On the Determination of Catechol‐O‐Methyltransferase Activity in Tissue Homogenates

A procedure for the spectrophotofluorimetric determination of catechol‐O‐methyltransferase (COMT) activity in crude tissue homogenates is described. The method is based on the known principle that the enzyme converts 3,4‐dihydroxyphenylacetic acid to homovanillic acid in the presence of S‐adenosyl methionine and magnesium ions. After incubation and simple organic solvent extractions, the end product is separated by paper chromatography before the development of the fluorophor. The characteristics of the enzyme kinetics using the present method were found to be in good agreement with those of previous studies using a purified COMT preparation. It was found that iodide could act as an inhibitor of COMT which has practical implications in that some commercial coenzyme preparations contain this ion. Some factors contributing to the discrepancies in the results obtained for COMT activity determinations by fluorimetric and radiochemical procedures were investigated and the sources of error discussed. The main advantage of the present fluorimetric method for COMT determinations is considered to be the applicability of the method to crude homogenates of tissues such as brain which have relatively low enzyme activity.