Subcellular distribution of cardiac fatty acid-binding protein in bovine heart muscle and quantitation with an enzyme-linked immunosorbent assay.

Several types of the 14-15 kDa fatty acid-binding proteins (FABPs) are known to occur in the cytosol of mammalian cells. With antibodies raised against the cardiac-type protein from bovine heart, immunoblots indicated a more widespread distribution of the cardiac FABP in subcellular fractions, such as mitochondria and nuclei. A detailed view was obtained when the post-embedding protein A-gold labeling method was applied to cross-sections of heart cells and isolated subcellular fractions. Cardiac FABP in myocytes was associated with myofibrils and localized within mitochondria and nuclei. After subfractionation of mitochondria, the binding protein was recovered with matrix proteins only. A non-competitive enzyme-linked immunosorbent assay (ELISA) of the direct type was developed specifically for bovine cardiac FABP. This assay was sensitive in the range of 0.05 to 1 ng, and concentrations of cardiac FABP per mg protein were found for cytosol, matrix and nuclei to be around 3.18, 0.18 and 0.03 micrograms, respectively. The newly found compartmentation of cardiac FABP in the heart cell must be considered when the true functions of the protein, yet to be defined, are studied.