Histidine Metabolism in the Human Adult: Histidine blood tolerance, and the effect of continued free L-histidine ingestion on the concentration of imidazole compounds in blood and urine

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Abstract
Relationships among the intermediary metabolites of histidine were studied in the human adult. Histidine and imidazolelactic acid were the only imidazole compounds identified in the fasting plasma of 3 healthy human adults fed a constant diet. Plasma histidine tolerance studies in which 5 g of L-histidine (free base) were fed, showed the peak increase in histidine to occur one hour after ingestion; imidazolelactic acid changes paralleled that of histidine. Eleven imidazole compounds were quantitatively identified in 24-hour urine samples collected for periods of 3 consecutive days before histidine ingestion, during ingestion of a 10-g daily load of free L-histidine, and after histidine ingestion. Histidine loading caused an increase in the excretion of histidine, urocanic acid, imidazolepropionic acid, imidazoleacetic acid and imidazolelactic acid; none of these returned to the pre-histidine ingestion level by the end of the 3-day post-histidine period. The amounts of 4-amino-5-imidazole carboxamide, carnosine, homocarnosine, anserine, 1-methylhistidine, and 3-methylhistidine either were not affected by histidine loading, or the results were erratic and varied among the subjects.