Studies on the Structuredness of Cytoplasm and Rates of Enzymatic Hydrolysis in Growing Yeast Cells

Summary

Changes in the structuredness of cytoplasm after irradiation with x-rays under oxic and hypoxic conditions in yeast cells were studied. These were followed by measuring the fluorescence polarization of fluorescein molecules produced by enzymatic hydrolysis of the non-fluorescent fluoresceindiacetate (FDA) in the cytoplasm of cells in logarithmic phase of growth. The fluorescence polarization value of fluorescein molecules in the cytoplasm of yeast cells in the initial part of logarithmic phase of growth is P = 0·165 ± 0·0086 at 27°c. This value increases progressively after irradiation in oxic conditions. After a dose of 16 krads the fluorescence polarization value increased by 105 per cent, indicating an increase in the structuredness or viscosity of the cytoplasm. Irradiation under hypoxic conditions decreased the fluorescence polarization by 10 per cent, indicating a decrease in the structuredness of cytoplasm. The decrease is constant from 1 krad to 30 krads. Rates for the enzymatic hydrolysis of FDA to fluorescein in vivo were measured. The first-order rate constants for the hydrolysis of FDA decreased (increased) with increasing (decreasing) structuredness of the cytoplasm. Good agreement with the theoretical correlation between viscosity of the matrix and reaction-rate constants of diffusion controlled reactions was obtained for the enzymatic hydrolysis of FDA in vivo. This means that the observed changes in rates of enzymatic hydrolysis are due to radiation-induced changes in the structuredness of cytoplasm. Effects of the structuredness of cell matrix on rates of enzyme reactions are discussed.