The activity of partially purified rat liver catechol-O-methyltransferase (COMT) was measured by an assay procedure in which 3,4-dihydroxybenzoic acid was used as a substrate for the enzyme. Optimal enzyme activity was present at a concentration of MgCl2 of 10-3 M. The effects on COMT activity of a series of alkaline earth compounds were determined in the presence of optimal concentrations of MgCl2. CaCl2 and Ca(NO3)2 at concentrations of 10-3 M reduced COMT activity by 63 and 59 per cent respectively. BaCl2 and Sr(NO3)2 (10-3 M) did not decrease enzyme activity, nor did additional MgCl2 to a final [Mg2+] of 2 × 10-3 M) did not decrease enzyme activity and 0·50 × 10-3 M CaCl2 in the presence of different concentrations of 3,4-dihydroxybenzoic acid, S-adenosyl-1-methionine and MgCl2 was determined. In each case Lineweaver-Burk plots of these data were compatible with noncompetitive or "mixed" inhibition.
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