Modification of the enzymatic isotopic assay of histamine and its application to measurement of histamine in tissues, serum and urine

Abstract

A modified enzymatic isotopic assay for histamine in tissue homogenates, serum and urine is described. The assay is based upon that of Snyder et al. 1 in which histamine is converted to [ 14C]methylhistamine by incubation with histamine- N-methyltransferase and S-adenosylmethionine-[ 14C]methyl but is modified to make the formation and recovery of [ 14C]-methylhistamine quantitative and to reduce the extraction of other 14C-containing material. The modifications enhance the sensitivity of the assay to 0.1 ng histamine and permit the simultaneous measurement of as many as 80 to 100 samples. The assay is particularly useful for determining the histamine content in tissues with low histamine levels where the fluorometric assay of Shore et al. 2 gives spuriously high results. The enzymatic assay also has the advantage that histamine can be measured directly in tissue homogenates, plasma, serum, and urine without the need for prior extraction of the histamine. Various amounts of histamine were detected in human tumors and normal tissues but none (less than 0.5 ng/ml) in human serum. Normal human urinary histamine excretion averaged 16 ± 14 (± SD) μg histamine/24 h.