[Enzymatic examination of citrate-isocitrate accumulation in yeasts]

The activities of the enzymes citrate synthase, aconitate hydratase, isocitrate dehydrogenase and isocitrate lyase in cell homogenates of n-paraffin grown citrate plus isocitrate accumulating yeasts (C. lipolytica high rate accumulating strain and C. guilliermondii low rate accumulating strain) were determined. It is shown that the activities of the enzymes decline after transition from tropho- to idiophase and remain constant with exception of isocitrate lyase which diminishes slowly. The decline in activity of the isocitrate lyase was greatest in the cells of C. guilliermondii. It is discussed that the differences of the enzymatic activities in the tropho- and idiophase, resp. may be artefacts due to changes in the structure of cellular envelope, but that the decline of lyase activity in the idiophase could be one factor determining the rate of citric- and isocitric acid overproduction. The diminishing of isocitrate lyase in the course of idiophase is interpreted as example of a disappearing enzyme no further needed for normal function of the cells.