This study examined whether the high sensitivity of plasma free metanephrines for diagnosis of pheochromocytoma may result from production of free metanephrines within tumors. Presence in pheochromocytomas of catechol-O-methyltransferase (COMT), the enzyme responsible for conversion of catecholamines to metanephrines, was confirmed by Western blot analysis, enzyme assay, and immunohistochemistry. Western blot analysis and enzyme assay indicated that membrane-bound and not soluble COMT was the

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... edominant form of the enzyme in pheochromocytoma. Immunohistochemistry revealed colocalization of COMT in the same chromaffin cells where catecholamines are translocated into storage vesicles by the vesicular monoamine transporter. Levels of free metanephrines in pheochromocytoma over 10,000 times higher than plasma concentrations in the same patients before removal of tumors indicated production of