Elucidation of antimicrobial activity and mechanism of action by N-substituted carbazole derivatives

Compounds belonging to a carbazole series have been identified as potent fungal plasma membrane proton adenosine triphophatase (H(+)-ATPase) inhibitors with a broad spectrum of antifungal activity. The carbazole compounds inhibit the adenosine triphosphate (ATP) hydrolysis activity of the essential fungal H(+)-ATPase, thereby functionally inhibiting the extrusion of protons and extracellular acidification, processes that are responsible for maintaining high plasma membrane potential. The compound class binds to and inhibits the H(+)-ATPase within minutes, leading to fungal death after 1–3 h of compound exposure in vitro. The tested compounds are not selective for the fungal H(+)-ATPase, exhibiting an overlap of inhibitory activity with the mammalian protein family of P-type ATPases; the sarco(endo)plasmic reticulum calcium ATPase (Ca(2+)-ATPase) and the sodium potassium ATPase (Na(+),K(+)-ATPase). The ion transport in the P-type ATPases is energized by the conversion of ATP to adenosine diphosphate (ADP) and phosphate and a general inhibitory mechanism mediated by the carbazole derivative could therefore be blocking of the active site. However, biochemical studies show that increased concentrations of ATP do not change the inhibitory activity of the carbazoles suggesting they act as allosteric inhibitors. Furthermore decreased levels of intracellular ATP would suggest that the compounds inhibit the H(+)-ATPase indirectly, but Candida albicans cells exposed to potent H(+)-ATPase-inhibitory carbazoles result in increased levels of intracellular ATP, indicating direct inhibition of H(+)-ATPase.