Comparative study of the effects of amphotericin B on the glucose metabolism in Saccharomyces cerevisiae in K +- and Na +-rich media

Abstract

In order to elucidate the effects of amphotericin B (AMB) on the glycolytic pathway, the metabolism of [1- 13C]glucose in glucose-grown repressed Saccharomyces cerevisiae was studied. The cells were aerobically suspended in pyrophosphate solutions of high potassium concentration with or without 10 −6 M amphotericin B and measurements were made using 1H-, 13C-NMR spectroscopy and biochemical methods. The results were compared with those obtained under the same experimental conditions but in a medium rich in sodium salts containing the same antibiotic concentration. In general the presence of 10 −6 M AMB reduces the glucose consumption and the ethanol production while favouring the glycerol and trehalose formation. These effects are greatly reduced when a high K + concentration was used. The AMB effects on the glucose consumption and the production of ethanol, glycerol and trehalose, observed in a suspension rich in Na +, can be fairly well explained by the leakage of K + through AMB membrane channels. This outflux induces a substantial decrease in the activity of some K +-dependent enzymes, such as aldolase, phosphofructokinase and pyruvate kinase. The intensities of the glutamate C2 and C4 signals are higher with a suspension rich in Na + than with a suspension rich in K +, suggesting that the Krebs cycle operates more effectively in a solution rich in Na +. In the absence of AMB, the passive diffusion of glycerol through the cell membrane is relatively slow and apparently depends on the ionic external medium: it is more efficient in solutions with a high K + than with a high Na + concentration. In the presence of 10 −6 M AMB, the glycerol C1,3 resonance drastically decreases at 20 min and then disappears in the noise. This rapid disappearance suggests that glycerol can easily pass through the pores arising from the interaction of AMB with the membrane sterols. However, the rate of pore formation is slow, independent of the external medium (Na + or K +) and this process is not completed within 20 min.