A rapid and direct method for the quantitative determination of tryptophan in the intact protein

1. A method is given for the quantitative determination of free tryptophan or tryptophan in the intact protein by treating with ninhydrin in a mixture of formic acid and hydrochloric acid (reagent b), for 10min at 100 degrees C. Glycyltryptophan was used as a standard for the determination of tryptophan in the intact protein. The extinction at 390nm was linear in the range 0.05-0.5mumol for free tryptophan (in7120) and 0.05-0.30mumol for glycyltryptophan (in15400). 2. Free tryptophan in the presence of protein may be determined by treating with ninhydrin in a mixture of acetic acid and 0.6m-phosphoric acid (reagent a) for 10min at 100 degrees C, the extinction being linear for tryptophan in the range 0.05-0.9mumol. N-Terminal tryptophan peptides also give the typical yellow product on treatment with reagent a. 3. Tryptophan content of several pure intact proteins when treated with the above method gave values in good agreement with those reported by others. A mean tryptophan content of 11.25 (s.e.m. +/-0.08) mumol/100mg of protein was found in rat brain during development from 1 to 82 days after birth.