On the possible iodometric estimation of insulin.

Abstract

It is generally agreed that insulin is destroyed by trypsin and other proteolytic enzymes. Recently, 1 we suggested that the physiologically inactive addition compound, which is formed between insulin and trypsin before the digestion of insulin starts, is probably the so-called intermediate compound between enzyme and substrate. Up to the present, conclusions as to the existence or non-existence of the enzyme-substrate compound have been drawn almost exclusively 2 from the analysis of kinetic studies. Further progress in the investigation of the insulin-enzyme reaction depends in part on the development of a suitable method for the chemical estimation of insulin.

Certain earlier observations 3 induced us to attempt an iodometric estimation of insulin. Our present knowledge of the chemical constitution of insulin is not opposed to the feasibility of such a method. We know from studies on the iodization of proteins 4 that iodine reacts with various groups of the protein molecule, among others with the lead-blackening sulphur. The presence of lead-blackening sulphur in the molecule of insulin seems to have been demonstrated by Abel and Geiling. 5 Whether the insulin molecule contains other groups that react with iodine cannot yet be decided. It has been recommended that the iodometric estimation of certain inorganic groups be carried out in the presence of neutral buffer. We have noticed that phenols, glucose, cyclic aminoacids, and other substances, which are supposed to react with I₂-KI solution only at an alkaline pH, take up iodine also in the presence of neutral buffer. 6