Circular dichroism analysis of the secondary structure of Z protein and its complexes with bilirubin and other organic anions

Conformational and binding properties of chicken liver basic fatty acid binding protein in solution

The conformation of basic fatty acid binding protein from chicken liver and the binding properties of the apo protein toward 11-dansylamino-undecanoic acid were investigated by CD and fluorescence spectroscopy. In one set of experiments the binding process was followed by the appearance of induced optical activity in the absorption region of the dansyl chromophore. In a second set of experiments the binding process was followed by the large enhancement of emission fluorescence of the dansyl fluorophore. From the saturation curves, the stoichiometry of the complex and the binding constant of the fatty acid to the protein were precisely determined. The values of the dissociation constant determined with the two methods were in excellent agreement: we obtained KD = (1.0 +/- 0.1) x 10(-6) M in a 0.9: 1 stoichiometry. The native conformation of the protein is remarkably stable in a variety of solvent systems, including acetonitrile-water, ethylene glycol-water, and dioxane-water of various compositions. The CD results also showed that the binding of the fatty acid does not induce any appreciable change in the protein conformation. In a mixture of water and 2,2,2-trifluoroethanol 1:9 (v/v), the native conformation collapses and a new ordered structure is formed, characterized by a high amount of alpha-helix.

Functions of fatty acid binding proteins

Cytosolic fatty acid binding proteins (FABP) belong to a gene family of which eight members have been conclusively identified. These 14-15 kDa proteins are abundantly expressed in a highly tissue-specific manner. Although the functions of the cytosolic FABP are not clearly established, they appear to enhance the transfer of long-chain fatty acids between artificial and native lipid membranes, and also to have a stimulatory effect on a number of enzymes of fatty acid metabolism in vitro. These findings, as well as the tissue expression, ligand binding properties, ontogeny and regulation of these proteins provide a considerable body of indirect evidence supporting a broad role for the FABP in the intracellular transport and metabolism of long-chain fatty acids. The available data also support the existence of structure- and tissue-specific specialization of function among different members of the FABP gene family. Moreover, FABP may also have a possible role in the modulation of cell growth and proliferation, possibly by virtue of their affinity for ligands such as prostaglandins, leukotrienes and fatty acids, which are known to influence cell growth activity. FABP structurally unrelated to the cytosolic gene family have also been identified in the plasma membranes of several tissues (FABPpm). These proteins have not been fully characterized to date, but strong evidence suggest that they function in the transport of long-chain fatty acids across the plasma membrane.

Adsorption of bilirubin with polypeptide-coated resins

Poly-L-lysine, poly-D-lysine and poly-L-ornithine have been coated covalently onto a water-swellable polyamide resin. These sorbents, especially the polylysine coated resins, exhibit high binding affinities for bilirubin in aqueous phosphate buffer solutions at 0 degrees C. The polylysine coated resins have an improved binding affinity for bilirubin over the sorbents with lysine containing oligopeptide pendants and much higher affinity than cholestyramine. The binding capacity is directly proportional to the number of lysine residues on the resin. Replacement of poly-L-lysine with poly-D-lysine does not affect the adsorption capacity.

Interactions of fatty acids with neutral fatty-acid-binding protein from bovine liver

Hepatic-type fatty-acid-binding protein (hFABP) from the cytosol of bovine liver is a 14.4-kDa neutral protein with a blocked N-terminus and a disulfide system located on the surface of the protein. It binds two molecules of fatty acid in one binding site, apparent dissociation constants of the oleic acid/hFABP complex are 0.24 microM and 2.15 microM. Computer analysis of circular dichroic spectra predicts that hFABP contains about 12% alpha-helix, 45% beta-structure, 15% beta-turn and 27% unordered structure. Ellipticities indicative of secondary structure are not affected by fatty acid binding. Cationic amino acid residues of hFABP (1 His, 15 Lys, 2 Arg) were screened for ionic fatty acid/protein interactions. His was excluded, as 1H-NMR analysis of His-C2 and His-C4 protons indicated that binding of oleic acid shifts the pK of His from 6.9 to 7.1 only in hFABP with the disulfide system in the oxidized state; acylation of His with diethylpyrocarbonate does not affect the binding of the fatty acid. Acetylation of Lys reduces binding marginally, whereas modification of Arg with phenylglyoxal lowers the binding activity by 65%. From 1H-NMR investigations, conformational changes within the protein, due to a sort of disaggregation of hFABP upon fatty acid binding, were derived. Most of the proton resonances sharpen up with ligand binding, and some of the methyl resonances shift positions, possibly because they are directly involved in the fatty acid/protein interaction.

A survey on cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds: from organic anion binders to new protein families

This review deals with recent advances in the research of cytosolic non-enzymic proteins involved in the metabolism of lipophilic compounds. Emphasis is given to the important contribution of structural data in the understanding of the functional properties of these proteins and in the emergence of new protein families. The possibility that many of the 'cytosolic' proteins might be structure-bound and structure-forming in the living cell is discussed, with references to so far available structural data and to recent investigations on the architecture and biochemical composition of the cytoplasm. The aim of this review is to present in a condensed form (227 references) the evolution in the study of cytosolic proteins binding and transferring lipophilic compounds and to enable interested investigators to become aware of current concepts and perspectives in this active and steadily growing area of research.

Bovine fatty acid binding proteins. Isolation and characterisation of two cardiac fatty acid binding proteins that are distinct from corresponding hepatic proteins

When a 100,000 X g supernatant from bovine heart was incubated with [1-14C]oleic acid and subjected to isoelectric focusing, two fatty acid binding proteins (FABPs) with isoelectric points at 4.9 and 5.1 were detected. The proteins were purified on a large scale first by heat and acid precipitation of a postmitochondrial supernatant, as well as fractionation with ammonium sulfate, then by alternate application of ion-exchange and gel chromatography. The procedure afforded around 60 mg pure proteins from 1.5 kg fresh heart muscle. Relative molecular masses of 15 300 +/- 1600 for both proteins were derived from sodium dodecyl sulfate/polyacrylamide gel electrophoresis, gel chromatography, sedimentation velocity as well as from amino acid analysis. Up to 50% of the proteins' secondary structures consisted of beta-sheet. N-termini of the peptide chains were blocked; the amino acid compositions of the two proteins were similar, but differed considerably from those of the two FABPs isolated from bovine liver [Haunerland et al. (1984) Hoppe Seyler's Z. Physiol. Chem. 365, 365-376]. Whereas hepatic FABPs changed their pI upon binding fatty acids, cardiac FABPs did not. Cardiac FABPs were immunologically identical, but did not cross-react with hepatic proteins. A reversible, concentration-dependent self-association reported for FABP from pig heart [Fournier et al. (1983) Biochemistry 22, 1863-1872] was not observed for FABP from bovine heart. Changes of concentration did not alter secondary structure, intrinsic fluorescence or the sedimentation coefficient of the protein.

Function and regulation of hepatic and intestinal fatty acid binding proteins

Two structurally different fatty acid binding proteins (FABP) have been isolated from rat liver and small intestinal epithelium. hFABP is a 14 184 Da protein found in abundance in both liver and small intestine, whereas gFABP (15 063 Da) is abundantly present only in small intestine. This review discusses studies which have provided insight into the physiological functions of these proteins. These include analyses of endogenous and exogenous ligand binding to FABP in vitro; examination of the modulating effect of FABP preparations on enzyme activities in vitro; exploration of relationships between alterations in cytosolic FABP content in response to hormonal, pharmacological, and dietary manipulations and changes in the rates of cellular fatty acid uptake and utilization; and studies of hFABP turnover and the mechanisms of FABP regulation. These experiments provide compelling evidence for a broad role of the FABPs in the transport, utilization and cellular economy of free fatty acids in the liver and small intestine, and also in protecting several aspects of cellular function against the modulatory effects of fatty acids, fatty acyl-CoA esters, and other ligands. Studies of FABP regulation also suggest a role in long-term rather than short-term modulation of hepatic fatty acid metabolism and indicate that hFABP and gFABP may perform different functions in the small intestine.

Analyzing the structures, functions and evolution of two abundant gastrointestinal fatty acid binding proteins with recombinant DNA and computational techniques

The structures of intestinal and liver fatty acid binding proteins (FABPs) have been determined from an analysis of the nucleotide sequences of cloned cDNAs. The primary translation product of intestinal FABP mRNA contains 132 residues (Mr = 15 124). Liver FABP mRNA encodes a 127 amino acid polypeptide (Mr = 14 273). In vitro co-translational cleavage and translocation assays showed that neither sequence has a cleavable signal peptide or signal peptide equivalent - suggesting that the FABPs do not enter the secretory apparatus but rather are targeted to the cytoplasm. A variety of computational techniques were used to compare the two FABP sequences. The results indicate that liver and intestinal FABP are paralogous homologues. A superfamily of proteins was defined which includes the FABPs, the cellular retinol and retinoic acid binding proteins, the P2 protein of peripheral nerve myelin, and a polypeptide known as 422 whose synthesis is induced during differentiation of 3T3-L1 cells to adipocytes. No sequence homologies were noted between any of these small molecular weight cytosolic proteins and nonspecific lipid transfer protein (sterol carrier protein 2), phosphatidylcholine transfer protein, serum albumin or apolipoprotein AI. The FABPs may have structural features responsible for lipid-protein interactions that are not present in these non-homologous sequences. The distribution of intestinal and liver FABP mRNAs in adult rat tissues and the changes in FABP gene expression which occur during gastrointestinal development support the notion that these proteins are involved in fatty acid uptake, transport and/or compartmentalization. However, differences in tissue distribution and periods of non-coordinate expression during gastrointestinal ontogeny suggest that the two FABPs have distinct functions. The relationship between intestinal and liver FABPs and similar sized cytosolic FABPs isolated from brain, skeletal and cardiac muscle remains unclear. Recombinant DNA techniques combined with comparative sequence analyses offer a useful approach for defining unique as well as general structure-function relationships in this group of fatty acid binding proteins.

Isolation and characterization of the three fractions (DE-I, DE-II and DE-III) of rat-liver Z-protein and the complete primary structure of DE-II

Three fractions (DE-I, DE-II and DE-III) of Z-protein (fatty acid binding protein) have been isolated from rat liver cytosol by DEAE-cellulose chromatography and characterized. They had the same molecular weight of 14000 and essentially identical amino acid composition. However, compositions of endogenous fatty acids were found to differ strikingly from one another. Long-chain fatty acids detected in DE-II were palmitic, stearic, oleic, linoleic and arachidonic acids. In contrast to DE-II, DE-III contained mainly arachidonic acid. Molar ratios of endogenous long-chain fatty acids to both DE-II and DE-III were estimated to be around 1.0. Unlike the latter two fractions, DE-I was virtually lipid-free. Analyses of the three fractions by polyacrylamide gel electrophoresis, electrofocusing and DEAE-cellulose chromatography before and after delipidation suggested that the difference between DE-I and DE-II was in part due to fatty acids bound to DE-II. In contrast, DE-III appeared to be somewhat different from these forms in its protein structure, though tryptic peptide mappings of the three fractions did not reveal clear differences among them. Analysis of the primary structure was made on the most abundant fraction, DE-II, to investigate the relationship among the three fractions and to other proteins. The protein was a single chain consisting of 127 amino acid residues and had a mostly acetylated NH2 terminus and a free sulfhydryl group. The complete sequence of Z-protein showed striking homology to cellular retinoid binding proteins and peripheral nerve myelin P2 protein, which indicated the presence of a new family of cellular lipid-binding proteins diverged from a common ancestor. A possible intragenic duplication of Z-protein was also suggested.

The influence of soluble binding proteins on lipophile transport and metabolism in hepatocytes

A theory is presented that deals with the involvement of the intracellular binding proteins ligandin and aminoazodye-binding protein A (otherwise known as Z-protein or fatty-acid-binding protein) on the uptake and intracellular transport and metabolism of their ligands. Equations are derived that combine steady-state diffusional fluxes of small molecules that are (a) free in the aqueous phase of the cell, (b) bound to the two proteins and (c) partitioned into intracellular membranes, for model systems that resemble conditions in the rat hepatocyte. These equations are then combined with expressions for the enzyme-catalysed metabolic reactions undergone by these small molecules to assess the influence of diffusion rats on the overall metabolic rates. It is concluded that ligandin and protein A can enhance the rate of intracellular of their ligands by an order of magnitude or more and that this could make the hepatocyte several times more efficient in metabolizing these ligands. Various ways of testing this theory are discussed.

A low molecular weight binding protein for organic anions (Z protein) from human hepatic cytosol: purification and quantitation

Human Z protein from liver was purified to homogeneity. The protein has a molecular weight of 11,000 an an isoelectric point of pH 5.8. Circular dichroism spectra of Z protein-bilirubin (unconjugated and diglucuronide) complexes revealed two ellipticity extrema, a negative peak at 460 nm, and a positive peak at 410 nm. Human serum albumin had a higher affinity for bilirubin than did Z protein. Fluorescence studies showed the approximate association constants of this protein and bilirubin, bromosulfophthalein, and indocyanine green were 10(6) M-1, 10(5) M-1, and 10(6) M-1, respectively. Immunofluorescence studies revealed that Z protein was localized in the cytoplasm of hepatocytes, proximal tubular epithelium, and epithelial cells of the small intestine. Radioimmunoassay studies were done to assess the amount of Z protein in controls and in various liver diseases. The highest concentrations of Z protein were found in the liver, kidney, heart muscle, and small intestine, in that order. In cases of acute and chronic hepatitis, hepatic Z protein concentrations were generally decreased, whereas serum Z protein concentrations were increased. In contrast, both serum and hepatic Z protein concentrations were decreased in cases of constitutional hyperbilirubinemia.

Purification of acidic Z protein from human liver

Preparation of Z protein from human liver is described. Z protein consists of 2 forms which have different isoelectric points, pI 5.8 and pI 8.7 respectively. The acidic Z protein has a molecular weight of about 11,000 and has binding affinity for BSP using gel filtration.

Spectroscopic studies of the binding of bilirubin by ligandin and aminoazo-dye-binding protein A

Ligandin and aminoazo-dye-binding protein A both bind bilirubin at a single site. Quantitative studies of the interactions using difference spectrophotometry show that at pH 7.0, protein A binds the tetrapyrrole with an association constant (K) greater than or equal to 2 X 10(7) litre/mol, whereas binding by ligandin is slightly weaker (K = 7 X 10(6) litre/mol) at this pH. The protein-bilirubin complexes give rise to absorption and fluorescence spectra quite different from those of unbound bilirubin and also to large Cotton effects. It appears that on binding to both proteins, the ligand is forced into a rigid twisted configuration in a hydrophobic environment. Ligandin and protein A resemble serum albumin in their interactions with bilirubin.

A low-molecular-weight protein from rat liver that resembles ligandin in its binding properties

A protein of S20,W 1.6S and mol.wt. 14000, which binds covalently a metabolite of the aminoazodye carcinogen NN-dimethyl-4-amino-3'-methylazobenzene, was isolated from rat liver cytosol from both carcinogen-treated and normal rats. The protein binds non-covalently palmitoyl-CoA, fatty acids, bilirubin, sex steroids and their sulphates, bile acids and salts, bromosulphophthalein, diethylstilboestrol and 20-methylcholanthrene with a wide range of affinities. The protein is isolated as three components with isoelectric points of 5.0, 5.9 and 7.6 by a method involving isoelectric focusing. All three components have closely similar amino acid analyses, tryptic-peptide 'maps' and u.v. spectra. Each single component redistributes into all three on further electrophoresis. However, the three forms differ in their binding characteristics, the form of pI 7.6 having much the highest affinity for compounds bound non-covalently. The protein was identified immunologically in rat liver, small intestine, adipose tissue, skeletal muscle, myocardium and testis. The protein was compared with other hepatic binding-protein preparations of similar molecular weight.