Isolation, amino acid sequence determination and binding properties of twofatty-acid-binding proteins from axolotl (Ambistoma mexicanum) liver - Evolutionary relationship

Up until now, the primary structure of fatty-acid-binding proteins (FABPs) from the livers of four mammalian (rat, human, cow and pig) and three nonma mmalian (chicken, catfish and iguana) species has been determined. Based on amino acid sequence comparisons, it has been suggested that mammalian and nonmammalian liver FABPs may be paralogous proteins that originated by gene duplication, rather than as a consequence of mutations of the same gene. I n this paper we report the isolation and amino acid sequence determination of two FABPs from axolotl (Ambistoma mexicanum) liver. One of them is simil ar to mammalian liver FABPs (L-FABPs) and the other to chicken, catfish and iguana liver FABPs (Lb-FABPs). The finding of both L-FABP and Lb-FABP in a single species, as reported here, indicates that they are paralogous prote ins. The time of divergence of these two liver FABP types is estimated to b e of approximate to 694 million years ago. The ligand-binding properties of axolotl liver FABPs were studied by means of parinaric-acid-binding and pa rinaric-acid-displacement assays. L-FABP binds two fatty acids per molecule but Lb-FABP displays a fatty-acid-conformation-dependent binding stoichiom etry; L-FABP shows a higher affinity for fatty acids, especially oleic acid , while Lb-FABP has a higher affinity for other hydrophobic ligands, especi ally retinoic acid. In addition, the tissue-expression pattern is different , L-FABP is present in liver and intestinal mucosa while the expression of Lb-FABP is restricted to liver. Data indicate distinct functional propertie s of both liver FABP types.