Quantification of urinary histamine by a new radioenzymatic assay: documentation of assay specificity and establishment of normal excretion rates.

The goals of this study were to evaluate the specificity of a new radioenzymatic assay for histamine when it is used for urinary-histamine determinations and to establish normal rates of urinary-histamine excretion for male and female subjects. Specificity of the assay was characterized by reacting urine samples at two different incubation temperatures and with varying amounts of highly purified histamine N-methyltransferase and S-adenosyl-L-[methyl-3H]methionine. The radiolabeled products were then separated by thin-layer chromatography and visualized by fluorescence-enhanced autoradiography. Our results indicate that at least one other substrate for histamine N-methyltransferase in addition to histamine is present in human urine, although the identity of this compound could not be determined. Optimization of enzyme-reaction conditions eliminated the methylation of this unidentified substrate while the methylation of histamine was maintained. The average urinary unidentified substrate while the methylation of histamine was maintained. The average urinary excretion of histamine determined for male subjects (N = 37) and female subjects (N = 111) was 18.7 micrograms/24 hr and 27.3 micrograms/24 hr, respectively. More than 12% of the normal female subjects examined had urinary-histamine excretion rates in excess of 50 micrograms/24 hr, which overlaps into the range previously considered to be indicative of abnormal mast cell secretion. In summary, this is the first radioenzymatic assay for histamine with documented specificity for the measurement of histamine in human urine.