Uptake and Binding of 14C-Ethambutol by Tubercle Bacilli and the Relation of Binding to Growth Inhibition

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Studies were designed to characterize ethambutol uptake by Mycobacterium tuberculosis (H37Ra) and to relate uptake to the time-dependent, concentration-independent nature of growth inhibition by ethambutol. When cells grown aerated at 37 C in Sauton medium were exposed for 7 hr to 0.2, 0.5, 1.0, 2.5, and 5.0 μg of 14C-ethambutol per ml, uptake increased with time and was a linear function of concentration. The process was inhibited at 22 C. Studies with chloramphenicol, sodium azide, and 2,4-dinitrophenol indicated that uptake is independent of requirements for protein synthesis and energy. The organism did not accumulate ethambutol against a concentration gradient. It can be concluded that ethambutol enters the cells in a passive manner. Kinetic studies of 14C loss from tubercle bacilli pretreated with labeled drug suggested the existence of two ethambutol fractions within the cell: a highly variable labile pool and a second fraction that is small and quite firmly bound. Levels of cell-bound drug may be independent of total uptake, but this possibility was not established unequivocally. Definitive evidence showing identity in the concentrations of bound drug regardless of total uptake could explain the apparent discrepancy between concentration-dependent uptake and concentration-independent growth inhibition.

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