Multi-site inhibition of human plasma cholinesterase by cationic phenoxazine and phenothiazine dyes

Two cationic phenoxazine dyes, meldola blue (MB) and nile blue (NB), and the structurally related phenothiazine, methylene blue (MethB), were found to act as complex inhibitors of human plasma cholinesterase (butyrylcholinesterase, BChE). Studied at 25 °C, in 100 mM MOPS buffer (pH 8.0), with butyrylthiocholine as substrate, the kinetic pattern of inhibition indicated cooperative I binding at 2 sites. Intrinsic K′ values (≡[I]0.52 extrapolated to [S] = 0) for MB, NB and MethB were 0.64 ± 0.05, 0.085 ± 0.026 and 0.42 ± 0.04 μM, respectively. Under the same experimental conditions the dyes acted as single-occupancy, hyperbolic-mixed inhibitors of electric eel acetylcholinesterase (AChE), with Ki = 0.035 ± 0.010, 0.026 ± 0.0034 and 0.017 ± 0.0063 μM (for MB, NB, MethB); α (coefficient of competitive interaction) = 1.8-2.4 and β (coefficient of noncompetitive interaction) = 0.15-0.28. The complexity of the BChE inhibitory effect of phenoxazine/phenothiazine dyes contrasted with that of conventional ChE inhibitors which cause single-occupancy (n = 1), competitive or mixed inhibition in both AChE and BChE and signaled novel modes of ligand interaction at (or remote from) the active site gorge of the latter enzyme.