An antibacterial from Hypericum acmosepalum inhibits ATP-dependent MurE ligase from Mycobacterium tuberculosis

Abstract

In a project to characterise new antibacterial chemotypes from plants, hyperenone A and hypercalin B were isolated from the hexane and chloroform extracts of the aerial parts of Hypericum acmosepalum. The structures of both compounds were characterised by extensive one- and two-dimensional nuclear magnetic resonance (NMR) spectroscopy and were confirmed by mass spectrometry. Hyperenone A and hypercalin B exhibited antibacterial activity against multidrug-resistant strains of Staphylococcus aureus, with minimum inhibition concentration ranges of 2-128 mg/L and 0.5-128 mg/L, respectively. Hyperenone A also showed growth-inhibitory activity against Mycobacterium tuberculosis H37Rv and Mycobacterium bovis BCG at 75 mg/L and 100mg/L. Neither hyperenone A nor hypercalin B inhibited the growth of Escherichia coli and both were non-toxic to cultured mammalian macrophage cells. Both compounds were tested for their ability to inhibit the ATP-dependent MurE ligase of M. tuberculosis, a crucial enzyme in the cytoplasmic steps of peptidoglycan biosynthesis. Hyperenone A inhibited MurE selectively, whereas hypercalin B did not have any effect on enzyme activity.

Fig. 1

Structures of hypercalin B (1) and hyperenone A (2).

Fig. 2

Effect of compound 2 on the growth of mycobacterial species using a spot culture growth inhibition assay. Approximately 500 Mycobacterium bovis BCG and Mycobacterium tuberculosis H37Rv cells were spotted on solidified Middlebrook 7H10 agar containing different concentrations of compound 2 in a six-well plate. Pictures were taken using a digital camera after 14 days of inoculation.

Fig. 3

Effect of compound 2 on MurE enzyme activity. The x-axis represents the different concentrations of the compound tested in the assay and the y-axis represents the percent inhibition at different concentrations of the compound. The assay was performed in triplicate.