The reactivity of the iminazole ring in proteins

Abstract

L,2,4-Fluorodinitrobenzene reacts with the iminazole ring of N1-acetyl-histidine to give a compound which does not yield histidine on acid hydrolysis. With the aid of this reaction it has been found that 2 of the 4 histidine residues in beta-lacto-globulin fail to react with fluorodinitrobenzene unless the protein is first denatured. In ovalbumin, serum albumin, hemoglobin and globin the histidine residues react fully whether the proteins are denatured or native. In crystalline zinc insulin 0.3-0.4 histidine residues of the 4 present per submolecule fail to react, and after heat precipitation or conversion to fibrous insulin, 1.8 residues become unreactive. The significance of these findings in relation to fiber formation is discussed. Unsuccessful attempts to use this reaction to establish the mode of linkage of heme to globin in native hemoglobin are described.