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Amino acid assay in "high-performance" analyzers.

Recent amino acid analyzers combine high-pressure technology and optimal elution conditions to speed the quantification of common amino acids in physiological fluids. However, operators must keep in mind that some infrequently observed amino acids may interfere with the accurate quantification of those that are clinically more relevant. Although many interfering substances have been identified and their retention times characterized for systems using sodium citrate buffers (1), there is still very little information on retention times for the same substances chromatographed with the newer lithium-based systems (2). We use a System 6300 High Performance Amino Acid Analyzer, with physiological buffers and elution parameters as supplied and recommended by the manufacturer (Beckman Instruments Inc., Palo Alto, CA). This system adequately separates the 39 amino acids present in the standard mixture supplied by the manufacturer. Under these conditions, we have observed that glycerophosphoethanolamine (Rgb, = 0.088, the retention time relative to that of glycine) is insufficiently resolved to allow accurate integration of urinary taurine peak areas (Rgiy = 0.097). Similarly, we find that homocitrulline (Rgj = 1.453), excreted by infants on condensed milk feedings (3), co-elutes with methionine (Rgiy = 1.448), making difficult the detection of clinical hypermethioninuria. When authentic penicillamine is run, the sulthydryl form elutes at Rgh = 0.911 and obscures the proline peak (Rgiy = 0.942).Penicillaminedisulfide elutes at Rg = 1.495; the peak overlaps methionine (Rgj = 1.481) and cystathionine (Rgi = 1.509) standards and follows closely on the cystine peak (Rgiv = 1.464). In cystinuric patients who are receiving therapy with penicillamine, the cysteine-penicillamine mixed disulfide and penicillamine disulfide are not resolved from cystine, thus making it impossible to determine the effectiveness of treatment. Finally, in two phenylketonuric sera, we have observed a spurious peak (Rg = 1.698), which may be cystinylglycine (4). It causes shouldering of the phenylalanine peak (Rgiy= 1.741) and makes quantificationfserum phenylalanine less accurate. We would like to encourage the reporting of similar observations. We would also welcome the publication of programs that obviate these problems or any others encountered by users of this system.