INFLUENCE OF STORAGE-CONDITIONS ON NORMAL PLASMA AMINO-ACID-CONCENTRATIONS

Conflicting information in the literature is given concerning the opti mal preparation and storage conditions of plasma samples for amino-aci d analysis. To assess the optimal pre-storage treatment, we compared s everal 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, n ot deproteinised plasma at this temperature, resulted in a 32% decreas e of arginine and a 30% increase in ornithine after 24 weeks. Deprotei nisation prohibited this arginine decay. At -20 degrees C, arginine de cay was even more pronounced, whereas glutamine decreased by 14% in un treated 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 resu lts in humans, we repeated this experiment with plasma of 6 volunteers . In contrast to rat plasma, we did not observe any changes in arginin e and ornithine concentrations in human plasma stored at -70 degrees C . At -20 degrees C the reduction in glutamine was only 4-5%. These res ults suggest that interspecies differences in enzymatic activity exist in plasma. Finally, having assessed the optimal treatment and storage conditions (deproteinisation followed by storage at -70 degrees C), s amples were obtained from a total of 112 human volunteers, stratified for age and sex, and amino-acids were measured. In the female group, w e found a tendency to a gradual increase in most amino-acid concentrat ions with advancing age, which however only reached significance for h istidine, citrulline, alanine and leucine. These observations demonstr ate that plasma samples for amino-acid analysis should be deproteinise d and stored at -70 degrees C. Also important interspecies differences appear to exist in plasma enzymatic activity. Finally, control sample s should be taken from an age and sex matched control group.