Characterization and primary structure of a fatty acid-binding protein and its isoforms from the liver of the amphibia, Rana catesbeiana

Fatty acid-binding protein (FABP) was purified from the liver of the Amphibia, Rana catesbeiana, by gel filtration and ion-exchange chromatography. The complete primary structure of the frog liver FABP was determined by protein analysis. Two isoforms, I and II, were separated by reversed phase HPLC, and found to differ by 10 atomic mass units as measured by ion-spray ionization mass spectrometry, A detailed analysis of enzymatic peptides revealed a single Pro (isoform I)/Ser (isoform II) replacement at position 16. It seems remarkable that a rather neutral point mutation results in the nearly complete separation of the two isoforms by reversed phase chromatography. Homology modeling suggests the location of this site on the first helix of the helix-turn-helix domain and the presence of a single thiol group of cysteine-91 at the inside of the ligand-binding cavity, Binding studies using a natural fluorescent fatty acid, cis-parinaric acid, showed a lower K-d value for the serine form and large enhancement of fluorescence intensity upon glutathione-thiolation at cysteine-91, Examination of phylogenetic relationships identified the frog liver protein as a mammalian liver type FABP, and suggested a change in the vertebrate liver FABP gene expression at the bony fish/cartilagenous fish boundary.