The paper we recently published in JPEN' and discussions about it since its publication raised our interest in the determination of cord blood tryptophan concentrations and total cysteine concentrations. Our initial attempts to analyze samples for these amino acids using standard techniques were unsuccessful. The modifications we have used are described below.Plasma samples were analyzed for total tryptophan by the fluorometric procedure of Denckla and Dewey' as modified by Bloxam and Warren' with further modifications for improved accuracy and precision by our phys-FIG. 1. Regression analysis of gestational age (weeks) at the time of sample and plasma tryptophan concentrations (~mole/dl).iological amino acid analysis laboratory. The major modifications include heating samples in closed vials and extending the heating period to 75 min in boiling water for maximum conversion of tryptophan to norharmon. The accuracy of our plasma tryptophan method was evaluated by comparing data from our method with data generated from a reverse-phase high performance liquid chromatography (HPLC) method. A paired t-test analysis showed no significant difference (p = 0.50) between pairs of tryptophan values obtained by the two analytical methods.Total plasma cyst(e)ine was determined by reverse phase HPLC with electrochemical detection. Dithiothreitol was added to plasma samples and thiols were detected TABLE I Age, tryptophan, and cysteine data FIG. 2. Regression analysis of gestational age (weeks) at the time of sample plasma total cyst{e)ine concentrations (~mole/dl).
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