Z protein in hepatic uptake and esterification of long-chain fatty acids

Thermodynamic and kinetic properties of fatty acid interactions with rat liver fatty acid-binding protein

Fatty acid-binding protein from rat liver (L-FABP) binds 2 fatty acids (FA) per protein, in contrast to FABPs from adipocyte, heart, and intestine, for which binding and structural studies are consistent with a single FA binding site. To understand better the unique characteristics of L-FABP, we have carried out equilibrium binding and kinetic measurements of long chain FA using the fluorescent probes of free fatty acids (FFA), ADIFAB and ADIFAB2, to monitor the concentration of FFA in the reaction of FA with L-FABP. We found that the dissociation constants (Kd) ranged from about 1 nM to 4 microM, being largest for myristate at 45 degrees C and smallest for oleate at 10 degrees C, and that 2 FA were bound per L-FABP for all temperatures and FA. The binding measurements also revealed that at temperatures below 37 degrees C, affinities for the two binding sites differ by between 5- and 20-fold but as the temperature was increased, the affinities converge toward equal values. Off-rate constants (koff) were similar for all FA and for temperatures between 10 and 45 degrees C, ranged from about 0.1 s-1 to 50 s-1. Moreover, for all FA, koff values for dissociation from both the high and low affinity sites were similar, indicating that binding affinity differences at the lower temperatures reflect lower on-rates for binding to the low affinity site. The temperature at which the affinities of the two sites become equivalent depends upon the FA; higher temperatures (45-50 degrees C) are required for the unsaturated FA and myristate than for the longer chain saturated FA (<37 degrees C). This transition from different to equivalent affinity binding sites at specific temperatures reflects a nonlinear van't Hoff behavior of the high affinity site, which in turn is a reflection of large heat capacity changes (between -0.6 and -1.2 kcal K-1 mol-1) that accompany FA binding to the high affinity site. These heat capacity changes, which are unique to L-FABP, do not appear to be correlated with a significant conformational change upon ligand binding. The differences between long chain saturated and unsaturated FA suggest that the conformation of FA bound to L-FABP may differ with both FA type and temperature, and that, in comparison to other FABPs, L-FABP may have distinctly different effects on saturated and unsaturated FA metabolism.

Kinetics of fatty acid interactions with fatty acid binding proteins from adipocyte, heart, and intestine

Rate constants for the interaction of fatty acids (FA) with fatty acid binding proteins (FABP) from adipocyte (A-FABP), heart (H-FABP), and intestine (I-FABP) were determined by using stopped-flow fluorometry and ADIFAB, the fluorescent probe of free fatty acids (FFA), or a new FFA probe, ADIFAB2, constructed by derivatizing with acrylodan the Leu72 --> Ala mutant of I-FABP. ADIFAB2, because its binding affinities are about 10-fold greater than ADIFAB, was found to be more accurate for monitoring the kinetics of the higher affinity reactions. On- (kappa on) and off- (kappa off) rate constants were determined as a function of temperature. Our results reveal that in all cases the FA-FABP equilibrium is achieved within 2 s at 37 degrees C and within 20 s at 10 degrees C. Off-rate constants varied by about 10-fold among the different underivatized FABPs; kappa off values were smallest for H-FABP and largest for A-FABP, while kappa on values for these proteins generally varied by less than 2-fold. The results show that the previously reported larger affinities of I- and H-FABPs as compared to A-FABP are primarily a reflection of larger kappa on values for I-FABP and smaller kappa off values for H-FABP. Eyring transition state theory was used to evaluate the activation thermodynamic parameters for both on- and off-reactions and the results show that in virtually all cases the rate-limiting steps are predominantly enthalpic. Activation free energies for binding to ADIFAB are generally composed of about 8 kcal/mol unfavorable enthalpy and about a 1 kcal/mol favorable entropic contribution. For the underivatized FABPs the activation free energies are all about 7 +/- 0.3 kcal/mol, suggesting that the transition state for entering or leaving the binding site involves a common protein structural change. We suggest that entering or leaving the FABP binding cavity involves similar mechanisms for all 3 FABPs and may involve amino acid residues located within the portal regions of these proteins.

Thermodynamics of fatty acid binding to fatty acid-binding proteins and fatty acid partition between water and membranes measured using the fluorescent probe ADIFAB

Using the fluorescent probe ADIFAB (acrylodan-derivatized intestinal fatty acid-binding protein) to determine the equilibrium concentration of the free (unbound) fatty acid (FFA), dissociation constants were measured between 10 and 50 degrees C for the interaction of five different long chain fatty acids (FA) with fatty acid-binding proteins (FABP) from adipocyte, intestine, and heart. Gibbs free energies (delta G) determined from the dissociation constants were between about -9 and -11 kcal/mol at 25 degrees C. Thermodynamic parameters for binding were determined using van't Hoff plots of the dissociation constants, which range, over the entire temperature region, between 2 and 3000 nM. For all the unlabeled FABPs, free energies of binding were dominated by large negative enthalpies that ranged from -7 to -12 kcal/mol, and the enthalpies tended to decrease with increasing FA unsaturation. The entropic contributions (-T detla S) at 25 degrees C between -4 and +2 kcal/mol and tended to increase with increasing FA unsaturation. To assess the role of FA aqueous solubility in FABP binding, measurements of the partition of FA between unilamellar lipid vesicles and water were also done using ADIFAB; the lipid/water partition coefficients (Kp) determined from these measurements were found to be independent of temperature. The binding of FA to FABP is governed by the sum of contributions of various interactions between FA, water, and FABP. An analysis of the individual contributions suggests that the net free energy of binding results from the canceling in part of a number of separate quite large contributions. The entropic contributions sum almost to zero for most FA and FABPs as a result of the canceling of a large increase in bulk solvent entropy by decreases in configurational entropy upon FA binding to FABP. The net, approximately -10 kcal/mol enthalpy of binding, probably results from an increase in FA configurational enthalpy upon binding to FABP plus a large negative enthalpy from the interaction between the FA and the FABP. This large enthalpy of the FA-FABP interaction suggests that in addition to previously identified specific interactions between the carboxylate portion of the FA and charged amino acids within the binding cavity, other significantly larger enthalpic interactions, presumably involving the hydrocarbon portion of the FA, must contribute to the binding energy.

Regulation of hepatic level of fatty-acid-binding protein by hormones and clofibric acid in the rat

Regulation of the hepatic level of fatty-acid-binding protein (FABP) by hormones and p-chlorophenoxyisobutyric acid (clofibric acid) was studied. The hepatic level of FABP, measured as the oleic acid-binding capacity of the cytosolic FABP fraction, was decreased in streptozotocin-diabetic rats. The level of FABP was markedly increased in adrenalectomized rats, and the elevation was prevented by the administration of dexamethasone. Hypothyroidism decreased the level of FABP and hyperthyroidism increased it. A high correlation between the incorporation of [14C]oleic acid in vivo into hepatic triacylglycerol and the level of FABP was found for normal, diabetic and adrenalectomized rats. The level of FABP was increased by administration of clofibric acid to rats in any altered hormonal states, as was microsomal 1-acylglycerophosphocholine (1-acyl-GPC) acyltransferase, a peroxisome-proliferator-responsive parameter. These results suggest that the hepatic level of FABP is under regulation by multiple hormones and that clofibric acid induces FABP and 1-acyl-GPC acyltransferase by a mechanism which may be distinct from that by which hormones regulate the level of FABP.

In situ binding of fatty acids to the liver fatty acid binding protein: analysis using 3-[125I]iodo-4-azido-N-hexadecylsalicylamide

A photoactivatable radioiodinated fatty acid analogue, 3-[125I]iodo-4-azido-N-hexadecylsalicylamide (125I-AHS) has been synthesized and used to investigate the involvement of cellular lipid carriers in hepatic fatty acid utilization. Photoactivation of Hep G2 internalized 125I-AHS revealed that several cellular proteins were crosslinked with the radiolabeled fatty acid analogue. Three predominant proteins in the membrane fraction of the cell with molecular masses 17, 50 and 127 kDa were crosslinked with the lipid analogue, as determined using autoradiography after SDS-PAGE. Three other proteins in the soluble fraction of the cell, with molecular masses 14, 24 and 35 kDa, were also labeled in situ. In contrast to the other labeled proteins, the fatty acid analogue accumulated on the cytoplasmic 14 kDa protein in a time and temperature dependent fashion. The in situ-labeled 14 kDa protein was identified from primary rat hepatocytes as the liver fatty acid binding protein by partial purification and its ability to be immunoprecipitated with immunospecific L-FABP antiserum. Collectively the results indicate that fatty acids traverse the plasma membrane and are bound cytoplasmically by the liver fatty acid binding protein, as well as other proteins in the cell. This represents the first demonstration in intact hepatocytes that the liver fatty acid binding protein participates in the process of intracellular fatty acid trafficking, and supports a model in which cytoplasmic lipid carriers solubilize fatty acids as a step in their metabolic utilization.

The effect of dietary alpha-bromopalmitate on blood lipids in the rat

When alpha-bromopalmitate was fed to rats for 9-30 days, the level of serum triacylglycerol increased up to 2-fold over the concentration of controls. alpha-Bromopalmitate treatment had no effect on concentration of complex lipids in liver, while the triacylglycerol level in heart was significantly enhanced. From metabolic studies using isolated hepatocytes and liver microsomes, it is suggested that the increased serum triacylglycerol level after alpha-bromopalmitate feeding is mainly due to reduced fatty acid oxidation in both liver and peripheral tissues, and to a lesser extent, to inhibited fatty acid uptake and esterification.

Effect of liver fatty acid binding protein on fatty acid movement between liposomes and rat liver microsomes

Although movement of fatty acids between bilayers can occur spontaneously, it has been postulated that intracellular movement is facilitated by a class of proteins named fatty acid binding proteins (FABP). In this study we have incorporated long chain fatty acids into multilamellar liposomes made of phosphatidylcholine, incubated them with rat liver microsomes containing an active acyl-CoA synthetase, and measured formation of acyl-CoA in the absence or presence of FABP purified from rat liver. FABP increased about 2-fold the accumulation of acyl-CoA when liposomes were the fatty acid donor. Using fatty acid incorporated into liposomes made either of egg yolk lecithin or of dipalmitoylphosphatidylcholine, it was found that the temperature dependence of acyl-CoA accumulation in the presence of FABP correlated with both the physical state of phospholipid molecules in the liposomes and the binding of fatty acid to FABP, suggesting that fatty acid must first desorb from the liposomes before FABP can have an effect. An FABP-fatty acid complex incubated with microsomes, in the absence of liposomes, resulted in greater acyl-CoA formation than when liposomes were present, suggesting that desorption of fatty acid from the membrane is rate-limiting in the accumulation of acyl-CoA by this system. Finally, an equilibrium dialysis cell separating liposomes from microsomes on opposite sides of a Nuclepore filter was used to show that liver FABP was required for the movement and activation of fatty acid between the compartments. These studies show that liver FABP interacts with fatty acid that desorbs from phospholipid bilayers, and promotes movement to a membrane-bound enzyme, suggesting that FABP may act intracellularly by increasing net desorption of fatty acid from cell membranes.

Fatty acid-binding capacity of cytosolic proteins of various rat tissues: effect of postnatal development, starvation, sex, clofibrate feeding and light cycle

Fatty acid-binding capacity of dealbuminized, delipidated cytosolic proteins from rat tissues was studied with a radiochemical binding assay. Oleate-binding capacity ranges from 1.6 to 4.4 pmol/micrograms cytosolic protein in liver, heart, kidney, adrenal, brain, skeletal muscle and diaphragm. Differences in binding affinity indicate the presence of different fatty acid-binding proteins in these tissues. No change in fatty acid-binding protein content of heart and liver cytosol was observed during postnatal development up to 70 days. Starvation did not affect the fatty acid-binding capacity of heart cytosol, but increased the oleate-binding capacity in liver cytosol. Sex-related differences of binding by heart and liver cytosolic proteins were found with oleate, but not with palmitate. Fatty acid-binding capacity of liver and heart cytosol did not show marked diurnal variation. Clofibrate treatment had different effects on the oleate-binding capacity of cytosolic proteins: an increase in liver and kidney, no change in skeletal muscle and a decrease in heart. The results are discussed in relation to data concerning fatty acid oxidation.

Fatty acid-binding protein in liver and small intestine of the preruminant calf

Cytosol obtained from differential centrifugation of homogenates from liver and small intestine mucosa was incubated with 1-[14C] oleic acid or 1-[14C] palmitic acid and filtered through Sephadex G-75. Elution profiles for both tissues showed radioactivity in two main peaks, the first corresponding to binding of fatty acid to high molecular weight proteins and the second to a protein fraction with a molecular weight of approximately 12,000 daltons. The low molecular weight fraction had high fatty acid-binding activity, which was greater for oleic than palmitic acid. The findings demonstrate the presence of fatty acid-binding protein in liver and intestinal mucosa of the preruminant calf.

Role of sterol carrier protein in cholesterol metabolism

This report summarizes our recent studies on the protein known as sterol carrier protein (SCP) or fatty acid binding protein (FABP). SCP is a highly abundant, ubiquitous protein with multifunctional roles in the regulation of lipid metabolism and transport. SCP in vitro activates membrane-bound enzymes catalyzing cholesterol synthesis and metabolism, as well as those catalyzing long chain fatty acid metabolism. SCP also binds cholesterol and fatty acids with high affinity and rapidly penetrates cholesterol containing model membranes. Studies in vivo showed SCP undergoes a remarkable diurnal cycle in level and synthesis, induced by hormones and regulated in liver by translational events. SCP rapidly responds in vivo to physiological events and manipulations affecting lipid metabolism by changes in level. Thus SCP appears to be an important regulator of lipid metabolism. Preliminary evidence is presented that SCP is secreted by liver and intestine into blood and then taken up by tissues requiring SCP but incapable of adequate SCP synthesis.

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.

Effects of long-term administration of clofibric acid on peroxisomal beta-oxidation, fatty acid-binding protein and cytosolic long-chain acyl-CoA hydrolases in rat liver

Long-term effects of rho-chlorophenoxyisobutyric acid (clofibric acid) on inductions of peroxisomal beta-oxidation, fatty acid-binding protein and cytosolic acyl-CoA hydrolases in rat liver were studied. Male rats were fed clofibric acid at a dietary concentration of 0.25% for 22 weeks. The induction of peroxisomal beta-oxidation activity lasted throughout the long-term treatment of rats, the activity being a half that of rats treated with clofibric acid for 2 weeks. cytosolic long-chain acyl-CoA hydrolase I and II were both induced by the long-term and the short-term treatment of age-matched rats with clofibric acid, although the ability to induce hydrolase I decreased greatly by aging of rats. There was little difference in the inducing effect on fatty acid-binding protein between the long-term treatment and the short-term treatment. These results suggest that the inductions of peroxisomal beta-oxidation, fatty acid-binding protein and two cytosolic long-chain acyl-CoA hydrolases are essential responses of rats to clofibric acid (but not the brief events which occur in only the first stage of the continuous treatment with clofibric acid).

Does Z-protein have a role in transport of bilirubin and bromosulfophthalein by isolated perfused rat liver?

Bilirubin and other organic anions are transported in serum avidly bound to albumin from which they are extracted and transferred into the hepatocyte where they bind to cytosolic proteins. Two abundant organic anion binding proteins, ligandin and Z-protein, were previously purified from liver cytosol and characterized. Other studies in isolated perfused rat liver revealed that selectively increased cytosolic ligandin concentration, following phenobarbital treatment or thyroidectomy, directly correlated with net bilirubin uptake which resulted from reduced bilirubin efflux. To clarify the role of Z-protein in hepatic organic anion transport, we have now determined the kinetics of bilirubin and bromosulfophthalein (BSP) uptake in isolated perfused liver of normal rats and compared results to rats in which Z-protein, but not ligandin, was selectively increased following treatment with clofibrate (ethylchlorophenoxy-isobutyrate). These studies revealed that despite a 147% induction of Z-protein in treated animals, there was no effect on influx or efflux of tracer doses of bilirubin or BSP. Addition of albumin to the protein-free 10% fluorocarbon perfusate reduced influx of 3H-bilirubin (p less than 0.03) and tended to reduce influx of BSP. In this situation, there was still no influence of Z-protein concentration on efflux. These studies indicate that Z-protein does not appear to play a role in the hepatic uptake of bilirubin and BSP.

Species difference of liver cytosolic fatty acid-binding protein in rat, mouse and guinea pig

Binding properties of liver cytosolic protein for oleic acid, palmitoyl-CoA and bromosulphophthalein (BSP) were compared for rat, mouse and guinea pig. Hepatic cytosol of rat, mouse and guinea pig contained proteins with a molecular weight of ca. 12,000 and had an affinity for [1-14C]-oleic acid. The concentration of fatty acid-binding protein (FABP) was almost the same in livers of the animals of the 3 species and was ca. 50 micrograms/mg cytosolic protein. Electrophoretic studies revealed that FABP from hepatic cytosol of rat, mouse and guinea pig, purified with affinity chromatography, are distinct from one another in terms of their charge. FABP of rat liver was capable of binding any 3 ligands--oleic acid, palmitoyl-CoA and BSP--at relatively high binding capacity. FABP of mouse liver also bound oleic acid and palmitoyl-CoA to a great extent, but its binding capacity for BSP was only one-third that of rat liver. FABP of guinea pig liver bound less oleic acid and palmitoyl-CoA than rat liver, whereas it had almost the same binding capacity for BSP as rat liver.

Regulation of Lipid Metabolism by a Lipid-Carrying Protein

The overall conclusion to be made from the information presented here is that for many reasons SCP is a highly unusual protein. Some of these reasons are, first, SCP serves as cofactor for a number of different membrane-bound enzymes catalyzing specific steps in lipid metabolism. Second, SCP is involved in intracellular transport or movement of both cholesterol and fatty acids. Third, SCP is remarkably abundant and ubiquitous; its structure is conserved throughout nature. Fourth, SCP is exported to the blood stream from its site of synthesis by some, perhaps unique, mechanism and then rapidly taken up by specific tissues, e.g., the adrenal. Fifth, SCP is free in the cytosol and can also move to the inner mitochondrial membrane, where it is tightly bound. Sixth, SCP undergoes a dramatic diurnal variation in amount, reflecting changes in synthetic rate. Its half-life is less than an hour. Seventh, the diurnal variation in amount is triggered by feeding and influenced by several hormones. The diurnal variation is lost but a high level of SCP is maintained in the face of debilitating conditions, i.e., starvation, diabetes. Eighth, malignant cells exhibit defects in the uptake, synthesis, or turnover of SCP. Ninth, the synthesis of SCP is regulated by the efficiency of translation of its ever abundant mRNA. Tenth, there is much more to be learned about the functions and regulation of SCP.

Concomitant increase in hepatic triacylglycerol biosynthesis and cytosolic fatty-acid-binding-protein content after feeding rats with a cholestyramine-containing diet

Cholestyramine feeding of rats increased the rate of palmitate and glycerol incorporation into triacylglycerols of isolated hepatocytes. Concomitantly an increase of fatty-acid binding by hepatic cytosolic proteins was observed, which could be attributed to an elevation of the content of the fatty-acid-binding protein (Mr 12000). The involvement of this protein in cholesterol, bile-acid and triacylglycerol metabolism is discussed.

Effects of peroxisome proliferators on fatty acid-binding protein in rat liver

The relationship between hepatic fatty acid-binding protein and peroxisomal beta-oxidation was studied. Rats were fed a diet containing p-chlorophenoxyisobutyric acid (clofibric acid), 2,2'-(decamethylenedithio)-diethanol (tiadenol), di-(2-ethylhexyl)-phthalate (DEHP), di-(2-ethylhexyl)-adipate (DEHA) and acetylsalicylic acid. On the administration of these peroxisome proliferators, both [1-14C]oleic acid-binding capacity and content of fatty acid-binding protein were increased, in association with an increase in peroxisomal beta-oxidation activity. The order of the increase in binding capacity and content of fatty acid-binding protein was tiadenol greater than DEHP greater than or equal to clofibric acid greater than DEHA = acetylsalicylic acid. The order of the increase in peroxisomal beta-oxidation activity in liver was tiadenol greater than clofibric acid greater than or equal to DEHP greater than DEHA = acetylsalicylic acid. Linear regression analysis between the binding capacity or content of fatty acid-binding protein and peroxisomal beta-oxidation activity was highly significant.

A trypsin-sensitive, heat-labile, N-ethylmaleimide-sensitive factor in adipocyte post-microsomal supernatant which affects the assay of adipocyte glycerol phosphate acyltransferase activities

Addition of adipocyte 100 000 g post-microsomal supernatant to assays of glycerol phosphate acyltransferase in isolated mitochondria or microsomal fractions decreased activity at lower concentrations of palmitoyl-CoA. At higher concentrations of palmitoyl-CoA, activation was observed on addition of post-microsomal supernatant. The effect of post-microsomal supernatant to decrease activity at lower [palmitoyl-CoA] was abolished by heating or by trypsin treatment, and was also abolished by addition of N-ethylmaleimide to assays or by pretreatment of post-microsomal supernatant with N-ethylmaleimide. The stimulatory effect seen at higher [palmitoyl-CoA] was not sensitive to heat or trypsin treatment. The effect of post-microsomal supernatant at lower [palmitoyl-CoA] cannot be attributed to palmitoyl-CoA hydrolase activity. It was found that brief treatment of adipocyte mitochondria with low concentrations of trypsin was an effective way to remove contaminating microsomal glycerol phosphate acyltransferase activity. Adipocyte post-microsomal supernatant was more effective than an equivalent quantity of liver post-microsomal supernatant protein in decreasing adipocyte microsomal glycerol phosphate acyltransferase activity. The effects of the supernatants from both tissues were decreased by flavaspidic acid. Semi-purified Z-protein fraction from rat liver did not mimic the effect of adipocyte post-microsomal supernatant to decrease glycerol phosphate acyltransferase at lower [palmitoyl-CoA]. Post-microsomal supernatants obtained from noradrenaline-treated adipocytes were less effective than those from control cells in decreasing glycerol phosphate acyltransferase activity in microsomal fractions at lower [palmitoyl-CoA]. It is suggested that adipocyte cytosol may contain an acyl-CoA-binding protein or proteins differing from Z-protein in some respects. The physiological significance of the findings is briefly discussed.

Cytosolic modulators of activities of microsomal enzymes of cholesterol biosynthesis. Effects of Acyl-CoA inhibition and cytosolic Z-protein

Physiological concentrations of long-chain fatty acyl-CoAs have now been shown to inhibit microsomal methyl sterol oxidase. Acyl-CoA inhibition of hydroxymethylglutaryl-CoA reductase as well as methyl sterol oxidase can be either prevented or reversed by the addition of purified Z-protein (fatty acid-binding protein). Concomitantly, Z-protein addition decreases the extent of binding of radioactively labeled oleoyl-CoA to microsomal membranes. Free heme also inhibits hydroxymethylglutaryl-CoA reductase, and Z-protein reverses the extent of observed inhibition by binding heme analogous to the effect observed with acyl-CoAs. Similarly, Z-protein reverses substrate inhibition of acyl-CoA:cholesterol acyltransferase at high concentrations of acyl-CoA substrate. All these observations are consistent with the suggestion that, by binding acyl-CoAs and other enzyme effectors such as free heme, Z-protein modulates the effects of fluctuations of concentrations of major cellular metabolites. Furthermore, because the concentration of Z-protein is very low in rapidly growing hepatomas, such tumors may be very poorly buffered against the effects of acyl-CoAs, free fatty acids, heme and other effectors that may vary markedly by either altered metabolism or release of metabolites from necrotic tumor tissue.

Liver cytosolic fatty acid-binding proteins. Effect of diabetes and starvation

The binding of [1-(14)C]oleate to rat liver cytosol was studied, using gel filtration on Sephadex G-75 and G-50. In liver cytosols from control rats, most of the high-affinity oleate binding was in the region of 12 000-dalton proteins. In liver cytosols from diabetic and starved rats, a second peak of radioactivity appeared in the void volume. This peak was shown to be associated with a component having the molecular weight of 400 000. Evidence suggesting that a change in the composition of cytosolic binding proteins is involved is presented.

Characterization and purification of fatty acid-binding protein in rat and human adipose tissue

A protein with properties similar to fatty acid-binding protein has been isolated from rat and human adipose tissue. Comparison of fatty acid-binding protein from rat liver and adipose tissue and human adipose tissue shows that all have approximately similar molecular weights. Immunologically, rat liver fatty acid-binding protein is similar to the protein characterized from rat adipose tissue. In isolated rat fat cells the fatty acid-binding protein was demonstrated to be involved in the uptake and esterification of long-chain fatty acids. These observations constitute evidence for a potential role of this protein in the fatty acid metabolism of adipocytes.

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.

Possible involvement of fatty acid binding protein in peroxisomal beta-oxidation of fatty acids

The localization of beta-oxidation of fatty acids in isolated peroxisomes from rat liver was investigated. The enzyme system is soluble in the luminal compartment and carnitine does not appear to be involved in the transfer of the CoA derivatives through the peroxisomal membrane. Experiments involving proteolysis, inhibitors and competitive inhibition suggest that a fatty acid binding protein is responsible for the carrier process. This carrier protein seems to be present in increased amounts both in the supernatant and in the peroxisomes after clofibrate induction.

Purification and characterization of a long chain, fatty-acid-binding protein supplying the mitochondrial beta-oxidative system in the heart

A fatty-acid-binding protein with a molecular weight of approximately 12 000 was purified from rat heart and the binding investigated by electron spin resonance. The stearic acid bound to the protein was found to be transferred to the mitochondrial beta-oxidative system, suggesting a role as transcytoplasmic fatty acid carrier for this protein. For the first time a physiological cytoplasmic protein was used as a carrier supplying the mitochondrial beta-oxidative system. A new mechanism of action is proposed to explain the control exerted by this type of protein in some membrane-linked enzymatic processes.

A direct radioimmunoassay for human renin substrate and identification of multiple substrate types in plasma

Plasma renin substrate, a widely measured parameter of the renin reaction, is quantitated indirectly by the measurement of liberated angiotensin I upon exhaustive incubation of plasma with added renin. To overcome methodological problems of this assay system, we have developed a direct radioimmunoassay for this plasma protein using renin substrate purified from pooled plasma of normotensive subjects as the antigen. Comparison of substrate quantitated by the two assay systems (direct and indirect) indicates a 1:1 correlation with the exception of certain subjects with elevated substrate levels induced by estrogen therapy. To study the possibility of multiple substrate forms, we have made a comparison of substrate quantitated by both radioimmunoassays in conjunction with electrophoresis of plasma on polyacrylamide gel. One major form of substrate with a retardation factor (Rf) = 0.60 was found in normotensive and essential hypertensive subjects which gave a 1:1 correspondence on quantitation by the two methods. In contrast, six of 16 women on oral contraceptives demonstrated three forms of substrate (Rf = 0.16, 0.35, and 0.60) on electrophoresis. Substrate with Rf = 0.16 and 0.35 did not cross-react with the antiserum prepared against substrate from normotensive subjects, implying structural differences in these proteins.