A RAPID COLORIMETRIC METHOD FOR THE DETERMINATION OF TRYPTOPHANE IN PROTEINS AND FOODS

Abstract

A survey of the literature shows that the values given by several investigators for the tryptophane content of certain proteins are twice those reported by others for the same protein.In practically all cases the high values were obtained by calorimetric methods in which the color developed was compared against that similarly obtained with casein, the value for the casein having been predetermined by the same procedure with free tryptophane as a standard.The tryptophane values for casein thus found ranged from 2 to 2.4 per cent.These high values for tryptophane in casein have been reported by May and Rose (1) and others (2-5) who used their method or modifications of it.On the other hand, Shaw and McFarlane (6), Block and Bolling (7), with different calorimetric procedures, and Greene and Black (8), by a microbiological method, found casein to contain from 1 to 1.20 per cent of tryptophane.Sullivan, Milone, and Everitt (4) developed a modification of the May and Rose method which greatly shortened the time required for the determination of tryptophane.When casein was heated with hydrochloric acid and p-dimethylaminobenzaldehyde at 85" for 15 minutes, a blue color developed after the addition of 0.3 per cent hydrogen peroxide, which remained permanent for 24 hours.The values for casein were identical with those obtained after 8 days digestion according to the May and Rose method.The color thus obtained was then compared with that given by a standard solution of free tryptophane, treated according to the May and Rose method.They found 2.4 per cent.tryptophane for casein.Shaw and McFarlane (9) made a critical study both of the May and Rose method and the rapid method of Sullivan, Milone, and Everitt, and showed that erroneous results are obtained by these methods, due to the fact that tryptophane, as combined in the protein molecule, gives more color with the p-dimethylaminobenzaldehyde reagent than does an equivalent amount of free tryptophane under the same conditions.They also showed that the * The determination of tryptophane in proteins and foodstuffs was begun as a collaborative study with Dr.