Intramitochondrial Movement of Adrenal Sterol Carrier Protein with Cholesterol in Response to Corticotropin

Sterol carrier protein (SCP) is a highly abundant, ubiquitous, low molecular weight protein that has a rapid turnover and multifunctional roles in lipid metabolism and transport. It is also known as the fatty acid-binding protein. These and other characteristics led to studies on the possible role of SCP in mediation of steroidogenic responses of the adrenal gland to corticotropin. To quantitate the level of SCP in adrenal tissue and subcellular fractions, a specific immunochemical assay was developed using an antibody to homogeneous liver SCP. SCP is a major protein in adrenal cells (>100 μ g/mg of total protein). The bulk of it is present in the soluble (60%) and mitochondrial (35%) fractions. Nearly all mitochondrial SCP is present in the inner membrane. Adrenal SCP undergoes a dramatic diurnal variation, varying from 2% to 12% of total protein. When corticotropin was administered in vivo in the presence of aminoglutethimide, an inhibitor of steroidogenesis, there was a parallel movement of SCP with cholesterol to the inner mitochondrial membrane. Other work showed mitochondrial SCP levels increase only in situations where there is an increase in cholesterol levels. These findings strongly suggest that one of the functions of adrenal SCP is to participate in the corticotropin-stimulated movement of cholesterol to the inner mitochondrial membrane for steroidogenesis.