Documenta Geigy

29 Jun

Free amino acids in urine

About one-third of each of the urinary amino acids is present in the free form, about one-third is in the non-peptide-bound form (e.g. hippuric, phenylacetylglutamine) and about one-third in the peptide bound form (peptides and proteins). The amino-acid content can be determined by measuring the α-amino nitrogen, though this method also embraces the free amino groups of the peptides and other substances; for this purpose, gasometric 9with ninhydrin or nitric acid), titrimetric (formol titration) and colorimetric (wuth ninhydrin or sodium β-napthoquinobe-4-sulphonate or as copper complex procedures have been devised, but these do not yield concordant results. The individual anino acids can be determined microbiologically or by electrophoretic, paper-chromatographic or column-chromatographic separation. The automatic column-chromatographic separation method of STEIN and MOORE reveals up to 80 substances reacting with ninhydrin, of which 29 are well-known amino acids. There are individual differences in the amino-acid content of the urine, but this is little affected by the diet of the urinary volume. On a protein-rich diet, the amount of methylhistidine increases, in hunger and malnutrition that of β-aminoisobutyric acid. When measured as the nitrogen content of the urine, the amino-acid excretion of the newborn and infants (particularly that of premature infants0 is greater than that of adults. In women the amino-acid content of the urine varies during the menstrual cycle, the histidine excretion showing a maximum oestrogen excretion. During pregnancy the amino-acid excretion, particularly that of histidine, is increased. It is pathologically increased when the amino-acid level of the blood is increased provided renal function is normal (for instance in liver diseases, increased protein catabolism, phenolketonuria) as well as in various renal disturbance ( poisoning, Rickets, various hereditary metabolic anomalies). There is a considerable literature on the amino-acid content of urine.
Genetic anomalies of the urinary amino acids are discussed on pages 448-450.

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