Histaminase activity in tumor and tissues from seven patients with medullary carcinoma of the thyroid was compared with that from control patients with other diseases. In control patients, tissue histaminase activity was high only in kidney and ileum. In one patient who died of widely disseminated medullary carcinoma, high histaminase activity was found in both solid tumor and tissues which did not have gross tumor. Histologic studies showed that the tissues with high enzyme activity contained microscopic foci of the tumor. In a second patient with widely disseminated medullary thyroid carcinoma, who had received the histaminase inhibitor aminoguanidine five hours before death, low enzyme activity was found in serum, tumor, kidney, ileum and other tissues. This patient had high serum histaminase activity before administration of aminoguanidine. It appeared that the drug had inhibited the enzyme activity in both serum and tissues. Tumors removed from four patients with localized medullary carcinoma of the thyroid had high enzyme activity. Pheochromocytomas from six patients had low histaminase activity and could thus be differentiated from medullary thyroid carcinoma. A pheochromocytoma from one patient, who died of disseminated medullary carcinoma, had high histaminase activity and microscopic foci of medullary carcinoma. Another patient with a tumor of the mediastinum and a pheochromocytoma had high histaminase activity in serum and the mediastinal tumor. This finding raised the possibility that the mediastinal tumor was an unusual primary tumor of the mediastinal parafollicular cells and may be related to medullary thyroid carcinoma. No other tumor examined had high histaminase activity. It was concluded that histaminase activity in surgical and autopsy specimens can serve as a specific biochemical marker for the presence of medullary thyroid carcinoma.
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