Inhibition of the carboxylesterase activity of recombinant human CES1 by 27-HC. The data were best fit to an equation that describes partially noncompetitive enzyme inhibition (A) : i = (1−β) * [I]/([I] + Ki). For a partially noncompetitive inhibitor it is assumed that the E S I complex is partially catalytically active (i.e., 0<β<1), and the affinity of substrate for enzyme is unchanged by the inhibitor (i.e., α=1). The combined data from 3–4 independent experiments are shown [error bars represent the intra-assay variation (SD) for each independent experiment, which were performed in triplicate]. Analysis of data using a Dixon plot (shown in B) followed by a replot of the slopes of the Dixon plot versus 1/[S] (shown in C) confirmed a non-competitive mode of inhibition .