Quantitative colorimetric determination of tryptophan.

The methods currently used for the estimation of tryptophan are based on one of the following principles: Reaction with aldehydes in acid solution (1, Z), with glyoxylic acid (3), with mercury salts and nitrous acid (4,5), by determination after isolation from its mercury salt (6)) or by the use of Marshal’s reagent (7). Of the above mentioned methods, those based on aldehyde reactions and on the use of Marshal’s reagent are not ideal for the purpose, since in biological fluids there are many other substances, unrelated to tryptophan, that interfere with the determination. Mercury salt precipitation and subsequent determination of tryptophan is a specific albeit time-consuming procedure. Moreover, some loss (12 to 18 per cent) is unavoidable. The best method for the tryptophan estimation seems to be Shaw’s (3) glyoxylic acid reaction. The preparation of the reagent from oxalic acid, or its maintenance in a stable condition when otherwise obtained, still presents a problem for the routine laboratory. Therefore, we have developed an extremely simple, but specific method for the quantitative determination of tryptophan. Our method is based on the reaction of Adamkiewitz (8), who used it for the detection of proteins (containing tryptophan) as a ring test. We have adopted his principles for the measurement of tryptophan in concentrations from 5 pg upwards, in a final volume of 3.5 ml.