FIGURE 4.

Kinetic characterization of O-methylation of representative PAH-catechols containing a bay region (B[a]P-7,8-catechol), a methylated bay region (5MC-1,2-catechol), and a fjord region (B[g]C-11,12-catechol). The initial velocity was estimated by the slope of a linear portion of the progress curve over 10 min. Kinetic analyses were performed by fitting the data to the Michaelis-Menten equation or the substrate inhibition equation. Reactions contained 10 mm KPO₄ buffer, pH 7.8, 1.0 mm dithiothreitol, 1.0 mm MgCl₂, 50 μm unlabeled AdoMet, 0–20 μm PAH o-quinone, and 0.225 or 0.45 μg of human recombinant COMT at 25 °C. A, velocity versus [S] curve for B[a]P-7,8-catechol; B, velocity versus [S] curve for 5MC-1,2-catechol; C, velocity versus [S] curve for B[g]C-11,12-catechol.