On the mechanism of long chain fatty acid transport in cardiomyocytes as facilitated by cytoplasmic fatty acid-binding protein.

Fatty acid-binding protein (FABP) is abundantly present in the cytoplasm of the cardiomyocyte, i.e. the cell which causes the contractile activity of the heart. Although FABP is thought to act as an intracellular long chain fatty acid (FA) carrier, definite experimental proof on this putative function has yet to be obtained. In the present study, experimental results from several authors were combined in an attempt to elucidate the precise physiological function of heart-type FABP in cardiac FA transport. It was calculated that, under normal conditions, the major part of FA in the cardiomyocyte is dissolved in lipid bilayers and that the presence of FABP in the heart enhances the aqueous solubility of FA more than 700-fold despite the fact that only a minor part (< 2%) of the total FABP content is then complexed with FA. Moreover, it is shown that, as a result of the enhanced cytoplasmic solubility, the FA flux from sarcolemma (the cellular membrane of the cardiomyocyte) to mitochondria is increased at least 17-fold in the presence of physiological amounts of FABP compared with the hypothetical situation in which FABP is absent. These calculations indicate the involvement of FABP in the transport of FA from the sarcolemma to those mitochondria lying in the innermost region of the cardiomyocyte. The extent to which FABP facilitates FA trafficking through the cytoplasm of the cardiomyocyte under physiological circumstances remains, however, to be established.