ISOTOPIC MICROASSAY OF HISTAMINE, HISTIDINE, HISTIDINE DECARBOXYLASE AND HISTAMINE METHYLTRANSFERASE IN BRAIN TISSUE

Abstract— Microassays are described for histamine, histidine, and the activities of the enzymes histidine decarboxylase (EC 4.1.1.22) and histamine niethyltransferase (EC 2.1.1.8) in brain tissue. The enzymic‐isotopic microassay for histamine is based on the methylation of tissue histamine by added histamine methyl‐transferase and [¹⁴C]‐ or [³H]‐labelled S‐adenosyl‐l‐methionine. In a double‐isotopic form of the assay, a tracer of [³H]histamine is employed along with [¹⁴C]S‐adenosyl‐l‐methionine, and the ratio [¹⁴C]:[³H] reflects the amount of histamine in the sample. Because the methylation of histamine is uniform in brain samples studied, a single isotopic assay with [³H]S‐adenosyl‐l‐methionine as the methyl donor is possible and increases sensitivity, so that 10 pg of tissue histamine can be estimated reliably. The assay for histidine involves decarboxylation of histidine by a bacterial histidine decarboxylase and measurement of the histamine formed by the enzymicisotopic procedure. In the histidine decarboxylase assay, histamine synthesized from added histidine is measured. The assay for histamine methyltransferase involves measuring the formation of [¹⁴C]methylhistamine with [¹⁴C]S‐adenosyl‐l‐methionine serving as the methyl donor.