxyl group can increase the potency o f a synthetic inhibitor many orders of magnitude.