Albumin bound nonesterified fatty acids inhibit in vitro lipid peroxidation

Individual nonesterified fatty acids were bound to albumin in vitro and these fatty acid albumin complexes were used to study their effect on lipid peroxidation in liver microsomes. Peroxidation was induced by various methods and malondialdehyde (MDA) was measured as an index of peroxidation. Among the fatty acids tested, albumin-bound monounsaturated fatty acids showed more inhibition of peroxidation as compared to other fatty acids. Increasing the concentration of iron in the peroxidizing system, partially reversed the inhibition by fatty acids. Moreover, albumin-bound fatty acid did not inhibit iron independent peroxidation. This suggests that, like nonesterified fatty acids, albumin-bound fatty acids inhibit peroxidation by chelating the iron. Albumin fatty acid complex, similar to the fatty acid composition present in the circulating albumin, also showed inhibition of peroxidation. These data indicate that nonesterified fatty acids even when bound to albumin are capable of inhibiting peroxidation and circulating albumin, which contains various fatty acids bound to it, may impart some antioxidant effect in addition to other plasma antioxidants.     

Mycoplasma growth factors in bovine serum fraction.

Mycoplasma growth factors in bovine serum fraction were separated by Sephadex G150 column chromatography and density ultracentrifugation. The major growth factor of bovine serum fraction eluted from the Sephadex column in the void volume. Its growth-supporting activity was greatly enhanced by the presence of bovine serum albumin in the mycoplasma culture media. Other investigators had previously identified the major growth factor in serum as an alpha-lipoprotein. Although density ultracentrifugation revealed the presence of traces of a high-density lipoprotein in bovine serum fraction, another, less dense component, isolated by ultracentrifugation (component 3) and containing cholesterol, cholesteryl esters, free fatty acids, triglycerides, and protein, but no lipoprotein, exhibited considerably more growth-supporting activity than did the high-density lipoprotein, thus indicating that at least two mycoplasma species do not require intact serum lipoprotein for growth. Both the high-density lipoprotein and component 3 exhibited maximum activity only in the presence of bovine serum albumin. A chloroform extract containing component 3 lipids combined with bovine serum albumin to form an effective, partially defined, less complex substitute for serum in mycoplasma culture media.     

CTP:phosphorylcholine cytidylyltransferase in rat lung. The effect of free fatty acids on the translocation of activity between microsomes and cytosol

Phosphatidylglycerol and oleic acid had differential effects on cytidylyltransferase activity in cytosol and microsomes. The low-molecular-weight cytidylyltransferase in cytosol was stimulated more by phosphatidylglycerol than by oleic acid, whereas microsomal activity was stimulated more by oleic acid than by phosphatidylglycerol. Microsomal activity was stimulated by several unsaturated fatty acids but was not stimulated by saturated fatty acids. Bovine serum albumin decreased cytidylyltransferase activity in microsomes in the presence or absence of oleic acid but did not alter the activity measured in the presence of phosphatidylglycerol. The addition of oleic acid to albumin/microsome mixtures in amounts exceeding the binding capacity of albumin lead to complete recovery of the oleic acid stimulation. The addition of oleic acid to postmitochondrial supernatants resulted in a translocation of cytidylyltransferase activity from cytosol to microsome. The magnitude of the shift was severalfold greater with fetal preparations than adult. The free fatty acid content of microsomes increased coincident with the translocation. Bovine serum albumin, added to postmitochondrial supernatants, caused a release of cytidylyltransferase from microsomes to cytosol and a corresponding decrease in microsomal free fatty acid content. The amount of cytidylyltransferase activity in microsomes increased shortly after birth. The increase was accompanied by an increase in free fatty acid content of the microsomes. The increase in cytidylyltransferase activity and free fatty acids which occurred in vivo following birth was nearly identical to that obtained by adding oleic acid to postmitochondrial supernatants from fetal lung. We conclude that free fatty acids may affect the intracellular activity of cytidylyltransferase by promoting the translocation of inactive cytosolic forms to microsomes as well as by stimulating microsomal bound activity.     

Utilization of free fatty acids complexed to human plasma lipoproteins by mammalian cell suspensions

The purpose of this study was to determine whether lipoprotein-bound free fatty acid could be utilized by isolated mammalian cells. Ehrlich ascites tumor cells were incubated in vitro with radioactive free fatty acids that were bound to human plasma lipoproteins. Under these conditions, lipoprotein-bound free fatty acids were readily taken up by the cells. After 2 min of incubation with free fatty acids bound to low density lipoproteins, most of the radioactivity that was associated with the cells was in the form of free fatty acids. As the incubation continued, increasing amounts of radioactivity were incorporated into CO(2) and cell lipids, particularly phospholipids. Most of the free fatty acid uptake was the result of fatty acid transfer from low density lipoproteins to the cell, not from irreversible incorporation of the intact free fatty acid-low density lipoprotein complex. Fatty acid uptake increased as the ratio of free fatty acid to low density lipoprotein was raised. When albumin was added to the medium, free fatty acid uptake decreased. A large percentage of the newly incorporated cellular radioactivity was released into the medium if the cells were exposed subsequently to a solution containing albumin. Most of the released radioactivity was in the form of free fatty acid. The results with this experimental model suggest that lipoprotein-bound free fatty acid, like albumin-bound free fatty acid, is readily available for uptake by isolated cells. The mechanism of free fatty acid utilization by the Ehrlich cell is similar when either low density lipoprotein or serum albumin serves as the fatty acid carrier.     

Composition and enzyme activities of Spiroplasma citri membranes.

Spiroplasma citri was cultured in three different media that supplied cholesterol and fatty acids from: (i) horse serum, (ii) pleuropneumonia-like organism (PPLO) serum fraction, or (iii) bovine serum albumin-fatty acid-cholesterol. The ability of PPLO serum fraction to support growth varied by lot number. Neither PPLO serum fraction nor the bovine serum albumin medium supported growth as well as the horse serum medium. Analysis of cholesterol, lipid phosphorus, and membrane protein showed the horse serum- and PPLO-grown cells to be indistinguishable, but the bovine serum albumin-grown cells were deficient in lipid phosphorus. The three cultures did not show markedly different fatty acid compositions, but, in all cases, the cultures preferentially incorporated palmitic acid and discriminated against linoleic acid. Cultures grown for different times from logarithmic growth through a degenerative phase showed relatively constant ratios of cholesterol/protein and lipid phosphorus/protein. Fatty acid composition was also relatively constant at the different stages. Adenosine triphosphatase and p-nitrophenyl phosphatase were mainly associated with the membrane, whereas reduced nicotinamide adenine dinucleotide oxidase was either readily removed or not associated with the membrane. The reduced nicotinamide adenine dinucleotide oxidase was inactivated at temperatures above 35 degrees C.     

Regulation of palmitic acid synthesis in cultured glial cells: effects of lipid on fatty acid synthetase, acetyl-CoA carboxylase, fatty acid and sterol synthesis.

Abstract— C6 glial cells in culture were utilized to study the regulation of the important lipogenic enzymes, fatty acid synthetase and acetyl-CoA carboxylase, and the synthesis of fatty acids and sterols. Regulation of these phenomena by lipid was demonstrated by the following observations. First, removal of serum from the culture medium was accompanied over the next five days by 2–3-fold increases in the lipogenic enzymatic activities and in 5–15-fold increases in rates of incorporation of acetate into fatty acids and sterols. Second, cells grown in delipidated serum exhibited approx 2-fold higher levels of activity of the lipogenic enzymes and 5–10-fold higher rates of synthesis of fatty acids and sterols than cells grown in normal calf serum. Third, cells grown in serum-free medium supplemented with concentrations of fatty acid comparable to those present in medium supplemented with serum exhibited activities of fatty acid synthetase comparable to those exhibited by cells grown in the serum-supplemented medium. The mechanism of these effects on fatty acid synthetase was shown by immunochemical techniques to involve alterations in content rather than in catalytic efficiency of the enzyme. The changes in content of the synthetase were caused by alterations in enzyme synthesis. In view of morphological and biochemical data suggesting that C6 cells are related to differentiating cells with properties of both astrocytes and oligodendroglia, the present data may indicate that regulation of palmitic acid synthesis by fatty acid or a product thereof occurs in brain during development.     

Release of free fatty acids from Ehrlich ascites tumor cells

Ehrlich ascites tumor cells release free fatty acids (FFA) during in vitro incubation in media that contain albumin. The released FFA are derived by lipolysis from endogenous lipid esters. Addition of glucose to the incubation medium greatly decreases the quantity of fatty acid released by the cells. Cyanide, which inhibits endogenous lipid oxidation but not lipolysis, increases the quantity of fatty acid released to media containing albumin and causes free fatty acid to accumulate in the cells in the absence of exogenous albumin. The release of fatty acid, either preformed or derived by lipolysis during prolonged incubations, occurs under conditions of net fatty acid uptake from the incubation medium. Net release of fatty acid from the cell occurs only when fatty acid-extracted albumin is present in the extracellular medium; extrapolation of the data suggests that net release will not occur under physiological conditions. It is postulated that free fatty acid uptake and release are independent processes, the direction of net fatty acid movement being determined by the relationship between cellular free fatty acid concentration (regulating efflux) and the molar ratio of free fatty acid to albumin in the extracellular medium (regulating uptake).     

Changes in cellular composition during magnesium deficiency.

1. Concentrations and compositions of liver, serum and milk lipids of cows were measured during 6 days' starvation and serum lipids during 60 days' re-feeding. 2. The concentration of free fatty acid in serum increased fivefold during starvation. 3. The content of total lipid in liver (g/100g of liver dry matter) doubled owing to a 20-fold increase in triglyceride, an eightfold increase in cholesterol ester, a three fold increase in free fatty acid and a 20% increase in cholesterol. There were no changes in the content or composition of liver phospholipids. 4. Starvation lowered the concentrations of total lipid, phospholipid and cholesterol ester of dextran sulphate-precipitable serum lipoproteins. Total lipid and cholesterol ester concentrations in lipoproteins of d greater than 1.055 and in lipoproteins not precipitable by dextran sulphate decreased from day 4 of the starvation period and during the first 20 days' re-feeding. 5. During starvation there were decreases in percentages of stearic acid and increases in oleic acid in serum free fatty acids and triglycerides and in liver neutral lipid. 6. Throughout starvation total milk lipid yield decreased, yields and percentages of C4-14 fatty acids decreased and percentages of C18 fatty acids increased. 7. It is suggested that accumulation of triglyceride in liver may be caused by increased uptake of plasma free fatty acids without corresponding increase in lipoprotein secretion.     

Effects of Three Bacterial Infections on Serum Lipids of Rabbits

Alteration of the rabbit serum lipids as a result of three bacterial infections was studied by quantitative thin-layer and gas-liquid chromatography. Anthrax infection slightly changed the serum lipid. Cholesterol did not change, though free fatty acids, triglycerides, and cholesteryl esters doubled, and lecithin increased threefold. Tularemia infection produced drastic changes in the serum lipid content of rabbits, increasing levels of cholesterol over 4-fold, free fatty acids 17-fold, triglycerides 11-fold, cholesteryl esters 2.5-fold, and lecithin almost 3-fold. Pneumococcus infection increased cholesterol 2.5 times, free fatty acids were more than doubled, triglycerides were increased 9.5 times, and lecithin was increased almost 4 times. Gas-liquid chromatographic analysis of the methyl esters of free fatty acids showed only quantitative changes in these acids due to infection. Some possible mechanisms of alteration of serum lipid content are discussed.     

Role of bovine albumin in a serum-free suspension cell culture medium.

A serum-free culture medium, supplemented with 1% bovine serum albumin, supported the growth of both primary and continuous suspension-type cultures of various mammalian tumor cells. The role of albumin added to the medium was also studied. Defatted albumin failed to support cell growth, unless reconstituted with its lipid extract. Similarly, defatted albumin when combined with oleic and linoleic acids, also supported cell growth. Therefore, albumin-bound fatty acids play an important growth-promoting role in serum-free medium.     

Lipid composition and lipid metabolism of Spiroplasma citri.

In a horse serum-based medium containing a full complement of fatty acids, cells of Spiroplasma citri were seen to preferentially incorporate palmitic acid. In the same medium, which had a steryl ester-to-sterol ratio of 3.64, a steryl ester-to-sterol ratio of 0.23 was seen in the cells, cholesterol being preferentially incorporated over cholesteryl ester. Like most other mycoplasmas, S. citri was shown to be unable to synthesize fatty acids or esterify cholesterol. The neutral lipids of S. citri grown in a medium containing horse serum consisted of free cholesterol, cholesteryl ester, free fatty acids, triglycerides and diglycerides. All polar lipids were phospholipids, with no glycolipids detected. These phospholipids, which are characteristic of many mycoplasmas, are phosphatidyl glycerol, diphosphatidyl glycerol, and their lyso derivatives. Sphingomyelin was also incorporated when cells were grown on horse serum. A sterol requirement for the growth of S. citri was confirmed using a serum-free medium supplemented with bovine serum albumin, palmitic acid, and various concentrations of sterols dissolved in Tween 80. The addition of palmitic acid stimulated growth but was not essential for growth. S citri was shown to grow best on cholesterol and beta-sitosterol and was able to grow on stigmasterol and ergosterol to a lesser degree. No growth was obtained using mevalonate, deoxycholate, or taurodeoxycholate as an alternative to sterol. S. citri was also able to grow when palmitic acid was replaced with oleic acid, linoleic acid, or linolenic acid. Alterations in the lipid composition of the growth medium and hence in the lipid composition of S. citri induced changes in the characteristic helical morphology of the cells, concurrent with loss of cell viability. Culture, age, and pH were also factors in determining cell morphology and viability.     

Cardiac-specific knock-out of lipoprotein lipase alters plasma lipoprotein triglyceride metabolism and cardiac gene expression

Fatty acids are the primary energy source for the heart. The heart acquires fatty acids associated with albumin or derived from lipoprotein lipase (LpL)-mediated hydrolysis of lipoprotein triglyceride (TG). We generated heart-specific LpL knock-out mice (hLpL0) to determine whether cardiac LpL modulates the actions of peroxisome proliferator-activated receptors and affects whole body lipid metabolism. Male hLpL0 mice had significantly elevated plasma TG levels and decreased clearance of postprandial lipids despite normal postheparin plasma LpL activity. Very large density lipoprotein-TG uptake was decreased by 72% in hLpL0 hearts. However, heart uptake of albumin-bound free fatty acids was not altered. Northern blot analysis revealed a decrease in the expression of peroxisome proliferator-activated receptor alpha-response genes involved in fatty acid beta-oxidation. Surprisingly, the expression of glucose transporters 1 and 4 and insulin receptor substrate 2 was increased and that of pyruvate dehydrogenase kinase 4 and insulin receptor substrate 1 was reduced. Basal glucose uptake was increased markedly in hLpL0 hearts. Thus, the loss of LpL in the heart leads to defective plasma metabolism of TG. Moreover, fatty acids derived from lipoprotein TG and not just albumin-associated fatty acids are important for cardiac lipid metabolism and gene regulation.     

Deficiency in hepatic ATP-citrate lyase affects VLDL-triglyceride mobilization and liver fatty acid composition in mice

ATP-citrate lyase (ACL) is a key lipogenic enzyme that converts citrate in the cytoplasm to acetyl-CoA, the initial precursor that yields malonyl-CoA for fatty acid biosynthesis. As cytosolic citrate is derived from the tricarboxylic acid cycle in the mitochondrion, ACL catalyzes a critical reaction linking cellular glucose catabolism and lipid synthesis. To investigate the metabolic action of ACL in lipid homeostasis, we specifically knocked down hepatic ACL expression by adenovirus-mediated RNA interference in mice maintained on a low-fat or high-fat diet. Hepatic ACL abrogation markedly reduced the liver abundance of both acetyl-CoA and malonyl-CoA regardless of dietary fat intake, which was paralleled with decreases in circulating levels of triglycerides and free fatty acids. Moreover, hepatic ACL knockdown resulted in diet-dependent changes in the expression of other lipogenic enzymes, accompanied by altered fatty acid compositions in the liver. Interestingly, ACL deficiency led to reduced serum VLDL-triglyceride levels but increased hepatic triglyceride content, resulting at least partially from decreased hepatic secretion of VLDL-containing apolipoprotein B-48. Together, these results demonstrate that hepatic ACL suppression exerts profound effects on triglyceride mobilization as well as fatty acid compositions in the liver, suggesting an important role for ACL in lipid metabolism.     

Studies on the mechanism of capacitation. II. Evidence for lipid transfer between plasma membrane of rat sperm and serum albumin during capacitation in vitro

1. Evidence has been provided for the transfer of phosphatidyl[14C]choline and [3H]cholesterol between bovine serum albumin and cauda epididymal rat spermatozoa in Krebs-Ringer bicarbonate medium, which can promote sperm capacitation. 2. An analysis of the lipid composition in both albumin and spermatozoa revealed that phospholipid levels decreased in the protein and increased by roughly comparable amounts in sperm cells during incubation in vitro. 3. Cholesterol (free + ester) increased in albumin and decreased in spermatozoa. Changes in the amount of esterified cholesterol were solely responsible for the increase associated with albumin, whereas whole sperm cell extracts showed a significant decline in free cholesterol. 4. The composition of albumin-bound fatty acids did not alter appreciably as a result of incubation with spermatozoa. 5. Rates of [14C]palmitic acid utilization by spermatozoa suggest that lipid synthesis accounted for less than 5% of the changes observed under the conditions of this study. 6. These results are interpreted as broadly supporting our previous proposal that lipid exchange between albumin and sperm cells is implicated in sperm capacitation in vitro. Specifically, the results are compatible with the idea that a decreased cholesterol/phospholipid ratio in the sperm plasma membrane facilitates this transformation.     

Lipid composition and cholesterol esterification in serum lipoprotein fraction of the horse, Equus caballus.

1. Changes in lipid components of lipoproteins during incubation of horse serum at 37 degrees C were investigated. In non-incubated serum, cholesterol and lecithin existed predominantly in alpha-lipoprotein or in high-density lipoprotein (HDL). Lysolecithin was mainly associated with the fraction with density above 1.21. 2. When serum was separated into alpha- and beta-lipoproteins by the heparin precipitation method after 1 hr incubation, the decrease in alpha-lipoprotein free cholesterol and lecithin was about four times that in beta-lipoprotein counterparts. 3. When serum lipoproteins were separated by ultracentrifugation, the decrease in each lipoprotein free cholesterol was closely paralleled with that in lecithin. 4. HDL appeared to be a preferential substrate for the lecithin: cholesterol acyltransferase reaction. 5. Disc electrophoretic patterns indicated significant differences in the composition of horse serum lipoproteins from those of human and rat.     

Prostaglandin generation in rabbit kidney. Hormone-activated selective lipolysis coupled to prostaglandin biosynthesis.

The endogenous release of prostaglandins and free fatty acids from the isolated perfused rabbit kidney in the absence or presence of stimulation by bradykinin or angiotensin-II was investigated. Basal (nonstimulated) release of prostaglandin-precursor arachidonic acid was 15-20-fold higher than that of prostaglandin E2 indicating a low conversion of released arachidonate to prostaglandins. Addition of bovine serum albumin to the perfusion medium caused a substantial (50-250%) increase in the release of all fatty acids except myristic and arachidonic acids, and no significant change in prostaglandin E2 generation. In contrast, administration of bradykinin (0.5 microgram) or angiotensin-II (1 microgram) caused a 10-15-fold increase in prostaglandin E2 release, and with albumin present, also a 2-3-fold selective increase in arachidonic acid release. Thus, unlike what was observed under basal conditions, arachidonic acid released following hormone stimulation is efficiently converted to prostaglandin E2. We conclude that administration of bradykinin or angiotensin-II into the perfused kidney activates a lipase which selectively releases arachidonic acid, probably from a unique lipid entity. This lipase reaction is tightly coupled to a prostaglandin generating system so that the released arachidonate is first made available to the prostaglandin cyclooxygenase, resulting in its substantial conversion to prostaglandins.     

Marked acceleration of exogenous fatty acid incorporation into cellular triglycerides by fetuin.

Fetuin belongs to a group of fetal glycoproteins whose specific function is not known. In this study we investigated the effect of bovine fetuin on exogenous fatty acid incorporation into lipid classes by fetal rabbit aortic smooth muscle cells (SMC) and human fetal skin fibroblasts. When compared with albumin, the addition of fetuin to the culture medium caused a dramatic increase in labeled fatty acid incorporation (nanomoles/mg of protein) by SMC into triglycerides (albumin (control) 2.8 +/- 0.3 + fetuin 178.3 +/- 13.7). This effect was noted at a wide range of fetuin concentrations (0.2-5%) at oleate:fetuin molar ratios of 3.3-0.13, respectively. Similar effects were noted using human fetal skin fibroblasts with both labeled oleic and arachidonic acids (0.1 mM) as substrates (arachidonic acid incorporation into triglycerides, albumin (control) 76.9 +/- 16.2 + fetuin 684.6 +/- 64.1). Stimulation of fatty acid incorporation into di- and monoglycerides was also noted. Although the amount of unbound fatty acid in the presence of fetuin was greater than with albumin, experiments done under conditions that create identical unbound oleate levels (by varying fatty acid concentration) still showed increased fatty acid incorporation into triglycerides by SMC when exposed to fetuin. This marked effect of fetuin on triglyceride accumulation in cells was confirmed by lipid analysis, strong positive staining with oil red O, and transmission of electron microscopy. Furthermore, the potential physiological role of fetuin in terms of fatty acid and transport was attested by (a) the presence of significant amounts of free fatty acids associated with fetuin; and (b) by the stimulatory effect of fetuin, even when added to culture media containing other fatty acid carriers. These results show that (a) fetuin is far more efficient than albumin in incorporating fatty acids into cells; and (b) this might represent a novel function for fetuin during development.     

Fatty acids increase the circulating levels of oxidative stress factors in mice with diet-induced obesity via redox changes of albumin

Plasma concentrations of free fatty acids are increased in metabolic syndrome, and the increased fatty acids may cause cellular damage via the induction of oxidative stress. The present study was designed to determine whether the increase in fatty acids can modify the free sulfhydryl group in position 34 of albumin (Cys34) and enhance the redox-cycling activity of the copper-albumin complex in high-fat diet-induced obese mice. The mice were fed with commercial normal diet or high-fat diet and water ad libitum for 3 months. The high-fat diet-fed mice developed obesity, hyperlipemia, and hyperglycemia. The plasma fatty acid/albumin ratio also significantly increased in high-fat diet-fed mice. The increased fatty acid/albumin ratio was associated with conformational changes in albumin and the oxidation of sulfhydryl groups. Moreover, an ascorbic acid radical, an index of redox-cycling activity of the copper-albumin complex, was detected only in the plasma from obese mice, whereas the plasma concentrations of ascorbic acid were not altered. Plasma thiobarbituric acid reactive substances were significantly increased in the high-fat diet group. These results indicate that the increased plasma fatty acids in the high-fat diet group resulted in the activated redox cycling of the copper-albumin complex and excessive lipid peroxidation.     

Change of serum L-tryptophan levels following the development and recovery of acute puromycin aminonucleoside nephrosis in rats

Summary It is known that total L-tryptophan (Trp) levels decrease with a decrease in albumin-bound Trp levels and an increase in free Trp levels in the plasma or serum of nephrotic children. We, therefore, examined the change of serum Trp levels following the development and recovery of acute nephrosis in 6-week-old male Wistar rats injected once with puromycin aminonucleoside (100mg/kg body weight) and checked the levels of 16 amino acids including Trp in the serum and the levels of Trp in the liver, kidney, and urine under nephrotic conditions. In this study, the development and recovery of nephrosis were checked by the changes of levels of urinary protein and serum protein and albumin. Total serum Trp and albumin-bound serum Trp levels decreased with the development of nephrosis and these decreased levels returned to the normal level with its recovery. In contrast, free serum Trp levels increased with the development of nephrosis and this increased level returned to the normal level with its recovery. In the serum of nephrotic rats, the decrease of albumin-bound Trp levels and the increase of free Trp levels were well consistent with a decrease in albumin levels and an increase in the level of non-esterified fatty acids which are known to weaken the binding of Trp to albumin and among 16 amino acids studied, only Trp showed a significant change in its levels. Trp levels increased in the liver and kidney but not in the urine under nephrotic conditions. These results indicate that the change of serum Trp levels should be closely related to the condition of nephrosis and that although serum Trp is lost under nephrotic conditions, the lost serum Trp is accumulated in the liver and kidney.     

Release of endothelial cell lipoprotein lipase by plasma lipoproteins and free fatty acids

Lipoprotein lipase (LPL) bound to the lumenal surface of vascular endothelial cells is responsible for the hydrolysis of triglycerides in plasma lipoproteins. Studies were performed to investigate whether human plasma lipoproteins and/or free fatty acids would release LPL which was bound to endothelial cells. Purified bovine milk LPL was incubated with cultured porcine aortic endothelial cells resulting in the association of enzyme activity with the cells. When the cells were then incubated with media containing chylomicrons or very low density lipoproteins (VLDL), a concentration-dependent decrease in the cell-associated LPL enzymatic activity was observed. In contrast, incubation with media containing low density lipoproteins or high density lipoproteins produced a much smaller decrease in the cell-associated enzymatic activity. The addition of increasing molar ratios of oleic acid:bovine serum albumin to the media also reduced enzyme activity associated with the endothelial cells. To determine whether the decrease in LPL activity was due to release of the enzyme from the cells or inactivation of the enzyme, studies were performed utilizing radioiodinated bovine LPL. Radiolabeled LPL protein was released from endothelial cells by chylomicrons, VLDL, and by free fatty acids (i.e. oleic acid bound to bovine serum albumin). The release of radiolabeled LPL by VLDL correlated with the generation of free fatty acids from the hydrolysis of VLDL triglyceride by LPL bound to the cells. Inhibition of LPL enzymatic activity by use of a specific monoclonal antibody, reduced the extent of release of 125I-LPL from the endothelial cells by the added VLDL. These results demonstrated that LPL enzymatic activity and protein were removed from endothelial cells by triglyceride-rich lipoproteins (chylomicrons and VLDL) and oleic acid. We postulate that similar mechanisms may be important in the regulation of LPL activity at the vascular endothelium.