The genome sequence of Mycobacterium tuberculosis has revealed the presence of 20 different
cytochrome P450 mono-
oxygenases (P450s) within this organism, and subsequent genome sequences of other mycobacteria and of Streptomyces coelicolor have indicated that these actinomycetes also have large complements of P450s, pointing to important physiological roles for these
enzymes. The actinomycete P450s include homologues of 14alpha-sterol demethylases, the targets for the
azole class of drugs in yeast and fungi. Previously, this type of P450 was considered to be absent from bacteria. When present at low concentrations in growth medium,
azole antifungal drugs were shown to be potent inhibitors of the growth of Mycobacterium smegmatis and of Streptomyces strains, indicating that one or more of the P450s in these bacteria were viable
drug targets. The drugs
econazole and
clotrimazole were most effective against M. smegmatis (MIC values of <0.2 and 0.3 micro M, respectively) and were superior inhibitors of mycobacterial growth compared to
rifampicin and
isoniazid (which had MIC values of 1.2 and 36.5 micro M, respectively). In contrast to their effects on the actinomycetes, the
azoles showed minimal effects on the growth of Escherichia coli, which is devoid of P450s.
Azole drugs coordinated tightly to the
haem iron in M.
tuberculosis H37Rv P450s encoded by genes Rv0764c (the
sterol demethylase CYP51) and Rv2276 (
CYP121). However, the
azoles had a higher affinity for M.
tuberculosis CYP121, with K(d) values broadly in line with the MIC values for M. smegmatis. This suggested that
CYP121 may be a more realistic target
enzyme for the
azole drugs than CYP51, particularly in light of the fact that an S. coelicolor DeltaCYP51 strain was viable and showed little difference in its sensitivity to
azole drugs compared to the wild-type. If the
azole drugs prove to inhibit a number of important P450s in M. smegmatis and S. coelicolor, then the likelihood of drug resistance developing in these species should be minimal. This suggests that
azole drug therapy may provide a novel
antibiotic strategy against strains of M.
tuberculosis that have already developed resistance to
isoniazid and other front-line drugs.