The activity of an esterified cholesterol transferring factor in human and rat serum.

In incubations containing mixtures of lipoproteins isolated either from humans or from rats, the addition of human lipoproteins-free serum (the dialysed 1.25 g/ml infranatant solution) was more than five times more effective than that of the rat in promoting the transfer of esterified [3H]cholesterol from high density lipoproteins to very low density lipoproteins, regardless of the species origin of the lipoproteins, implying the existence of an esterified cholesterol transferring factor of much greater activity in human than in rat serum.     

The effect of gossypol on serum lipoproteins of adult rats

It has been shown that gossypol treatment brings about marked changes in plasma lipoproteins of certain species causing a marked fall in total cholesterol and LDL-cholesterol levels. Using Helena electrophoretic system the lipoprotein pattern in serum of adult rats treated with gossypol was studied. Gossypol treatment for 1 day and 7 days continuously did bring about changes in serum lipoprotein profile. A marked increase in VLDL-cholesterol and fall in LDL-cholesterol was observed in rats treated with gossypol which was more pronounced in the 7 days treated group. It did not bring about any marked change in total cholesterol level. It did cause an increase in serum triglyceride level. Heparin injection which brought about a fall in VLDL-cholesterol with a concomitant increase in HDL-cholesterol in control rats did not elicit changes in gossypol-treated rats. Thus, gossypol treatment appears to alter serum lipoprotein metabolism in rats also.     

Benzo[a]pyrene uptake by serum lipids: correlation with triglyceride concentration

An in vitro study of the relationship between benzo[a]pyrene (BaP) association with serum lipoproteins (LP) and LP composition was conducted using human subjects. BaP partitioning into different serum LP ranged from 53 to 71% of available BaP. Efficiency of BaP partitioning was examined for the relationship with lipid components of different sera. The data indicate that triglyceride (TG) concentrations were more directly correlated with BaP uptake than were concentrations of other LP components. Adjusting sera to a uniform TG concentration (96.5 mg/dl) resulted in the same BaP uptake for each serum type, while adjusting sera to contain a uniform cholesteryl ester concentration (104.6 mg/dl) did not result in similar BaP uptake among serum types. Analysis of serum LP composition suggested that marked differences in both BaP uptake and serum TG concentrations among the subjects were due mainly to differences in serum very low density lipoprotein (VLDL) concentrations. A correlation study using 14 human subjects showed that serum TG concentration was the best predictor (r = 0.973, P less than 0.001) for BaP uptake by serum, followed by phospholipid (r = 0.658, P less than 0.01) and total cholesterol (r = 0.514, P less than 0.05) concentrations. The results indicate that serum TG concentration (typically VLDL-TG) may be the primary factor affecting BaP uptake by serum LP, and suggest that a small change in serum TG concentration could cause a significant increase in BaP uptake by serum LP, contributing to an increased level of circulating carcinogen.     

Formation of cholesterol- and apoprotein E-enriched high density lipoproteins in vitro

The delivery of cholesterol to canine serum or plasma altered the distribution of cholesterol and apoproteins in subclasses of high density lipoproteins (HDL). In these experiments, two in vitro systems were employed. The first system used cholesterol-celite particles to deliver cholesterol to canine plasma during 4-h incubations. When the cholesterol distribution in the lipoproteins was analyzed by Geon-Pevikon electrophoresis, an increase in cholesterol content was found in the slower migrating subclasses of HDL (HDL1 and HDLc). A large increase in apoprotein E (apo-E) was also observed in the lipoproteins. Densitometric analysis of lipid-stained, 4 to 30% gradient acrylamide gels of canine plasma after incubation with cholesterol-celite revealed that the concentration of the major high density lipoproteins (HDL3) decreased, and the concentration of subclasses of HDL-with apo-E (HDL1 and HDLc) increased 2- to 5-fold. In the second system, cholesterol-loaded mouse peritoneal macrophages released cholesterol to HDL in an incubation medium containing 10 to 20% canine serum. The HDL1 and HDLc, which demonstrated slower electrophoretic mobility as determined by Geon-Pevikon block electrophoresis, became enriched in cholesterol and cholesteryl esters. Gradient gel electrophoresis showed substantial increases in these subclasses of HDL-with apo-E. The cholesterol-loaded mouse peritoneal macrophages synthesized and secreted apo-E into the medium. When L-[35S]methionine was used as a precursor, 65 to 90% of the 35S-labeled protein associated with the lipoproteins in the 1.02 to 1.09 density range was immunoprecipitated with antibody directed against rat apo-E. Gradient gel electrophoresis of density fractions demonstrated the presence of HDL1 and HDLc as the major lipoproteins. In addition, when canine 125I-HDL3 (primarily apo-A-I-containing HDL) were added to canine serum and incubated with cholesterol-loaded macrophages, the appearance of HDL1 and HDLc was associated with a marked increase in the 125I label in these newly formed, cholesteryl ester-rich lipoproteins. There was a corresponding marked reduction in the 125I-HDL3 in the serum. Similar results were observed using human HDL3 and human serum.     

Effects of high cholesterol diets on rat plasma lipoproteins and lipoprotein-cell interactions

High fat, high cholesterol diets do not produce atherosclerotic lesions in some animal species such as the rat; however, when combined with experimentally induced hypothyroidism, such diets do produce lesions. While the diets or hypothyroidism each induce significant alterations in plasma lipoproteins, the combination produces marked hypercholesterolemia. If the atherosclerosis is related to the hyperlipidemia, the combination regimen could be provoking changes in the structure or compositions of lipoproteins which are not noted with either regimen alone. To test this hypothesis, Sprague-Dawley male rats (approximately 250 g) were treated as follows: Diet(a) = chow + 5% lard and 0.3% Na taurocholate; Diet(b) = Diet(a) + 2% cholesterol; Diet(c) = Diet(b) + 0.1% propylthiouracil (PTU). The major findings were as follows. 1) With Diet(b), slow floating very low density lipoprotein (VLDL) (pre-beta) enriched in cholesteryl esters accumulated in plasma and low density lipoprotein (LDL) disappeared from its usual flotation position. 2) With Diet(c), changes in plasma concentration were more marked but were also qualitatively different. More VLDL accumulated, and distribution of VLDL was shifted toward even slower floating cholesteryl ester-rich particles. VLDL had "broad beta" mobility. Also, a beta-migrating intermediate density lipoprotein (IDL) population appeared. 3) Lipoprotein (d less than 1.019 g/ml) and zonal subfractions of d less than 1.019 g/ml lipoproteins (isolated from rats on cholesterol Diet (b] stimulated [3H]oleate incorporation into cholesteryl esters of fibroblasts and macrophages, while the d less than 1.019 g/ml fractions of 5% fat (Diet(a]-fed rats did not. 4) The major finding of this study was that identically prepared d less than 1.019 g/ml fractions of Chol + PTU-treated rats (Diet(c] were approximately 2.5-fold more stimulatory than the lipoproteins of cholesterol-fed rats. The results could not be explained by differences in cholesterol contents of the cholesterol-rich lipoproteins, but significant differences in the apoprotein compositions of the fraction were found which could be important. The most active fractions had higher apoBL/apoBS and apoE/apoC ratios than less active fractions. Thus, the combination regimen of cholesterol and PTU produced changes in lipoprotein structure and composition which enhanced the abilities of the lipoproteins to interact with cells. The results suggest that analysis of lipoprotein-cell interactions in vitro may be predictive of the atherogenic potential of lipoproteins in vivo and that euthyroidism in rat protects against atherogenic hyperlipidemia.     

Serum lipoproteins of normal and cholesterol-fed rats

The density distribution of lipoproteins in rats fed chow or chow containing 1% cholesterol and 10% olive oil was studied. Lipoprotein fractions were prepared in the ultra-centrifuge between narrow density bands within the density range of 1.006-1.21 and were analyzed by chemical, electrophoretic, and immunological methods. In serum from normal rats there were three major lipoprotein fractions, with densities less than 1.006, 1.030-1.063, and 1.063-1.21. Almost no lipoprotein was found between d 1.006 and 1.030. Most of the low density lipoprotein appeared between a density of 1.04 and 1.05. In the density range 1.05-1.07, small amounts of both low density and high density lipoprotein were found. Feeding a diet high in cholesterol resulted in a marked increase in the concentration of lipoproteins of density less than 1.006, and a new lipoprotein fraction appeared between d 1.006 and 1.030; this fraction contained immunologically demonstrable low density and high density lipoproteins. In addition, there was a decrease in the high density lipoprotein fraction between d 1.070 and 1.21.     

Intestinal lipoprotein synthesis. Comparison of nascent Golgi lipoproteins from chow-fed and hypercholesterolemic rats

Hypercholesterolemia, induced by a cholesterol-enriched diet, is associated with distinctive modifications in the serum lipoproteins of a variety of species. Present in the serum of these animals are several classes of lipoproteins enriched in cholesteryl esters and apolipoprotein E. To investigate the role of intestinal lipoprotein synthesis in diet-induced hypercholesterolemia, we characterized nascent lipoproteins retrieved from Golgi apparatus-rich fractions of intestinal epithelial cells from chow-fed control and hypercholesterolemic rats. To eliminate chylomicrons from the preparations, rats were fasted overnight prior to the experiments. Golgi very low density lipoproteins (d less than 1.006 g/ml) from control rats were triglyceride-rich lipoproteins that migrated slightly slower than pre-beta migrating serum very low density lipoproteins. These particles contained apoproteins B-240, A-IV, and A-I. Golgi very low density lipoproteins from hypercholesterolemic rats were likewise triglyceride-rich lipoproteins migrating electrophoretically like control Golgi very low density lipoproteins and they contained apoproteins B-240, A-IV, and A-I. However, these latter particles contained less triglyceride and more cholesterol compared to control Golgi very low density lipoproteins. In addition, by radioisotope incorporation studies, Golgi very low density lipoproteins from hypercholesterolemic rats contained relatively more apoprotein A-IV (21.6 vs. 11.0%) and less apoprotein B-240 (17.0 vs. 27.0%) than found in control Golgi very low density lipoproteins. Approximately 60% of the total apoprotein radioactivity was found in apoprotein A-I in both preparations. We conclude that intestinal lipoprotein synthesis is modified by diet-induced hypercholesterolemia. The significance of these modifications with respect to the marked hypercholesterolemia observed in these animals remains to be determined.     

Isolation of plasma lipoproteins by zonal ultracentrifugation in the B14 and B15 titanium rotors

Lipoproteins were isolated from plasma of man, dog, rabbit, rat, and chicken by ultracentrifugation in continuous density gradients using the B14 titanium and B15 titanium zonal rotors. Both the VLDL and the LDL of human plasma were separated easily from the HDL and from the other more plentiful plasma proteins by centrifugation for only 1 or 2 hr in the B14 or B15 rotor, respectively. Satisfactory separation of the HDL from the more dense plasma proteins was not achieved with these rotors. The human LDL achieved isopycnic equilibrium (d 1.04) on prolonged periods (> 24 hr) of centrifugation in a sucrose-KBr density gradient. The pattern of distribution of cholesterol and phospholipid throughout the density gradient coincided with the pattern of distribution of the lipoprotein-protein measured spectrophotometrically or chemically. The concentration of cholesterol and phospholipid in the lipoproteins isolated by zonal ultracentrifugation agreed with analyses reported for lipoproteins isolated by sequential centrifugation in solutions of increasing density. The lipoproteins isolated by zonal ultracentrifugation were characterized further by their electrophoretic behavior. The fractions which were identified as the LDL (d 1.04-1.05) from all species migrated on paper as a beta-globulin; the LDL from plasma of dogs contained an additional component which has been designated as an alpha(2)-globulin. The fractions which were identified as the HDL from all species migrated as an alpha(1)-globulin. Reaction of human LDL with either rabbit antihuman beta-lipoprotein or rabbit antihuman serum resulted in a single immunodiffusion band. The S(f, 1.063) of the human LDL was calculated to be 6.0. When plasma from humans or rabbits was centrifuged in the B15 rotor, the HDL was not visible as a distinct peak and was not separable from the bulk of the more dense plasma proteins; when plasma from dogs or chickens was centrifuged under identical conditions, the HDL was clearly detectable. Even though the mean density of the HDL from dogs or chickens was not different from that of man or rabbits, the visibility of this lipoprotein in dogs and chickens was probably due to its high concentration in the plasma of these species. When plasma from the rat was centrifuged under similar conditions, the HDL was also clearly in evidence. Although rat plasma contained a relatively small concentration of HDL, the lipoprotein had a lower mean density than did the HDL of the other species and was therefore more easily separable from the dense plasma proteins. The procedure of zonal ultracentrifugation for the isolation of lipoproteins by flotation is simultaneously preparative and analytical and should find useful application in the investigation of the soluble lipoproteins from plasma and tissues.     

Relationships between cholesterol and phospholipid concentrations in the serum of humans, calves, rabbits and chickens

In humans, calves and rabbits very strong correlations were found between the concentrations of serum total cholesterol and phospholipids; in these species more than 70% of the variance in the level of serum phospholipids is accounted for by the variance in the serum cholesterol concentration. The regression coefficients in the equations relating the concentration of serum cholesterol to that of phospholipids were almost identical in humans and rabbits, but differed from those observed in calves and chickens. It is suggested that the rabbit is a suitable model animal for studying the relation between cholesterol and phospholipids in human serum.     

The lipid composition of serum lipoproteins in the harbour seal, Phoca vitulina

The density profile of serum lipoproteins and their lipid composition was studied in 12 adult, female harbour seals. The animals were sampled after an approximate 20 hr fast. The density profile of lipoproteins showed that the harbour seals displayed a distinct VLDL (density less than 1.006 g/ml) and HDL band (density about 1.125 g/ml), but no clear LDL band. There was a rather diffuse population of lipoproteins in the density range of 1.019-1.100 g/ml. Mean serum total cholesterol concentration was 5.7 mmol/l; about 60% of this cholesterol was located in the HDL fraction (density greater than 1.063 g/ml). The fasted seals were found to carry 4% of serum total lipids in chylomicrons. These lipoproteins consisted of 51% of triaclyglycerols (on the basis of total chylomicron lipids). The LDL (defined as heparin-manganese precipitable lipoproteins in VLDL and chylomicron-deficient serum) contained 49% of cholesterol and 43% of phospholipids (on the basis of total LDL lipids). The HDL (defined as heparin-manganese soluble lipoproteins in VLDL and chylomicron-deficient serum) contained 36% of cholesterol and 58% of phospholipids (on the basis of total HDL lipids).     

Feeding unsaponifiable compounds from rice bran oil does not alter hepatic mRNA abundance for cholesterol metabolism-related proteins in hypercholesterolemic rats

The hypocholesterolemic effect of rice bran oil (RBO) is defined in human and animal experiments which indicate the presence of active component(s) in the unsaponifiable fraction, but the detailed mechanism is not known yet. Exogenously hypercholesterolemic (ExHC) rats were fed for 2 weeks on a 0.5% cholesterol diet supplemented with 10% each of RBO, RBO-simulated oil (RBOSO) in its fatty acid composition, or RBOSO plus 0.25% unsaponifiable compounds (UC) from RBO. Rats fed RBO or the UC resulted in lowing serum and liver cholesterol concentration and preventing reduction of high density lipoproteinic-cholesterol. Dietary RBO or the UC led to an elevation of fecal neutral sterol excretion, but no significant change in fecal bile acid excretion or in hepatic abundance of mRNAs for 3-hydroxy-3-methylglutaryl-CoA reductase, cholesterol-7alpha-hydroxylase, and low density lipoprotein receptor. Besides, serum and liver alpha-tocopherol concentrations were lowered in RBO or the UC-fed rats. These results show that the UC in RBO leads to a decreased serum cholesterol concentration by interrupting the absorption of intestinal hydrophobic compounds rather than by modifying cholesterol metabolism in the liver.     

Stimulation of cholesterol esterification in hepatic microsomes by lipoproteins from normal and hypercholesterolemic rabbit serum.

Incubation of plasma lipoproteins with rabbit hepatic microsomes enriched the microsomes with free cholesterol and stimulated cholesterol esterification. The rate of cholesterol esterification correlated well (r = 0.96) with the concentration of microsomal free cholesterol. Lipoproteins from normal and hypercholesterolemic serum varied in their propensity to stimulate cholesterol esterification. Among the normal lipoproteins, low density lipoproteins was more stimulatory than either high density lipoproteins or intermediate density lipoproteins. However, the intermediate density lipoproteins fraction from hypercholesterolemic serum was consistently more stimulatory than any of the normal lipoproteins. The augmentation of cholesterol content, when microsomes were exposed to mixed hyperlipidemic lipoproteins, was proportionately much greater than augementation of phospholipid or protein concentration.     

Glucocorticoid-induced elevation of serum high-density lipoprotein-cholesterol and its reversal by adrenocorticotropin in the rat

The effect of administration of a high dose of glucocorticoid (triamcinolone) on serum lipids and lipoproteins was studied in rats. Changes in serum lipids, especially cholesterol, were most marked when 5 mg/kg body weight of triamcinolone was injected daily for 5 days. Serum lipoproteins were separated by ultracentrifugation followed by gel-filtration chromatography. Cholesterol distribution between apolipoprotein B-containing lipoproteins (very-low-density and low-density lipoproteins), high-density lipoprotein1 (HDL1), and HDL2 was determined after administration of triamcinolone with or without additional treatment with adrenocorticotropin (ACTH; Cortrosyn, 6 IU/rat). When triamcinolone was administered, cholesterol concentrations in HDL1 and HDL2 were elevated in a dose-dependent manner, but there was no significant change in apolipoprotein B-containing lipoprotein cholesterol levels. When ACTH was administered in combination with triamcinolone, the concentrations of all serum lipids except triacylglycerol were significantly lowered compared with rats treated with triamcinolone alone. HDL1-cholesterol concentration in serum was significantly (P less than 0.001) lowered from 69 +/- 13 mg/dl (mean +/- S.D.) in triamcinolone-treated rats to 36 +/- 4 mg/dl by the administration of ACTH plus triamcinolone. The additional administration of ACTH in triamcinolone-treated rats caused a slight, but significant, decrease in cholesterol concentration in apolipoprotein B-containing lipoproteins; however, HDL2-cholesterol level was not significantly affected, although there was a tendency for it to be lowered.     

Transfer of excess cholesterol from human red blood cells to plasma in vitro

This study examined the ability of plasma and plasma fractions from normolipidaemic subjects and plasma from a patient with homozygous familial high density lipoprotein deficiency (Tangier disease) to promote loss of excess cholesterol from red blood cells in vitro. Isolated high density lipoproteins were the most potent plasma fraction for removing excess cellular cholesterol. Lipoprotein-deficient plasma and human serum albumin, but not very low density lipoproteins and low density lipoproteins, also removed excess cholesterol from the red blood cells. The near absence of high density lipoproteins in plasma from the patient with Tangier disease did not result in an abnormally low rate of cholesterol loss from the enriched red blood cells. These results suggest that normal levels of high density lipoproteins are not vital for the removal of excess cholesterol from red blood cells by plasma.     

Distribution of glycosphingolipids in the serum lipoproteins of normal human subjects and patients with hypo- and hyperlipidemias.

Five glycosphingolipids (GSL), glucosylceramide, lactosylceramide, trihexosylceramide, globoside, and hematoside (GM3) were studied in serum from normal human subjects and patients with dyslipoproteinemia and found to be exclusively associated with the various classes of serum lipoproteins. Based on a unit weight of lipoprotein protein, the total amount of GSL in serum normal subjects was twice as high in very low density lipoprotein (VLDL) (d less than 1.006 g/ml) and low density lipoprotein (LDL) (d 1.019-1.063 g/ml) as in high density lipoproteins HDL2 (d 1.063-1.125 g/ml) or HDL3 (d 1.125-1.21 g/ml). In abetalipoproteinemia the levels of serum GSL were slightly reduced when compared to normal serum and were all found in the only existing lipoprotein, HDL; this contained 2-3 moles of GSL/ mole of lipoprotein as compared to 0.5 GSL/mole in normal HDL. In hypobetalipoproteinemia and Tangier disease, the serum glycosphingolipids were 10 to 30% reduced in concentration compared to the 75% reduction in other lipids, and were again found to be associated only with the serum lipoproteins. The relative proportions of GSL did not vary substantially in the normo- and hypolipidemic subjects studied. Only in patients with homozygous familial hypercholesterolemia was there a significant (3-4-fold) elevation of all of the five GSL species and this elevation of all of the five GSL species and this elevation correlated well with that of the circulating cholesterol and LDL. On a molar basis the LDL of these patients contained the same amount of GSL as normal subjects (5 moles GSL/mole protein). It is concluded that: (1) glycosphingolipids are associated only with the major lipoprotein classes in both normal and dyslipoproteinemic serum; (2) the relative proportions of the five glycosphingolipids are not significantly affected by dyslipoproteinemia; (3) only in severe hypolipoproteinemia do the remaining serum lipoproteins carry a complement of glycosphingolipids greater than normal. Although our results establish that glycosphingolipids are intimately associated with serum lipoproteins, the mode of association or the structural and functional significance of such an association remains undetermined.     

Regulation of lipogenesis in isolated hepatocytes by triglyceride-rich lipoproteins.

Very low density lipoproteins, chylomicrons, and remnants caused, within an hour, significant inhibition of fatty acid synthesis but not cholesterol synthesis in hepatocytes isolated from meal-fed rats. In contrast, low density lipoproteins, high density lipoproteins, and the serum fraction of density greater than 1.21 failed to significantly inhibit either fatty acid or cholesterol synthesis within 1 h. The Scatchard plots of specific binding showed that rat and human very low density lipoproteins interact with the high affinity sites on the hepatocytes with the apparent dissociation constants of 64 and 106 nM, respectively. These data also indicated that each hepatocyte was capable of binding 6 X 10(5) molecules of very low density lipoproteins.     

Eritadenine-induced alterations of plasma lipoprotein lipid concentrations and phosphatidylcholine molecular species profile in rats fed cholesterol-free and cholesterol-enriched diets

The effects of dietary eritadenine on the concentration of plasma lipoprotein lipids and the molecular species profile of plasma lipoprotein phosphatidylcholine (PC) were investigated in rats fed cholesterol-free and cholesterol-enriched diets to obtain insights into the relationship between the changes in PC molecular species profile and the hypocholesterolemic action of eritadenine. The effect of eritadenine on the secretion rate of very low density lipoprotein (VLDL) from the liver was also estimated. Rats were fed the control or eritadenine-supplemented (50 mg/kg) diets with or without exogenous cholesterol for 14 d. Eritadenine supplementation significantly decreased the cholesterol of major plasma lipoproteins, high density lipoprotein and VLDL, in rats fed cholesterol-free and cholesterol-enriched diets, respectively. The ratio of PC to phosphatidylethanolamine, delta6-desaturase activity, and the ratio of arachidonic acid to linoleic acid in liver microsomes were markedly decreased by eritadenine irrespective of the presence or absence of exogenous cholesterol. Dietary eritadenine increased the proportion of 16:0-18:2 molecular species with a decrease in 18:0-20:4 in plasma lipoprotein PC in both rats fed cholesterol-free and cholesterol-enriched diets. Eritadenine did not depress the secretion rate of VLDL in rats fed a cholesterol-free diet containing a high level of choline. The results indicate that dietary eritadenine elicits its hypocholesterolemic action with modulations of the fatty acid and molecular species profiles of PC irrespective of the presence or absence of exogenous cholesterol. The eritadenine-induced alteration of PC molecular species profile is discussed in relation to the hypocholesterolemic action of eritadenine.     

Oxidized cholesterol in the diet is a source of oxidized lipoproteins in human serum

The aim of this study was to determine in humans whether oxidized cholesterol in the diet is absorbed and contributes to the pool of oxidized lipids in circulating lipoproteins. When a meal containing 400 mg cholestan-5alpha,6alpha-epoxy-3beta-ol (alpha-epoxy cholesterol) was fed to six controls and three subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol in serum was found in chylomicron/chylomicron remnants (CM/RM) and endogenous (VLDL, LDL, and HDL) lipoproteins. In controls, alpha-epoxy cholesterol in CM/RM was decreased by 10 h, whereas in endogenous lipoproteins it remained in the circulation for 72 h. In subjects with Type III hyperlipoproteinemia, alpha-epoxy cholesterol was mainly in CM/RM. In vitro incubation of the CM/RM fraction containing alpha-epoxy cholesterol with human LDL and HDL that did not contain alpha-epoxy cholesterol resulted in a rapid transfer of oxidized cholesterol from CM/RM to both LDL and HDL. In contrast, no transfer was observed when human serum was substituted with rat serum, suggesting that cholesteryl ester transfer protein is mediating the transfer. Thus, alpha-epoxy cholesterol in the diet is incorporated into the CM/RM fraction and then transferred to LDL and HDL, contributing to lipoprotein oxidation. Moreover, LDL containing alpha-epoxy cholesterol displayed increased susceptibility to further copper oxidation in vitro. It is possible that oxidized cholesterol in the diet accelerates atherosclerosis by increasing oxidized cholesterol levels in circulating LDL and chylomicron remnants.     

Chloroquine-induced interference with degradation of serum lipoproteins in rat liver, studied in vivo and in vitro.

The effect of chloroquine, an inhibitor of certain lysosomal enzymes including cathepsin B (EC 3.4.22.1), on the degradation of serum lipoproteins in rat liver was studied in vivo and in liver homogenates. Chloroquine had no effect on the clearance from the circulation of 125I-labeled rat or human very low density lipoproteins or human low density lipoproteins. Pretreatment with chloroquine for 3 h, resulted in a 2-2.5 fold increase in 125i-labeled very low density lipoprotein recovered in the liver 45 min after injection of the homologous and heterologous lipoproteins. This effect was evident on both the 125I-labeled protein and 125I-labeled lipid moiety. 30 min after the injection of [3H]-cholesterol linoleate-labeled very low density lipoproteins, 70% of the injected label was recovered in the liver, both in control and chloroquine-treated rats. Since the perl and 20% in the experimental group, it was concluded that chloroquine interferes with the hydrolysis of [3H]cholesterol linoleate. Following injection of 125I-labeled human low density lipoproteins only 4% of the injected lipoprotein was recovered in the liver of control rats and not more than 10% after chloroquine treatment, when about 50% had been cleared from the circulation. Hence, while very low density lipoprotein protein and cholesterol ester are catabolized in the liver, the catabolism of low density lipoproteins occurs mainly in extra-hepatic tissues. Using post-nuclear liver suprnatant, optimal degradation of various serum lipoproteins was found at pH 4.4, and chloroquine inhibited their degradation. Degradation of very low density and low density lipoproteins was completely inhibited at 0.05 M chloroquine, while less pronounced inhibition was seen with high density lipoproteins, apolipoproteins and apolipoprotein AI. These results indicate that liver acid hydrolases in vivo participate in the degradation of serum lipoproteins. Cathepsin B is apparently responsible for the degradation of aplipoprotein B, while other cathepsins might also be active in the degradation of this and the other apolipoproteins.