The subcellular distribution of the nonspecific lipid transfer protein (sterol carrier protein 2) in rat liver and adrenal gland

Abstract

The distribution of the nonspecific lipid transfer protein (i.e., sterol carrier protein 2) over the various subcellular fractions from rat liver and adrenal gland was determined by enzyme immunoassay and immunoblotting. This distribution is very different in each of these two tissues. In liver, 66% of the transfer protein is present in the membrane-free cytosol as compared to 19% in the adrenal gland. In the latter tissue, the transfer protein is mainly found in the lysomal/peroxisomal and the microsomal fraction at a level of 1093 and 582 ng per mg total protein, respectively (i.e, 17% and 35% of the total), and to a lesser extent in the mitochondrial fraction (11% of the total). Of all the membrane fractions isolated, the microsomal fraction from the liver and the mitochondrial fraction from the adrenal gland have the lowest levels of the transfer protein (i.e., 168 ng and 126 ng per mg total protein, respectively). These low levels correlate poorly with the active role proposed for this transfer protein in the conversion of cholesterol into bile acids and steroid hormones in these fractions. Using immunoblotting, it was demonstrated that in addition to the transfer protein (14 kDa) a cross-reactive 58 kD protein was present in the supernatant and the membrane fractions of both tissues. Cytochemical visualization in adrenal tissue with specific antibodies against the nonspecific lipid transfer protein showed that immunoreactive protein(s) were present mainly in the peroxisome-like structures.