HYDROGEN-PEROXIDE METABOLISM IN YEASTS

A catalase-negative mutant of the yeast Hansenula polymorpha consumed methanol in the presence of glucose when the organism was grown in carbon-limited chemostat cultures. The organism was apparently able to decompose the H2O2 generated in the oxidation of methanol by alcohol oxidase. Not only H2O2 generated intracellularly but also H2O2 added extracellularly was effectively destroyed by the catalase-negative mutant. From the rate of H2O2 consumption during growth in chemostat cultures on mixtures of glucose and H2O2, it appeared that the mutant was capable of decomposing H2O2 at a rate as high as 8 mmol .cntdot. g of cells-1 .cntdot. h-1. Glutathione peroxidase (EC 1.11.1.9) was absent under all growth conditions. However, cytochrome c peroxidase (CCP; EC 1.11.1.5) increased to very high levels in cells which decomposed H2O2. When wild-type H. polymorpha was grown on mixtures of glucose and methanol, the CCP level was independent of the rate of methanol utilization, whereas the level of catalase increased with increasing amounts of methanol in the substrate feed. Also, the wild type decomposed H2O2 at a high rate when cells were grown on mixtures of glucose and H2O2. In this case, an increase of both CCP and catalase was observed. When Saccharomyces cerevisiae was grown on mixtures of glucose and H2O2, the level of catalase remained low, but CCP increased with increasing rates of H2O2 utilization. From these observations and an analysis of cell yields under the various conditions, two conclusions can be drawn. (i) CCP is a key enzyme of H2O2 detoxification in yeasts. (ii) Catalase can effectively compete with mitochondrial CCP for hydrogen peroxide only if hydrogen peroxide is generated at the site where catalase is located, namely in the peroxisomes.