Whole-blood and plasma amino acid analysis: gas-liquid and cation-exchange chromatography compared.

We analyzed for free amino acids in normal human venous plasma and whole blood by gas-liquid chromatography (GLC) and by the standard cation-exchange technique.The semi-automatically prepared n-propyl N-acetyl derivatives were used for GLC. Using this method, we found higher concentrations in whole blood than in plasma for glycine, leucine, threonine, serine, aspartic #{224}cid/asparagine, methionine, and glutamic aqid/glutamin#{233}. The sum of the 17 amino acids measured in whole blood exceeded that inplasma by 592 mol/L, corresponding to an intracellular free amino acid concentration approximately one-third of that in plasma. The bulk of this difference (464 tmol/L) was accounted for by glycine, aspartic acid/asparagine, and glutamic acid/glutamine. When a group of samples was analyzed both by GLC and by ion-exchange we saw four consistent discrepancies, clearly exceeding analytical variation. For both whole blood and plasma, values for leucine and serine were higher by GLC, whereas values for glycine and tyrosine were lower by GLC. The observed differences ranged from 17.5 to 44.5%, and all were highly significant. Neither the reasons for these discrepancies nor the true values for the amino acids in question are known. The good agreement between GLC and ion-exchange chromatography for most of the measured amino acids suggests that the choice of methodology can be based primarily on considerations of start-up costs and speed of analysis.