Properties of a non-specific phospholipid-transfer protein purified from rat lung

Prognostic significance of low serum levels of Clara cell phospholipid-binding protein in occupational aluminium neurotoxicity

The relationship between respiratory and neurological effects of exposure to aluminium (Al) was investigated in a group of foundry workers exposed to Al at concentrations below the threshold limit value (TLV) binding in Poland (2.0 mg Al2O3 m(-3)). Neurological and neurophysiological parameters indicated subclinical effects of Al exposure on the nervous system. The measurement of serum anti-inflammatory Clara cell protein (CC16) was employed as a peripheral marker of the lung epithelium function. There was a strong inverse relationship between serum Al (Al-S) and CC16 concentrations (p = 0.006). The lowest CC16 concentrations were found in serum of workers characterised by subjective symptoms of the central nervous system (CNS) and abnormal results of neurophysiological examinations (EEG and VEP). Low serum CC16 concentrations and enhanced Al and iron (Fe) levels were also observed in the younger age group of workers with the subjective CNS symptoms and abnormal VEP results, which suggests that Fe is implicated in strengthening of the neurotoxic Al potential. The results of our study support the hypothesis that subclinical neurological symptoms (especially abnormal VEP) are most likely associated with internalisation of Al ions with lipid fractions of the lung epithelium, which in turn may help Al ions overcome the blood-brain barrier. Low serum CC16 concentrations (<10 microg L(-1)) were noted in workers with the abnormal results of neurological (CNS) and neurophysiological (EEG and VEP) examinations as well as with Al body burden manifested by urinary excretion (Al-U) below 60 microg L(-1) and Al-S concentration of 2 microg L(-1). This concentration may be considered as a threshold allowable biological concentration of aluminium.

Phosphatidylinositol transfer protein in lung: cellular and subcellular localization

Determination of the cellular distribution of phosphatidylinositol transfer protein in rat lung by immunocytochemistry revealed that the protein is more readily observed in the nonciliated bronchial epithelial cells (Clara cells) than in other lung cells. By light microscopy, the phosphatidylinositol transfer protein (PtdIns-TP) was localized to the dome-shaped apical region of Clara cells that were identified by staining with an antibody to Clara cell protein. Further investigation by electron microscopy revealed that the PtdIns-TP accumulated at the limiting membrane surrounding secretory granules and at the apical plasma membrane. This localization is compatible with the proposed roles for PtdIns-TP in formation of vesicles and exocytosis of secretory granules and, when considered in the context of the proposed role of PtdIns-TP in phosphatidylinositide metabolism, suggests that phosphatidylinositides may be involved in the mechanisms regulating Clara cell secretion.

Purification, characterization and substrate specificity of rabbit lung phospholipid transfer proteins

Three phospholipid transfer proteins, namely proteins I, II and III, were purified from the rabbit lung cytosolic fraction. The molecular masses of phospholipid transfer proteins I, II and III are 32 kilodaltons (kDa), 22 kDa and 32 kDa, respectively; their isoelectric point values are 6.5, 7.0 and 6.8, respectively. Phospholipid transfer proteins I and III transferred phosphatidylcholine (PC) and phosphatidylinositol (PI) from donor unilamellar liposomes to acceptor multilamellar liposomes; protein II transferred PC but not PI. All the three phospholipid transfer proteins transferred phosphatidylethanolamine poorly and showed no tendency to transfer triolein. The transfer of [14C]PC from unilamellar liposomes to multilamellar liposomes facilitated by each protein was affected differently by the presence of acidic phospholipids in the PC unilamellar liposomes. In an equal molar ratio of acidic phospholipid and PC, phosphatidylglycerol (PG) reduced the activities of proteins I and III by 70% (P = 0.0004 and 0.0032, respectively) whereas PI and phosphatidylserine (PS) had an insignificant effect. In contrast, the protein II activity was stimulated 2-3-times more by either PG (P = 0.0024), PI (P = 0.0006) or PS (P = 0.0038). In addition, protein II transferred dioleoylPC (DOPC) about 2-times more effectively than dipalmitoylPC (DPPC) (P = 0.0002), whereas proteins I and III transferred DPPC 20-40% more effectively than DOPC but this was statistically insignificant. The markedly different substrate specificities of the three lung phospholipid transfer proteins suggest that these proteins may play an important role in sorting intracellular membrane phospholipids, possibly including lung surfactant phospholipids.

Lipid transfer proteins

Translocations of various lipid species between membranes have been extensively studied. The transport of water-insoluble lipids is thought to require the participation of lipid transfer proteins (LTP). Several LTP, differing in their physiochemical properties and substrate specificities, have been purified to homogeneity from blood plasma, eucaryotic and procaryotic cells. Depending on their site of activity, they can be classified as extracellular and intracellular LTP. Extracellular LTP are found in the blood plasma and intracellular LTP, which were originally characterized as phospholipid exchange proteins, are ubiquitous in nature. Despite the enormous knowledge about their physicochemical properties and their function in vitro their physiological role has not been clearly demonstrated. However, their ubiquitous occurrence indicates an important role in cellular events. This review gives an overview of this interesting category of proteins, which are able to catalyze inter-membrane transfer and exchange of lipids.

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.

Amino-terminal sequence of a phospholipid transfer protein from rat lung

A phospholipid transfer protein from rat lung has been characterized in terms of the amino-terminal sequence. The sequence is Val-Leu-Leu-Lys-Glu-Tyr-Arg-Val-Ile-Leu-Pro-(Val)-His-Val-Asp-Glu-Tyr-Gln-Val- Gly. Comparison of the amino-terminal sequence of the protein from lung with sequences from phosphatidylcholine transfer protein and non-specific phospholipid transfer protein from bovine liver revealed no apparent sequence homology. The sequence showed no homology with fatty acid binding proteins or cellular retinoid binding proteins.

Evidence for a high activity of sphingomyelin biosynthesis by phosphocholine transfer from phosphatidylcholine to ceramides in lung lamellar bodies

Biosynthesis of sphingomyelin from ceramides was investigated in lung subcellular fractions by incubating a lyophilized mixture of albumin and subcellular fraction (0.1-0.2 mg of protein) coated with [acyl-14C]-ceramide and phosphatidyl[methyl-3H]choline in Tris-buffer. The lamellar-body-rich fraction exhibited the highest specific activity for sphingomyelin biosynthesis measured by 14C incorporation into sphingomyelins or by [3H]phosphocholine transfer from phosphatidylcholines. Plasma membranes formed the next most active fraction, followed by the 'smooth' and, then, the 'rough' endoplasmic reticulum. Sphingomyelin biosynthesis by lamellar bodies was optimum at pH 7.4 and was inhibited by sphingomyelins formed. Slight inhibitory effects were also observed with Mn2+, Ca2+ and lysophosphatidylcholine. Phosphocholine transfer from CDPcholine was not observed under the reaction conditions employed. Ceramide conversion and phosphocholine transfer increased with ceramide concentration to reach a maximum at about 0.06 mM. The highest conversion rate was observed when 18:1 ceramide was used as an acceptor. When 1-palmitoyl-2-oleoylphosphatidylcholine was the phosphocholine donor, the overall biosynthesis of sphingomyelin was much higher than when using dipalmitoylphosphatidylcholine. These results suggest the possible involvement of the studied reaction in the control of the degree of saturation of the surfactant phosphatidylcholine.

Phospholipid transfer proteins: mechanism of action

Phospholipid transfer proteins are generally localized in the cytosolic fraction of cells and are capable of catalyzing the flux of phospholipid molecules among membranes. Artificial membranes also participate in protein-catalyzed phospholipid movements. In this review the major phospholipid transfer proteins are discussed with respect to their phospholipid substrate specificity and the contributions of membrane physical properties to this process. The phenomenon of net transfer of phospholipids is described. The use of various kinetic approaches to the study of these catalysts is reviewed. A detailed consideration of the distinct phospholipid binding and membrane interaction domains of one phospholipid transfer protein is presented. Finally, some recent applications of phospholipid transfer proteins to the examination of membrane structure and function and further directions for the continued research activity with this class of proteins are summarized.

Phospholipid transfer proteins from lung, properties and possible physiological functions

Phospholipid transfer proteins have been found in lung just as they have in tissues throughout the body. There is speculation that the proteins are involved in membrane biogenesis and in determining the phospholipid composition of membranes. For this reason the lung, which contains subcellular organelles of distinct phospholipid composition, is of interest in terms of its complement of phospholipid transfer proteins. The lamellar bodies of pulmonary type II alveolar cells have a phospholipid composition unique in terms of the proportions of dipalmitoyl phosphatidylcholine and phosphatidylglycerol. Studies of the phospholipid transfer proteins in lung have demonstrated two molecular species of the transfer proteins that differ significantly from those found in liver and other tissues. These proteins show specificity for the transfer of dipalmitoyl phosphatidylcholine and phosphatidylglycerol.

Purification and characterization of fatty acid binding protein in mammalian lung

A fatty acid-binding protein (FABP) has been isolated and characterized from rat lung tissue. Rat lung FABP has a slightly higher molecular weight than liver FABP, but immunologically, lung FABP is similar to that of liver FABP. Long chain acyl CoA synthetase, a key enzyme in fatty acid metabolism is stimulated by partially purified lung FABP, suggesting a physiologic role of the protein in the activation of long chain fatty acids in pulmonary tissue.

Protein composition of rabbit alveolar surfactant subfractions

The goal of this investigation was to characterize the proteins in subfractions of alveolar surfactant obtained by lung lavage and separated by differential centrifugation. It was previously demonstrated that the material in the more sedimentable fraction, which was enriched in tubular-myelin and was surface-active may be a precursor to the less sedimentable, vesicular, inactive material [1]. Separation of the proteins by polyacrylamide gel electrophoresis showed that the more sedimentable subfractions and rabbit surfactant isolated by conventional methods contained proteins with molecular weights comparable to those previously reported for alveolar surface active material (approximately 36 000 and 10 000). The less sedimentable subfractions contained less of these proteins. Immunoblots with anti-dog surfactant apoprotein antibodies, which cross-react with rabbit proteins, supported these observations. Immunoblots also showed that all of the subfractions contained serum proteins and secretory IgA, with the less sedimentable subfractions containing more secretory IgA. These results suggested that changes in protein composition may accompany functional changes in surfactant in the alveoli.

Acyl-chain specificity and membrane fluidity. Factors which influence the activity of a purified phospholipid-transfer protein from lung

A purified phospholipid-transfer protein from rat lung has been characterized in terms of the specificity of the protein for phosphatidylcholine molecules with different apolar moieties. The study demonstrated that the lung-phospholipid-transfer protein discriminates between dipalmitoylphosphatidylcholine and molecular species of phosphatidylcholine with unsaturated acyl chains. The initial rate of transfer of dipalmitoylphosphatidylcholine is 1.5-fold greater than the rate of transfer of dioleoylphosphatidylcholine, 1-palmitoyl-2- arachidonylphosphatidylcholine , or egg phosphatidylcholine under most assay conditions. Although the protein preferentially transfers dipalmitoylphosphatidylcholine, the incorporation of increasing mole percentages of dipalmitoylphosphatidylcholine into unilamellar phosphatidylcholine vesicles profoundly affects their effectiveness as donors for phosphatidylcholine transfer by the transfer protein. At 60 mol% dipalmitoylphosphatidylcholine, the rate of transfer is one-third that observed when vesicles are composed of 100% egg phosphatidylcholine. Decreases in membrane fluidity as estimated by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene correlate with decreases in the effectiveness of the vesicles as donors in the phospholipid-transfer reaction. The conclusion from these studies is that the rate of transfer of phosphatidylcholine by the purified phospholipid-transfer protein from lung is determined by physical properties of membrane interfaces with which the protein interacts, as well as by the specificity of the phospholipid-transfer protein for different molecular species of phosphatidylcholine.