The disulphide bonds of insulin

Abstract

In order to deduce the unique structure of ox insulin it is necessary to know its molecular weight. In previous papers a value of 12 000 was assumedc since physical measurements suggested that this was the weight of the smallest unit that existed in solution, Recently, however, Harfenist & Craig (1952) have used a new chemical method and have found a value of approximately 6000. It is difficult to see how this result could be wrong and some recent physical measurements have shown that dissociation into units of molecular weight lower than 12 000 definitely occurs (Fredericq, 1953; Kupke & Linderstrom-Lang, 1954). It may thus safely be concluded that the molecular weight of insulin is 5734, on the basis of condensation of the amino acids present with normal elimination of water. This molecule is composed of two poly- peptide chains joined together by the disulphide bridges of three cystine residues. Treatment with performic acid splits the insulin to two fractions A and B, which are the oxidized forms of the glycyl and the phenylalanyl chain respectively (Sanger, 1949a). The sequence of amino acids in these two polypeptide chains has been determined by partial hydrolysis methods (Sanger, 1949b; Sanger & Tuppy, 1951a, b; Sanger & Thompson, 1953a, b; Sanger, Thompson & Kitai, 1955) and is shown in 'core' was about 65 mg. It was further purified by dissolving Vol. 6o 543Q