title: Influence of storage conditions on normal plasma amino-acid concentrations.
abstract: Conflicting information in the literature is given concerning the optimal preparation and storage conditions of plasma samples for amino-acid analysis. To assess the optimal pre-storage treatment, we compared several methods and studied their influence on plasma amino-acid levels of rats and humans, stored at different temperatures. In rat plasma, the frequently reported degradation of glutamine was not measurable at a storage temperature of -70 degrees C. However, storage of native, not deproteinised plasma at this temperature, resulted in a 32% decrease of arginine and a 30% increase in ornithine after 24 weeks. Deproteinisation prohibited this arginine decay. At -20 degrees C, arginine decay was even more pronounced, whereas glutamine decreased by 14% in untreated plasma, by 10% in sulfosalicylic acid deproteinised plasma and by 3% if the deproteinisation was followed by removal of the protein pellet and subsequent neutralisation. To confirm these unexpected results in humans, we repeated this experiment with plasma of 6 volunteers. In contrast to rat plasma, we did not observe any changes in arginine and ornithine concentrations in human plasma stored at -70 degrees C. At -20 degrees C the reduction in glutamine was only 4-5%. These results suggest that interspecies differences in enxymatic activity exist in plasma. Finally, having assessed the optimal treatment and storage conditions (deproteinisation followed by storage at -70 degrees C), samples were obtained from a total of 112 human volunteers, stratified for age and sex, and amino-acids were measured. In the female group, we found a tendency to a gradual increase in most amino-acid concentrations with advancing age, which however only reached significance for histidine, citrulline, alanine and leucine. These observations demonstrate that plasma samples for amino-acid analysis should be deproteinised and stored at -70 degrees C. Also important interspecies differences appear to exist in plasma enzymatic activity. Finally, control samples should be taken from an age and sex matched control group.
authors.affiliation: Department of Surgery, Biomedical Center, University of Limburg, P.O. Box 616, NL-6200 MD Maastricht, The Netherlands.
authors.forename: H M
authors.initials: HM
authors.lastname: Van Eijk
authors.forename: C H
authors.initials: CH
authors.lastname: Dejong
authors.forename: N E
authors.initials: NE
authors.lastname: Deutz
authors.forename: P B
authors.initials: PB
authors.lastname: Soeters