A collaborative study on the determination of free amino acids in blood plasma

Evaluation of systematic errors due to deproteinization, calibration and storage of plasma for amino acid assay by ion-exchange chromatography

Three factors contributing to inter-laboratory variation in the determination of amino acids in plasma, i.e. deproteinization, calibration and storage conditions, were evaluated in this study. Deproteinization clearly enlarged the coefficient of variation in the determination of cystine, aspartic acid and tryptophan. During this process losses of hydrophobic amino acids occurred, in particular, when the volume of the supernatant was small. Correction for this effect, using an internal standard, was not possible. Delaying the removal of the supernatant for 1 h decreased the concentration of tryptophan. Correction for this effect, using an internal standard, was not possible. The use of different commercial standards also led to systematic errors during the calibration of samples. The amino acid concentrations in deproteinized plasma remained for a least 1 year when stored at a temperature of -40 degrees C or lower. Above this temperature, glutamine and asparagine were found to be degraded. This degradation could be minimized by neutralizing the samples before storage. The concentration of cystine decreased considerably during storage of non-deproteinized plasma. Correction for these changes due to storage is not advised and, in most cases, is impossible.

A scheme for the interpretation of primary and secondary disturbances of plasma and urinary amino acid profiles. A possible way to an expert system

A general scheme for the interpretation of primary and secondary abnormalities of plasma and urine amino acid concentrations is described. The key steps of this scheme are: analytical assessment of the measurements, comparison of results obtained with the reference values expressed in absolute and/or relative concentrations and identification of abnormally increased ninhydrin-positive compounds. The interpretation of results takes account of the various abnormalities induced by drugs or diet. The origins of these abnormalities are ordered by their frequency. A part of the proposed scheme is now computerized as the first step in the development of an expert system.

Evaluation of liquid chromatographic analysis of nutritionally important amino acids in food and physiological samples

Significant advances have been made in standardizing methods for amino acid analysis of foods. The methods included standardized hydrolysis of the food proteins followed by separation and quantitation of the released amino acids by ion-exchange chromatography (IEC). IEC is still the main method in use. Its use is, however, being replaced by the faster higher-performance liquid chromatographic (HPLC) methods of derivatized amino acids. The HPLC separation of precolumn phenylisothiocyanate (PITC) derivatives has been adapted for rapid analysis of all amino acids in protein hydrolysates (12 min) and nutritionally important amino acids in deproteinized physiological samples (20 min). The inter-laboratory variability of the PITC derivatization method has not been determined although the intra-laboratory variation of the HPLC method was found to be similar to that of IEC. When similar hydrolytic conditions were used in preparing protein hydrolysates, amino acid results obtained with the PITC derivatization method were generally in close agreement with those obtained by IEC. There is, however, room for improvement in the HPLC analysis of amino acids in physiological samples.

Measurement of free amino acid levels in ultrafiltrates of blood plasma by high-performance liquid chromatography with automatic pre-column derivatization

Although there are many techniques available for the analysis of amino acids, deproteinization is still one of the major problems in the analysis of amino acids in physiological fluids. The method used to prepare the plasma and to remove the plasma protein has a marked effect on the final results. The most widely used method of deproteinization is precipitation with 5-sulphosalicyclic acid followed by centrifugation to remove the precipitated protein. We have not had success in using this deproteinization agent for the analysis of plasma amino acids by a high-performance liquid chromatographic method with automatic pre-column o-phthaldialdehyde-3-mercaptopropionic acid and 9-fluorenylmethyl chloroformate derivatization because of the adverse effect of the sulphosalicyclic acid supernatant on the quantitation and separation. Ultrafiltration was used as an alternative method for the preparation of plasma samples in this experiment. The results were satisfactory for the analysis of plasma amino acids in 1500 samples during a period of four years. Some factors that might influence the results of the ultrafiltration were investigated.

General problems associated with the analysis of amino acids by automated ion-exchange chromatography

Some of the general problems commonly encountered with the analysis of amino acids have been described. These include problems associated with the preparation and storage of samples prior to analysis and those associated with the analytical stage itself. The effects on the accuracy of the determination of amino acids have been discussed, together with possible solutions to some of the problems.

Gas chromatographic analysis of amino acids in physiological fluids: a critique

The available methodology for sample purification prior to gas chromatographic analysis of amino acids in physiological fluids and tissue extracts is analysed. It would appear that over the past ten years the method of choice is that of an ion-exchange purification step, and little, if any, progress has been achieved in sample purification procedures. The inherent disadvantages of such a methodology are not only perpetuated but also cast some considerable doubt on the accuracy of the quantitative analysis of amino acids.

Availability of lysine in vegetable protein concentrates as determined by the slope-ratio assay with growing pigs and rats and by chemical techniques

The availability of lysine in nine vegetable-protein concentrates was assessed using the slope-ratio assay for growing pigs and rats. Diets were equalized for crude fibre using solka floc to minimize any possible effects of variation in fibre content on availability estimates. The availability of lysine in the nine proteins for pigs, using food conversion efficiency (FCE) on a carcass basis as the criterion of response were (proportion of total): cottonseed meal 0.39, lupin (Lupinus angustifolius) seed meal no. 1 0.37, no. 2 0.65, no. 3 0.54, no. 4 0.54, field peas (Pisum sativum) 0.93, peanut (groundnut) meal 0.57, soya-bean meal no. 1 0.98, no. 2 0.89. Estimates of available lysine for rats as assessed by the slope-ratio assay using FCE on a carcass basis were in close agreement with the pig estimates for cottonseed meal (0.35) and soya-bean meal no. 1 (0.91) and no. 2 (0.89), higher for lupin-seed meals (range 0.70-0.94 with a mean of 0.81) and peanut meal (0.76) and lower for field peas (0.76). The differences in available lysine were not detected by the chemical Silcock available-lysine test (Roach et al. 1967) or by the direct 1-fluoro-2,4-dinitrobenzene procedure (Carpenter, 1960).