Monoamine oxidase and catechol-o-methyltransferase activity in hamster and rat insulinomas

Hamster and rat insulinomas were assayed for norepinephrine, dopamine and serotonin concentration and for monoamine oxidase and catechol-o-ethyltransferase (COMT) activity. The concentration of norepinephrine (mean 0.55 mumol/kg, range less than 0.20 to 2.64 mumol/kg) and serotonin (mean 5.22 mucol/kg, rang less than 0.6 to 26.5 mumol/kg) in hamster insulinomas were comparable to previously reported concentrations. Dopamine conentration (mean 0.34 mumol/kg, range less than 0.20 to 0.95 mumol/kg) was only 2 to 2.5% of that reported previously. Monoamine oxidase activity of the hamster and rat insulinomas were comparable to those of normal hamster islets. In contrast, the COMT activity of both insulinomas was much greater than the COMT activity of normal pancreatic islets of both species and was greater than in several other tissues and tumours. The tumour COMT, which was predominantly in the cytosol, was Mg2+ dependent and had a comparable sensitivity to inhibition by tropolone as purified beef-liver COMT. Hamster insulinoma monoamine oxidase was more sensitive than rat insulinoma monoamine oxidase to inhibition by tranylcypromine and deprenyl, while rat insulinoma monoamine oxidase was more sensitive to inhibition by clorgyline and was more heat labile.