SUBMICELLAR BILE-SALTS STIMULATE PHOSPHATIDYLCHOLINE TRANSFER ACTIVITY OF STEROL CARRIER PROTEIN-2

To explore a potential role for sterol carrier protein 2 (SCP2, also k nown as non-specific lipid transfer protein) in hepatocellular phospho lipid trafficking, we examined the influence of submicellar bile salt concentrations on phosphatidylcholine (PC) transfer activity of SCP2, We measured rate constants for first-order transfer of m-l palmitoyl, sn-2 parinaroyl PC, a naturally fluorescent self-quenching phospholipi d between model membranes. Purified bovine liver SCP2 promoted transfe r of PC from donor to acceptor small unilamellar vesicles. Taurine- an d glycine-conjugated bile salts (anionic steroid detergent-like molecu les), at concentrations well below their critical micellar concentrati ons, stimulated PC transfer activity of SCP2 80- to 140-fold. Rate con stants increased in proportion to bile salt concentration, temperature , and bile salt-membrane binding affinity. Sodium taurofusidate, a con jugated fungal bile salt analog, also activated PC transfer whereas no effect was observed with the anionic and nonionic straight chain dete rgents sodium dodecyl sulfate and octylglucoside, respectively. Thermo dynamic and kinetic analyses of PC transfer support a mechanism in whi ch bile salts stimulate SCPS activity by partitioning into donor vesic les and enhancing membrane association of SCP2, These results imply th at under physiological conditions, SCP2 may contribute to hepatocellul ar selection and transport of biliary PCs.