Intestinal monoamine oxidase: does it have a role in histamine catabolism?

The importance of intestinal diamine oxidase in histamine catabolism was proved in several series of experiments. However, intestinal monoamine oxidase might also be involved in histamine degradation either by direct deamination or by the deamination of methylated products. The soluble fraction of intestinal monoamine oxidase was purified and tested for the properties and substrate specificity by three different methods which are described in detail. Using 0.15 M phosphate buffer the optimum pH was 7.4--7.6. The Km values for serotonin and tyramine were 0.2 and 0.3 X 10(-3) M. The most favoured substrates of the enzyme were tyramine, tryptamine and serotonin, but it was not possible to classify the enzyme as a type A or B monoamine oxidase only by its substrate specificity. Histamine and ring methylated derivatives were not attacked by intestinal monoamine oxidase. This means that in the intestinal mucosa by the oxidative pathway of histamine is completely catalysed by diamine oxidase.