The two main metabolic pathways involved in sugar metabolism, i.e., the pentose phosphate pathway (PPP) and the glycolytic pathway (GP), were amperometrically monitored using a double-mediator system composed of menadione and ferricyanide. With the use of the Saccharomyces cerevisiae deletion mutant, EBY44, lacking the gene encoding for the branch point enzyme phosphoglucose isomerize, selective amperometric monitoring of the PPP, mainly producing NADPH, and the GP, mainly producing NADH, could be achieved. It was found that the bioelectrocatalytic current was primarily originating from NADPH. This conclusion was supported by metabolite flux analysis, confirming that, in the presence of menadione, the cells increase the rate of NADPH-producing reactions although these processes might be detrimental to cell survival. The higher rate of in vivo NADPH-dependent menadione reduction can be ascribed to the fact that the intracellular NADPH/NADP+ ratio is much higher than NADH/NAD+ as well as that the former ratio is more tightly controlled. This tight control over the cofactor ratios is lost upon cell disintegration as observed from spectrophotometric assays using crude cell extract, and amperometric investigations of permeabilized cells indicate a higher rate of NADH- than NADPH-dependent menadione reduction. These in vitro experiments show a higher activity of NADH-dependent than NADPH-dependent menadione-reducing dehydrogenases in S. cerevisiae cells.