The Effect of Cycloheximide on Membrane Transport in Euglena

Cycloheximide and nigericin have been found to inhibit the uptake of 2,4-dinitrophenol (DNP) and glucose in Euglena gracilis with the inhibition of uptake of DNP being dependent upon a prior incubation of the cells with the antibiotics. The effect that cycloheximide exhibited upon the uptake of DNP and glucose was related to the growth phase of the cell. In log phase cells 3.6, 36, and 360 µm cycloheximide inhibited the uptake of DNP by 90%, whereas in carbon-starved cells the uptake of DNP was inhibited only by 5 to 15% by 360 µm cycloheximide but 3.6 µm cycloheximide was as effective in the inhibition of uptake in starved cells as in log phase cells. Although 360 µm cycloheximide exhibited little effect upon the uptake of DNP and glucose in starved cells, the inhibition of protein synthesis was virtually complete. This observation coupled with the finding that after removal of the antibiotic the inhibition of protein synthesis and the inhibition of uptake of DNP exhibited different recovery patterns suggested that the inhibition of membrane transport by cycloheximide was not a consequence of the inhibition of protein synthesis. A decrease in the polyribosome content, with a corresponding increase in the amount of monosomes, was observed when log phase cells were incubated with either 3.6 µm cycloheximide or 1.4 µm nigericin. It was suggested that the inhibition of uptake of DNP by cycloheximide and nigericin was due either to a decrease in the pH gradient existing between the inside and outside of the cell or to an effect upon membrane function such that the un-ionized form of DNP was no longer permeable. The mechanism by which the inhibition of membrane transport was manifested by the two antibiotics was not viewed as being identical.