A new colorimetric method for the estimation of histidine and histamine.

Histidine occurs widely in nature both in the free and combined form. In order to study histidine in its various roles it is necessary therefore to have a sensitive and specific method for its estimation. The colorimetric method described initially by Pauly (1904) and since modified by various workers (Koessler & Hanke, 1919; Jorpes, 1932; Macpherson, 1942; Sanger & Tuppy, 1951), which depends on the reaction of histidine with an aromatic diazonium salt to form a red azo dye, is widely used but suffers from its low specificity. Kapeller-Adler (1933) developed a quantitative colorimetric method based on a reaction discovered by Knoop (1908), which depends on a bromination of histidine in acetic acid. This method, which is specific for histidine and closely related compounds, has not proved entirely satisfactory and many modifications have been suggested (Tsuverkalov & Torban, 1951; Hunter, 1952, 1955; Voigt, 1959). We have discovered a reaction of histidine wvith iodine and pyridine in which a violet colour is developed. This has led to a convenient and reliable method for the estimation of histidine and histamine which may be used for the analysis of solutions containing as little as 1 ,eg./ml.