Ammonium uptake by Chlorella.

The preincubation of Chlorella cells with glucose caused a tenfold increase of the maximal uptake rate of ammonium without change in the K m (2 μM). A similar stimulation of ammonium uptake was found when the cells were transferred to nitrogen-free growth medium. The time-course of uptake stimulation by glucose revealed a lag period of 10-20 min. The turnover of the ammonium transport system is characterized by a half-life time of 5-10 h, but in the presence of light 30% of uptake activity stayed even after 50 h. 6-Deoxyglucose was not able to increase the ammonium uptake rate. These data together were interpreted as evidence for induction of an ammonium transport system by a metabolite of glucose. Mechanistic studies of the ammonium transport system provided evidence for the electrogenic uptake of the ammonium ion. The charge compensation for NH 4 (+) entry was achieved by immediate K(+) efflux from the cells, and this was followed after 1 min by H(+) extrusion. Ammonium accumulated in the cells; the rate of uptake was sensitive to p-trifluoromethoxy-carbonylcyanide-phenylhydrazon and insensitive to methionine-sulfoxime. Uptake studies with methylamine revealed that methylamine transport is obviously catalyzed by the ammonium transport system and, therefore, also increased in glucose-treated Chlorella cells.