Alteration in the characters of CDP-choline synthetase and phospholipid-choline exchange enzyme upon choline starvation in Chinese hamster ovary cells.

When CHO-K1 cells are cultivated under choline-deficient conditions, the specific activity of CDP-choline synthetase increases and conversely phospholipid-choline exchange enzyme activity decreases, whereas the other three known enzyme activities related to synthesis of phosphatidylcholine remain unchanged. The changes of the former two enzyme activities take place immediately after removal of choline from the medium. The altered activities readily revert to the control levels upon resupplementation of choline to the starved cell culture. The changes upon choline starvation are sensitive to cycloheximide, while the restoration processes are insensitive to the drug. The activity of CDP-choline synthetase in unstarved control cells is found in both the soluble and membrane fractions. The Km value of the enzyme in the soluble fraction for choline phosphate differs from that in the membrane fraction. Asolectin alters the Km value of the former to a value close to that of the latter and raises its Vmax value, whereas it hardly affects the Km and Vmax values of the latter. In choline-starved cells, the activity is exclusively found in the membrane fraction. The change in the subcellular distribution of the activity upon choline starvation is sensitive to cycloheximide. The altered subcellular distribution reverts to the initial status upon resupplementation of choline even in the presence of cycloheximide. The activity of the phospholipid-choline exchange enzyme is exclusively found in the membrane fraction for both starved and control cells. The properties of the enzyme are altered upon choline starvation with respect to the Vmax value for choline and the Km and Vmax values for Ca2+. These altered kinetic parameters are changed by egg yolk phosphatidylcholine so as to be indistinguishable from those in unstarved control cells. We discuss the mechanism of the alterations in the characters of both enzymes in response to choline starvation.