Inhibition of human plasma cholinesterase by malachite green and related triarylmethane dyes: Mechanistic implications

The inhibitory effects of the cationic triarylmethane (TAM+) dyes, pararosaniline (PR+), malachite green (MG+), and methyl green (MeG+) on human plasma cholinesterase (BChE) were studied at 25 °C in 100 mM Mops, pH 8.0, with butyrylthiocholine as substrate. PR+ and MG+ caused linear mixed inhibition of enzyme activity. The respective inhibitory parameters were Ki = 1.9 ± 0.23 μM, α = 13 ± 48, β = 0 and Ki = 0.28 ± 0.037 μM, α = 23 ± 7.4, β = 0. MeG+ acted as a competitive inhibitor with Ki = 0.12 ± 0.017 μM (α, ∞, β, not applicable). The Ki values were within the same range reported for a number of ChE inhibitors including propidium ion, donepezil, and the phenothiazines, suggesting that TAM+s are active site ligands. On the other hand, the α values failed to correlate with values previously reported for a number of ChE inhibitors. It appears that mixed inhibition is the combined result of more than one type of binding and S–I interference. The impact of ligands at the choline-specific and peripheral anionic sites (or, possibly, accessory structural domains) on BChE activity needs to be studied in further detail.