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

High concentrations of codeine necessary to demonstrate histamine release frog 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 l4C-histamine was investigated as as internal standard . This isotope serves as as internal standard and accurately predicts the recovery of histamine is the presence of up to 70Z enzyme inhibition . The use of this reverse double-isotope procedure allows the assay of histamine without direct external recovery measurements is the presence of various drug concentrations .