Thiamine transport in Escherichia coli: the mechanism of inhibition by the sulfhydryl-specific modifier N-ethylmaleimide

Active transport of thiamin (vitamin B(1)) into Escherichia coli occurs through a member of the superfamily of transporters known as ATP-binding cassette (ABC) transporters. Although it was demonstrated that the sulfhydryl-specific modifier N-ethylmaleimide (NEM) inhibited thiamin transport, the exact mechanism of this inhibition is unknown. Therefore, we have carried out a kinetic analysis of thiamin transport to determine the mechanism of inhibition by NEM. Thiamin transport in vivo exhibits Michaelis-Menten kinetics with K(M)=15 nM and V(max)=46 U mg(-1). Treatment of intact E. coli KG33 with saturating NEM exhibited apparent noncompetitive inhibition, decreasing V(max) by approximately 50% without effecting K(M) or the apparent first-order rate constant (k(obsd)). Apparent noncompetitive inhibition is consistent with an irreversible covalent modification of a cysteine(s) that is critical for the transport process. A primary amino acid analysis of the subunits of the thiamin permease combined with our kinetic analysis suggests that inhibition of thiamin transport by NEM is different from other ABC transporters and occurs at the level of protein-protein interactions between the membrane-bound carrier protein and the ATPase subunit.