Sites of modification of human angiogenin by bromoacetate at pH 5.5

Human angiogenin is inactivated by treatment with bromoacetate at pH 5.5. Use of [14C]bromoacetate and tryptic peptide mapping have identified the sites of carboxymethylation as His-13 and His-114, with His-114 reacting approximately 1.5-fold more rapidly than His-13. At later stages in the reaction, both His-13 and -114 become modified with His-114 in part forming a bis derivative. Comparison with carboxymethylhistidine derivatives of known structure obtained from bovine pancreatic ribonuclease A indicates that the reaction order is N-1 of His-114 greater than N-3 of His-13 greater than N-3 of His-114.