The CPSase activity in the amino acid-depleted cells. HEK293 cells transfected with the myc-CAD vector were cultured in the absence of serum for 16 h, and further incubated under different conditions for 2 h. Cells were harvested as shown in the bottom of the figure. 1: after serum starvation for 16 h, 2: after further incubation with serum, 3: after further incubation without serum, 4: after further incubation without serum and amino acids. The immunoprecipitates with the anti-myc antibody from the cells after each treatment were subjected to the CPSase assay. The relative CPSase activity normalized to the amount of immunoprecipitated myc-CAD are employing the activity of the serum-starved cells as 100%. **, P < 0.01.