Quantitative determination of tryptophan at the picomole level is described, using the ATP-[32P]PPi isotopic exchange reaction catalyzed by tryptophanyl-tRNA synthetase. Sensitivity limits of 500 fmol were obtained. The presence of other amino acids at a 1000-fold excess over tryptophan did not interfere significantly with the quantitative determination of tryptophan. The specificity of the reaction was checked using five tryptophan analogs. These analogs did not prevent the determination of tryptophan when present in the same concentration range as tryptophan. When sensitive determination of a single amino acid is needed, the ATP-[32P]PPi exchange reaction catalyzed by aminoacyl-tRNA synthetases is suggested as a general method and as an alternative to HPLC procedures.
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