Table 1.

Effect of Phospholipids and Sterols on ATPase Activity of Native G5G8 Isolated from Mouse Liver^a

lipid (2.5 µg)	G5G8 ATP hydrolysis activity [µmol (mg of protein)−1 min−1]
none	0.39 ± 0.05
phosphotidylcholine	0.38 ± 0.01
phosphotidylserine	0.35 ± 0.10
cholesterol	0.20 ± 0.02
sitosterol	0.21 ± 0.05
lipid mixture, as described   in the Experimental Procedures	0.33 ± 0.10

^aSterols in ethanol (100%) were added to a glass tube and dried under N₂. Purified native G5G8 (35 ng) and the assay buffer were added to the tube, and the sterols were suspended by gentle mixing using a pipet. The individual phospholipids and cholesterol/phospholipid mixtures were prepared as liposome solutions and added directly to the assay mixture. For each assay, the ATP hydrolysis activity of purified G5G8 was measured after incubation at 37 °C for 60 min in the presence or absence of the lipid, as described in the Experimental Procedures. A parallel set of tubes containing only the lipid served as controls for the experiments. The background counts from the lipid-alone controls were subtracted from the values obtained after addition of the enzyme to obtain the G5G8-dependent ATPase activity.