Inhibition of InhA, the enoyl reductase from Mycobacterium tuberculosis, by triclosan and isoniazid

Structural and genetic studies indicate that the antibacterial compound tri closan, an additive in many personal care products, is an inhibitor of EnvM , the enoyl reductase from Escherichia coli. Here we show that triclosan sp ecifically inhibits InhA, the enoyl reductase from Mycobacterium tuberculos is and a target for the antitubercular drug isoniazid. Binding of triclosan to wild-type InhA is uncompetitive with respect to both NADH and trans-2-d odecenoyl-CoA, with K-i' values of 0.22 +/- 0.02 and 0.21 +/- 0.01 mu M, re spectively. Replacement of Y158, the catalytic tyrosine residue, with Phe, reduces the affinity of triclosan for the enzyme and results in noncompetit ive inhibition, with Ki and Ki' values of 36 +/- 5 and 47 +/- 5 mu M, respe ctively. Consequently, the Y158 hydroxyl group is important for triclosan b inding, suggesting that triclosan binds in similar ways to both InhA and En vM. In addition, the M161V and A124V InhA mutants, which result in resistan ce of Mycobacterium smegmatis to triclosan, show significantly reduced affi nity for triclosan. Inhibition of M161V is noncompetitive with K-i' = 4.3 /- 0.5 mu M and K-i = 4.4 +/- 0.9 mu M, while inhibition of A124V is uncomp etitive with K-i' = 0.81 +/- 0.11 mu M. These data support the hypothesis t hat the mycobacterial enoyl reductases are targets for triclosan. The M161V and A124V enzymes are also much less sensitive to isoniazid compared to th e wild-type enzyme, indicating that triclosan can stimulate the emergence o f isoniazid-resistant enoyl reductases. In contrast, 147T and 121V, two Inh A mutations that occur in isoniazid-resistant clinical isolates of M. tuber culosis, show unimpaired inhibition by triclosan, with uncompetitive inhibi tion constants (K-i') of 0.18 +/- 0.01 and 0.12 +/- 0.01 mu M, respectively . The latter result indicates that InhA inhibitors targeted at the enoyl su bstrate binding site may be effective against existing isoniazid-resistant strains of M. tuberculosis.