Isoniazid is a pro-drug that requires activation in isoniazid-susceptible mycobacterial species. Based on
in vitro experiments, it has been proposed [
13] that the activation of isoniazid results in a number of highly reactive species that are capable of either oxidizing or acylating groups in proteins. However, the actual form of isoniazid that is active
in vivo is still unknown. It was observed soon after isoniazid was introduced in the 1950s that isoniazid-resistant clinical isolates frequently lost catalase and peroxidase activity [
14]. However, the association of this enzyme with isoniazid activation was not proven until the early 1990s, when the primary mycobacterial catalase-peroxidase gene (
katG) was cloned and sequenced [
15]. That study and others [
8,
16] revealed that mutations in this gene are found in 42–58% of isoniazid-resistant clinical isolates. A large number of different mutations have been described thus far; however, the Ser315Thr mutation is found most often, occurring in approximately 40% of all isoniazid-resistant strains [
8,
16,
17]. The Ser315Thr mutation results in an enzyme without the ability to activate isoniazid, but retains approximately 50% of its catalase-peroxidase activity [
18]. Thus, the altered catalase-peroxidase provides high-level resistance to isoniazid, while retaining a level of oxidative protection that is sufficient to enable the organism to maintain detoxifying activity against host antibacterial radicals. Isolates that carry other, less frequently occurring mutations in
katG have been described as exhibiting varying levels of isoniazid resistance and catalase-peroxidase activity [
8,[
16‐
18]].
Significant evidence supports the concept that isoniazid blocks the synthesis of cell-wall mycolic acids, the major components of the envelope of
M tuberculosis (Fig.
1). Two intracellular targets for the drug are currently being actively investigated [
19,
20]: the fatty-acid enoyl-acyl carrier protein reductase (InhA), and a complex of an acyl carrier protein (AcpM) and a β-ketoacyl-ACP synthase (KasA). These enzymes are involved in synthesis of mycolic acids, and mutations have been found in the promoter regions, or less commonly in the genes that encode these proteins (
inhA,
acpM, and
kasA), in clinical isolates that exhibit low-level resistance to isoniazid (for review [
8]). It is proposed that over-expression of one or more of these target proteins may be the reason for isoniazid resistance in these strains. However, the role of
kasA mutations in isoniazid resistance is presently unclear, because similar mutations were also found in isoniazid-susceptible isolates, and, in cases of isoniazid resistance, mutations were also found in
katG or
inhA [
21,
22].
Mutations in the promoter region of a gene that encodes an alkyl hydroperoxidase reductase (
ahpC) have been found in approximately 10% of isoniazid-resistant isolates, but mutations in
katG were also found in these isolates [
8,
16,
23]. The resulting over-expression of alkyl hydroperoxidase reductase may compensate for the loss of catalase-peroxidase activity in these mycobacteria [
24].