, Volume 31, Issue 1, pp 7-20

Interpretation of plasma amino acids in the follow-up of patients: The impact of compartmentation

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Summary

Results of plasma or urinary amino acids are used for suspicion, confirmation or exclusion of diagnosis, monitoring of treatment, prevention and prognosis in inborn errors of amino acid metabolism. The concentrations in plasma or whole blood do not necessarily reflect the relevant metabolite concentrations in organs such as the brain or in cell compartments; this is especially the case in disorders that are not solely expressed in liver and/or in those which also affect nonessential amino acids. Basic biochemical knowledge has added much to the understanding of zonation and compartmentation of expressed proteins and metabolites in organs, cells and cell organelles. In this paper, selected old and new biochemical findings in PKU, urea cycle disorders and nonketotic hyperglycinaemia are reviewed; the aim is to show that integrating the knowledge gained in the last decades on enzymes and transporters related to amino acid metabolism allows a more extensive interpretation of biochemical results obtained for diagnosis and follow-up of patients and may help to pose new questions and to avoid pitfalls. The analysis and interpretation of amino acid measurements in physiological fluids should not be restricted to a few amino acids but should encompass the whole quantitative profile and include other pathophysiological markers. This is important if the patient appears not to respond as expected to treatment and is needed when investigating new therapies. We suggest that amino acid imbalance in the relevant compartments caused by over-zealous or protocol-driven treatment that is not adjusted to the individual patient’s needs may prolong catabolism and must be corrected.

Communicating editor: Georg Hoffmann
Competing interests: None declared
This paper is based in the European Metabolic Group (EMG) Lecture given at the 39th EMG Meeting in Warsaw on 3 June 2007 - organized and sponsored by Milupa Metabolics.
References to electronic databases: Phenylketonuria: OMIM 262600. Urea cycle enzymes, mitochondrial: EC 2.3.1.1, 2.1.3.3, 6.3.4.16. Urea cycle enzymes, cytosolic: EC 3.5.3.1, 4.3.2.1, 6.3.4.5. HHH syndrome: OMIM 238970. Citrin deficiency: OMIM 603859. Lysinuric protein intolerance: OMIM 222700. Branched-chain aminotransferase: EC 2.6.1.42. Non-ketotic hyperglycinaemia: OMIM 605899. Hereditary hyperekplexia: OMIM 149400.