INCREASED SENSITIVITY OF THE ENZYMATIC ISOTOPIC ASSAY OF HISTAMINE - MEASUREMENT OF HISTAMINE IN PLASMA AND SERUM

The use of rat kidney instead of guinea pig brain as the source of histamine-N-methyltransferase for the enzymatic assay of histamine was found to improve the sensitivity of the assay. A partially purified preparation (ammonium sulfate fractionation) of the kidney enzyme was 20- to 50-fold more active than the guinea pig preparation, and sufficient enzyme for 14,000 assays could be prepared from six rats. The kidney enzyme, unlike the guinea pig brain enzyme, was free of interfering enzyme activities and gave low values for assay blanks. The two enzymes otherwise had similar properties. The low blank values permitted direct measurement of histamine in normal plasma without the need to isolate and concentrate histamine from the sample. Plasma histamine levels in normal individuals ranged from 0.2-1.4 (mean 0.6, n = 19) ng/ml. © 1979.