The reaction of mercuric acetate with histidine and tyrosine

• Published in: Journal of inorganic biochemistry Vol. 10; no. 3; pp. 235 - 255
• Main Authors: Korn, Alex P., Ottensmeyer, F.Peter, Jack, Thomas R.
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 1979
• ISSN: 0162-0134; 1873-3344
• DOI: 10.1016/S0162-0134(00)80283-4

The reaction of mercuric acetate with polypeptides in an appropriate buffer system has been found to result in the selective binding of two atoms of mercury to each tyrosine and histidine residue. These heavy atom labels are stable to the high chloride concentrations used to displace the excess mercury (II) from other binding sites on the polypeptide. The kinetics and stoichiometry of these reactions have been studied by the binding of radioactive ( 203Hg) mercuric acetate to synthetic polymers of these amino acids and by ultraviolet-visible spectroscopy. For a polymer containing tyrosine the mercuration kinetics closely match those for the following mechanism: ▪ At 60° C, and in a buffer containing 0.05M TRIS-acetate, k 1 was determined to be 9.47 ± 0.27 M −1 min −1. The best match to the data was for k 1/k 1 = 5.5 It was discovered that there is an inverse relationship between k 1 and the TRIS buffer concentration. The activation energy of k 1 was determined to be 18.9 ± 0.1 kcal/mole. Chemical analyses of the products obtained from the reaction of mercuric acetate with tyrosine amide, L(-)-histidine and the methyl ester of L(-)-histidine have established that mercuration results in the formation of a Hg-C bond at the C 3 and C 5 sites on the phenolic ring in tyrosine and at the C 4 site in the imidazole ring in histidine. The site of the second mercury retained by histidine in the presence of high chloride concentrations is uncertain but does involve the amine functions of the imidazole ring. The reaction conditions employed also cause the oxidation of methionine and cysteine to methionine sulfoxide and cysteic acid, respectively. Cystine, however, resists oxidation.