Kinetic analysis of the histamine N-methyltransferase reaction as used in the histamine radioenzymatic assay: optimization of assay specificity.

The specificity of the histamine N-methyltransferase (HNMT) based radioenzymatic assay for histamine has been questioned since N-alpha-methylhistamine is also a substrate for this enzyme. Purification of HNMT for use in the radioenzymatic assay improves sensitivity and specificity of this procedure. In this investigation, further improvements in specificity, with respect to other HNMT substrates, were attained by optimization of reaction conditions based on the evaluation of HNMT kinetic parameters. These studies demonstrate that appropriate control of reaction temperature and concentration of both the enzyme and the radiolabeled methyl donor improve the specificity of this assay for histamine.