Quantitative Methods for Amino Acid Analysis in Biological Fluids

Abstract

We must be able to provide accurate quantitative analyses of free amino acids in biological fluids. These are needed to: diagnose inherited metabolic disorders and to control their treatment; to monitor patients requiring long-term nutritional support; to evaluate new baby milks and parenteral or formulated enteral feeds; to detect renal drug toxicity; and to monitor bone or muscle turnover. Sometimes it is enough to analyse only one or a small group of amino acids, but more often, a full amino acid profile is required. This is an extremely difficult task as there are some 40 or 50 amino acids or amino acid conjugates in biological fluids, I with more than a WOO-fold range of concentrations. In order to separate amino acids for analyses, we have to exploit very small differences in structure and ionisation. Success depends upon fastidious attention to detail, not only in the analyses, but also in the collection and preparation of samples.? The results can only be interpreted against a sound knowledge of metabolism. Many clinicians lean heavily upon the laboratories for interpretation, and it is essential that the data are presented in a way that is helpful. Quantitative analyses for clinical use became feasible 40 years ago when Moore and Stein reported the separation of standard and plasma amino acids with a sulfonated polystyrene resin column.v' Spackman et al? automated this procedure. Among clinical laboratories, cationexchange is still by far the commonest method