Comparison of in vitro and in vivo histamine methylation by tissues

A method for determining the histamine-methylating enzyme (HME) using crude enzyme, and minute quantities of the substrate, was applied to tissues of mice, guinea-pigs and rats. Since high levels of endogenous histamine can affect the results, tissue homogenates were dialyzed prior to incubation. Findings were compared with in vivo data on methylating ability of individual tissues; most of this in vivo data is published but a new test of guinea-pig tissues was made using amodiaquine as an inhibitor. The correlation was good, better than that obtained by other procedures. It was observed that dialysis caused an increase in HME for some guinea-pig tissues, but a loss for some mouse tissues. Possible explanations are considered. Quinacrine N-mustard, a derivative of a known HME inhibitor, was tested in mice; it altered the distribution of injected 14C-histamine but showed no evidence of HME inhibition.