Isolation, characterization and binding properties of two rat liver fatty acid-binding protein isoforms

Mammalian liver has only one fatty acid-binding protein (L-FABP) while the liver of non-mammalian vertebrates expresses a liver basic FABP (Lb-FABP) i n addition to other members of the FABP family. We explore the possibility that L-FABP isoforms accomplish, in the liver of mammals, the metabolic fun ctions corresponding to the different FABPs present in the liver of non-mam malian vertebrates. We have isolated isoforms I and II which have a differe nt residue 105, Asn in the former and Asp in the latter. We made a conforma tional comparison of the apo-isoforms by intrinsic fluorescence emission an d fourth-derivative spectroscopy, native-state proteolysis and unfolding cu rves. Ligand affinity was studied by measuring cis-parinaric acid displacem ent by different ligands. They have differences in their molecular conforma tion, including the environment of the binding site. Isoform II has probabl y a more open conformation than isoform I, thus allowing the binding of a g reater variety of ligands. The affinity of isoform II for lysophospholipids , prostaglandins, retinoids, bilirubin and bile salts is greater than that of isoform I. These characteristics of rat L-FABP isoforms I and II suggest that they may accomplish different functions as happens with those of the different FABP types in non-mammalian species. (C) 2000 Elsevier Science B. V. All rights reserved.