Three fatty acid-binding proteins (FABPs) from the liver of the shark Halaetunus bivius were isolated and characterized: one of them belongs to the liver-type FABP family and the other two to the heart-type FABP family. The complete primary structure of the first FABP, and partial primary structures of the two others, were determined. The liver-type FABP constitutes 69% of the total FABPs, and its amino acid sequence presents the highest identity with chicken, catfish, iguana and elephant fish liver basic FABPs. The L-FABP protein has low affinity for palmitic and oleic acids and high affinity for linoleic and arachidonic acids and other hydrophobic ligands, all of them important for the metabolic functions of the liver. In contrast, both heart-type FABPs have the highest affinity for palmitic acid, the principal fatty acid mobilized from fat deposits for b-oxidation.Keywords: shark; Halaetunus bivius; fatty acid-binding protein (FABP); liver basic FABP.Fatty acid-binding proteins (FABPs) isolated from the same tissue of different mammals show greater primary structure similarity than FABPs isolated from different tissues of the same species [1,2]. On the other hand, a given FABP type can be expressed in more than one tissue, such as the heart-type (H-FABP) which is also found in kidney, skeletal muscle, aorta, lung, mammary gland, placenta, brain, stomach, testes and adrenal glands [3]. We have also recently found a H-FABP in a mammalian Harderian gland [4]. However, fish seem not to be included in this mammalian tissue specificity, as the first FABP isolated from a nonmammalian vertebrate (the liver of the nurse shark Ginglymostoma cirratum) is structurally more closely related to mammalian H-FABPs than to the members of the liver fatty acid-binding protein (L-FABP) subfamily [5]. This finding was attributed to the fact that, in elasmobranchs, fat reserves are concentrated in the liver [6] and that shark liver behaves, in some aspects, like mammalian adipose tissue [5]. Another possible reason put forward is that differentiation of liver and heart FABPs occurred after the fish-mammalian split [7], though the evolutionary trees proposed by Mendzihradsky [5] do not support this possibility. Previously [8,9], we analyzed the evolutionary relationship of 51 and 63 family members, respectively, from vertebrates and invertebrates and agree with the hypothesis that gene duplication leading to mammalian liver and heart FABPs occurred before the vertebrate-invertebrate split.In an attempt to shed some light on the presence of only one H-FABP in shark liver, we investigated the presence of FABPs in the liver tissues of two fish: catfish (Rhamdia sapo) [10±12], whose fat reserves are not concentrated in the liver (as is the case for the shark), and elephant fish (Callorhynchus callorhynchus), a chimera fish belonging, together with elasmobranchs, to the ancient condrictius class. Three FABPs were found in catfish liver, one of them a liver basic FABP (Lb-FABP) having a primary structure [10,12] closely related to those o...
