V. THAVER ET AL.
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( % of control)
Figure 2. The effect of 7-methyljuglone (7-MJ) on the levels
of ATP in the H37Rv strain of MTB. The data shown are
that of one experiment with five replicates for each concen-
tration and are representative of 3 different experiments
showing similar trends. The results are expressed as the
mean percentages of the corresponding compound-free
control systems SD; the absolute value of the control sys-
tem is 2,306,410 relative light units. *P values < 0.05 when
compared to the solvent control.
clinical development, 7-MJ is a noteworthy exception .
The MIC value of this agent for MTB is 0.5 mg/L, which
is significantly lower than its IC50 value (30.0 mg/L) for
eukaryotic cell lines in vitro . Nonetheless, relatively
little is known about either the primary targets or rapidity
of onset of anti-mycobacterial activity of this agent.
In the current study, a relatively brief exposure (30 -
60 min) of MTB to 7-MJ resulted in significant, dose
-related inhibition of both microbial energy metabolism
and uptake of K+, which, in both cases was maximal at
concentrations close to the MIC value. In the case of
ATP levels, exposure of MTB to 7-MJ at a concentration
of 0.0263 mg/L resulted in a significant increase in ATP,
with a progressive, dose-related decrease at higher con-
centrations. The increase may represent a stress response
to moderate oxidative trauma caused by low concentra-
tions of 7-MJ as described for other types of antimicro-
bial agents [6,7], while at higher concentrations, ire-
versible ROS-mediated toxicity predominates. Alterna-
tively, albeit speculatively, 7-MJ may interfere with the
activity of mycobacterial type 2 NADH: quinone oxi-
doreductase, an early step in the mycobacterial respira-
tory chain .
Inhibition of mycobacterial K+ transport by 7-MJ
closely paralleled interference with microbial energy
metabolism, and is probably secondary to ATP depletion.
MTB possesses two major K+ uptake systems. These are
the Kdp and Trk A/B systems, driven by ATP and proton
motive force, respectively . The experimental condi-
tions used in the current study (low K+ medium) are
likely to favour preferential utilisation of the inducible,
high-affinity, Kdp system, accounting for the susceptibil-
ity of K+ transport to ATP depletion, which in turn may
lead to inactivation of the Trk A/B system due to dissipa-
tion of the membrane potential.
In conclusion, 7-MJ appears to target mycobacterial
energy metabolism, leading to secondary membrane dys-
function and inhibition of bacterial growth. This agent
may serve as a prototype for the development of novel
naphthoquinone-based anti-tuberculosis agents.
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