Analysis of plasma histamine: a modification of the enzymatic isotopic assay.

The enzymatic isotopic analysis of histamine requires extraction and concentration of [3H]-1-methylhistamine from the reaction mixture. The assay was modified to include enzymatic reaction at 0 degrees C to reduce blank values and direct application of protein-free reaction mixture to a thin-layer chromatography plate for isolating of the product. The analysis was quantitative down to 100 pg/ml of histamine, adequate for monitoring plasma histamine content.