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

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.