7. HISTAMINE METABOLISM The response of histamine degrading enzymes to nematode infection

The response of intestinal mucosal enzymes which metabolize histamine i.e. diamine oxidase (DAO), histamine N-methyltransferase (HMT), and monoamine oxidase (MAO), to infection with Nippostrongylus brasiliensis has been examined in mice and compared to the changes evoked by in vivo administration of compound 48/80. Infection with the parasite resulted in a significant decrease in the concentration of both amine oxidases, followed by recovery of MAO and an overshoot in DAO activity. HMT activity was enhanced at the beginning of infection, then decreased markedly by days 11 to 15, and sharply increased thereafter. Histamine levels were on average only 20% higher than the basal levels over the entire period studied, except on day 4 when they were slightly reduced. Histamine is alleged to be a potential inducing factor for degrading enzymes. Consistently, the histamine releaser 48/80 significantly elevated intestinal mucosal DAO and in some of the mice also increased HMT activity.