Spontaneous and StarD4-enhanced transfer of Ch and 7α-OOH from liposomes to mitochondria. Reaction mixtures in pH 7.5 MS buffer at 25 °C contained POPC/[14C]Ch or [14C]7α-OOH (8:2 mol/mol) SUVs (0.06 mM total lipid; 100 nCi/ml), liver mitochondria (2.8 mg protein/ml, 0.6 mM total lipid) and either StarD4 [1 µM (▲) or 2 µM (▼)] or BSA (2 µM (●)]. Spontaneous transfer for mixtures lacking StarD4 or BSA is also represented (x). At the indicated incubation times, samples were removed, centrifuged to separate SUVs from mitochondria, and radioactivity in the latter was determined by scintillation counting. (A) First-order kinetic plot for [14C]Ch acquisition by mitochondria; (B) First-order plot for [14C]7α-OOH acquisition by mitochondria. A∞, Ao, and At denote analyte level at infinite time, time zero and time t, respectively. Plotted values are means ± SD (n = 3).