The second-order rate constants (at pH 7, 25 degrees C for the reduction of three cationic triarylmethane dyes [pararosaniline (PR+), malachite green (MG(+)), methyl green (MeG(+))] by NADH were 1.4 x 10(-2) to 6.7 x 10(-2) mM(-1) min(-1). Based on these values the intracellular nonenzymatic reduction of TAM(+) to TAM-H by endogenous NADH was estimated to proceed with an average half-life of 30 min. Rapid and significant adduct formation was observed with the thiol, 3-mercaptopropionic acid (MPA), suggesting that the primary intracellular form of the dyes must be a thiol adduct and that the conversion to adduct form takes place within ms-s. These time frames, when compared to the min-h time frame for microbial clearance of triarylmethanes from culture media, suggest that transport must be the rate-limiting step in nonadsorptive (chemical) clearance of the dyes and that the presence of enzymes to complement the nonenzymatic reductive and adduct-forming activities cited serves a kinetically limited purpose. It appears that a superior catalytic scavenger will be one with a superior transport capacity. (c) 2007 Elsevier B.V. All rights reserved.