Investigation of the apolipoprotein fraction of isolated rat adrenal and bovine adrenocortical lipid droplets.

The lipid droplet fractions from rat adrenal and bovine adrenocortical tissue were isolated by density ultracentrifugation. The droplet fractions were delipidated and the protein components investigated by SDS-polyacrylamide slab gel electrophoresis. The adrenal lipid droplets from both species displayed a qualitatively similar protein profile, and both contained a major apolipoprotein subunit of Mr 40 000. Incubation of intact, non-delipidated lipid droplets with [gamma-32P]ATP in vitro resulted in the phosphorylation of the Mr 40 000 apolipoprotein subunit in the case of rat lipid droplets, but not in the case of bovine lipid droplets. However, following delipidation of the droplets with diethyl ether/ethanol, the Mr 40 000 apolipoprotein subunit was phosphorylated in both cases upon incubation of the delipidated protein fractions with [gamma-32P]ATP in vitro. Labelling with [gamma-32P]ATP and [3H]diisopropyl phosphorofluoridate indicated that the cholesterol ester hydrolase enzyme protein was not a major constituent of the adrenal lipid droplet protein fraction.