title: The reaction of iodine and thiol-blocking reagents with human complement components C2 and factor B. Purification and N-terminal amino acid sequence of a peptide from C2a containing a free thiol group.
abstract: Human complement components C2 and Factor B each contain one free thiol group/molecule. Reaction with p-chloromercuribenzoate destroyed the haemolytic activity of C2 but had no effect on Factor B. Reaction of C2 with I2 gave a 16-fold enhancement of its haemolytic activity. The pH optimum for the reaction was 7.0. The I2 reacted at the thiol group in C2 with a stoicheiometry of 1 mol of I2/mol of C2. The product of the reaction was unaffected by millimolar concentrations of dithiothreitol; however, azide and cyanide were inhibitory. Reaction with azide did not result in re-expression of the thiol group. Mild oxidation of the thiol group with m-chloroperbenzoic acid did not enhance the haemolytic activity. The results suggest that reaction with I2 causes intramolecular covalent, but not disulphide, bond formation. I2 reacted with Factor B at the free thiol group without affecting the haemolytic activity. A CNBr-cleavage peptide from C2a (obtained by cleavage of C2 by subcomponent C1s) containing the free thiol group was isolated. Automated Edman degradation of the peptide showed that it was the N-terminal peptide of C2a. The free thiol group was identified at position 18.
mesh_terms: D000595:Amino Acid Sequence; D000596:Amino Acids / Q000032:analysis; D001386:Azides / Q000494:pharmacology; D002729:Chloromercuribenzoates / Q000494:pharmacology; D002850:Chromatography, Gel; D002851:Chromatography, High Pressure Liquid; D003175:Complement C2 / Q000378:metabolism*; D011415:Complement Factor B / Q000378:metabolism*; D004792:Enzyme Precursors / Q000378:metabolism*; D006801:Humans; D007455:Iodine / Q000494:pharmacology*; D010084:Oxidation-Reduction; D010446:Peptide Fragments / Q000302:isolation & purification; D019810:Sodium Azide; D013439:Sulfhydryl Reagents / Q000494:pharmacology*; D020245:p-Chloromercuribenzoic Acid
chemical_list: D000596:Amino Acids; D001386:Azides; D002729:Chloromercuribenzoates; D003175:Complement C2; D004792:Enzyme Precursors; D010446:Peptide Fragments; D013439:Sulfhydryl Reagents; D020245:p-Chloromercuribenzoic Acid; D007455:Iodine; D019810:Sodium Azide; D011415:Complement Factor B