Monoamine and diamine oxidase activity in the diagnosis of carcinoid tumors.

This study determines if one could distinguish foregut from midgut carcinoid tumors by quantitative measurement of the monoamine oxidase (MAO) and diamine oxidase (DAO) activities in homogenates of tumors. The MAO activity of 16 foregut carcinoid tumors (1850 +/- 342 pmol/mg/minute) was significantly higher than the MAO activity of 11 midgut carcinoid tumors (407 +/- 43 pmol/mg/minute, P less than 0.01) with no overlap between the groups. Although all ten of the midgut carcinoids had measurable DAO activity (720 +/- 190 pmol/mg/minute), with the exception of one duodenal carcinoid tumor (33 pmol/mg/minute) the nine foregut carcinoid tumors evaluated did not have detectable DAO activity. The MAO activity of all of the foregut carcinoids was higher than that of 6 islet cell tumors, 28 paragangliomas, and 12 medullary carcinomas of the thyroid. Quantitative MAO and DAO activity may be useful in distinguishing foregut carcinoid tumors from other related tumors.