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.