A reverse double-isotope enzymatic histamine assay: advantages over single-isotope methods.

High concentrations of codeine necessary to demonstrate histamine release from leukocytes interfere with histamine methylation, producing a concentration-dependent flattening of the histamine standard curves obtained from single-isotope (³H) assays. For this reason, the use of uniformly labeled ¹⁴C-histamine was investigated as an internal standard. This isotope serves as an internal standard and accurately predicts the recovery of histamine in the presence of up to 70% enzyme inhibition. The use of this reverse double-isotope procedure allows the assay of histamine without direct external recovery measurements in the presence of various drug concentrations.