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WhenSaccharomyces cerevisiaecells, grown in galactose, glucose or mannose, were treated with 1.5 mMhydrogen peroxide (H2O2) for 30 min, an important decrease in the ATP, and a less extensive decrease in the GTP, CTP, UTP and ADP-ribose levels was estimated. Concomitantly a net increase in the inosine levels was observed. Treatment with 83mMmenadionepromoted theappearanceof acompound similar to adenosine but no appreciable changes in the nucleotide content of yeast cells, grown either in glucose or galactose | Eur. J. Biochem. 270 1578-1589 2003 FEBS 2003 doi 10.1046 j.1432-1033.2003.03529.x H2O2 but not menadione provokes a decrease in the ATP and an increase in the inosine levels in Saccharomyces cerevisiae An experimental and theoretical approach Hugo Osorio1 2 Elisabete Carvalho1 Mercedes del Valle1 Maria A. Gunther Sillero1 Pedro Moradas-Ferreira2 and Antonio Sillero1 1 Departamento de Bioquimica Instituto de Investigaciones Biomedicas Alberto Sols UAM CSIC Facultad de Medicina Madrid Spain 2Instituto de Biologia Molecular e Celular Instituto de Ciencias Biomedicas Abel Salazar Universidade do Porto Portugal When Saccharomyces cerevisiae cells grown in galactose glucose or mannose were treated with 1.5 mM hydrogen peroxide H2O2 for 30 min an important decrease in the ATP and a less extensive decrease in the GTP CTP UTP and ADP-ribose levels was estimated. Concomitantly a net increase in the inosine levels was observed. Treatment with 83 mM menadione promoted the appearance of a compound similar to adenosine but no appreciable changes in the nucleotide content of yeast cells grown either in glucose or galactose. Changes in the specific activities of the enzymes involved in the pathway from ATP to inosine in yeast extracts from un treated cells could not explain the effect of H2O2 on the levels of ATP and inosine. Application of a mathematical model of differential equations previously developed in this laboratory pointed to a potential inhibition of glycolysis as the main reason for that effect. This theoretical consideration was reinforced both by the lack of an appreciable effect of 1.5 mM or even higher concentrations H2O2 on yeast grown in the presence of ethanol or glycerol and by the observed inhibition of the synthesis of ethanol promoted by H2O2. Normal values for the adenylic charge ATP and inosine levels were reached at 5 30 and 120 min respectively after removal of H2O2 from the culture medium. The strong decrease in the ATP level upon H2O2 treatment is an