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S Parthasarathy

Publications and source records attributed to S Parthasarathy.

At least 145 records · Page 8Linked to original sources
PubMed1990

Oxidation of low-density lipoprotein by Cu2+ and lipoxygenase: an electron spin resonance study.

The aim of this work was to obtain spectroscopic evidence for free radicals formed during copper ion- and lipoxygenase-catalyzed oxidation of the low-density lipoprotein. During the initial oxidation phase, a free-radical metabolite derived from the endogenous alpha-tocopherol present in the low-density lipoprotein was detected by the electron spin resonance technique. The divalent copper ions were bound to the residual EDTA present in the low-density lipoprotein and to the protein. Production of the alpha-tocopherol radical was suppressed in the presence of spin traps. Evidence for the low-density lipoprotein-lipid derived radicals was obtained by ESR-spin trapping methods. Implications of these findings in the oxidative modification of the low-density lipoprotein are discussed.

Copper↗
PubMed1990

High-density lipoprotein inhibits the oxidative modification of low-density lipoprotein.

Oxidatively modified low-density lipoprotein (LDL), generated as a result of incubation of LDL with specific cells (e.g., endothelial cells, EC) or redox metals like copper, has been suggested to be an atherogenic form of LDL. Epidemiological evidence suggests that higher concentrations of plasma high-density lipoprotein (HDL) are protective against the disease. The effect of HDL on the generation of the oxidatively modified LDL is described in the current study. Incubation of HDL with endothelial cells, or with copper, produced much lower amounts of thiobarbituric acid-reactive products (TBARS) as compared to incubations that contained LDL at equal protein concentrations. Such incubations also did not result in an enhanced degradation of the incubated HDL by macrophages in contrast to similarly incubated LDL. On the other hand, inclusion of HDL in the incubations that contained labeled LDL had a profound inhibitory effect on the subsequent degradation of the incubated LDL by the macrophages while having no effect on the generation of TBARS or the formation of conjugated dienes. This inhibition was not due to the modification of HDL as suggested by the following findings. (A) There was no enhanced macrophage degradation of the HDL incubated with EC or copper alone, together with LDL, despite an increased generation of TBARS. (B) HDL with the lysine groups blocked (acetyl HDL, malondialdehyde (MDA) HDL) was still able to prevent the modification of LDL and (C) acetyl HDL and MDA-HDL competed poorly for the degradation of oxidatively modified LDL. It is suggested that HDL may play a protective role in atherogenesis by preventing the generation of an oxidatively modified LDL. The mechanism of action of HDL may involve exchange of lipid peroxidation products between the lipoproteins.

Animals↗
PubMed1990

Lyso platelet activating factor (LysoPAF) and its enantiomer. Total synthesis and carbon-13 NMR spectroscopy.

Described is a reaction sequence for the total synthesis of lyso platelet activating factor (lysoPAF; 1-O-alkyl-sn-glycero-3-phosphocholine) and its enantiomer. The procedure is versatile and yields optically pure isomers of defined chain length. The synthesis is equally suited for the preparation of lysoPAF analogues and its enantiomers with unsaturation in the long aliphatic chain. First, rac-1(3)-O-alkylglycerol is prepared by alkylation of rac-isopropylideneglycerol with alkyl methanesulfonate followed by acid-catalyzed removal of the ketal group. The primary hydroxy group of alkylglycerol is then protected by tritylation, the secondary hydroxy group is acylated, and the protective trityl group is removed under mild acidic conditions with boric acid on silicic acid, essentially without acyl migration. Condensation of the diradylglycerol with bromoethyl dichlorophosphate in diethyl ether, hydrolysis of the resulting chloride, and nucleophilic displacement of the bromine with trimethylamine gives rac-1-O-alkyl-2-acylglycero-3-phosphocholine in good overall yield. The racemic alkylacylglycerophosphocholine is finally treated with snake venom phospholipase A2 (Ophiophagus hannah) which affords 1-O-alkyl-sn-glycero-3-phosphocholine (lysoPAF) of natural configuration in optically pure form. The "unnatural" 3-O-alkyl-2-O-acyl-sn-glycero-1-phosphocholine enantiomer, which is not susceptible to phospholipase A2 cleavage, gives 3-O-alkyl-sn-glycero-1-phosphocholine upon deacylation with methanolic sodium hydroxide. Homogeneity and structure of the intermediates and final products were ascertained by carbon-13 nuclear magnetic resonance spectroscopy on monomeric solutions.

Acylation↗
PubMed1990

Low density lipoprotein rich in oleic acid is protected against oxidative modification: implications for dietary prevention of atherosclerosis.

Oxidative modification of low density lipoprotein (LDL) enhances its potential atherogenicity in several ways, notably by enhancing its uptake into macrophages. In vivo studies in the rabbit show that inhibition of LDL oxidation slows the progression of atherosclerotic lesions. In the present studies, rabbits were fed either a newly developed variant sunflower oil (Trisun 80), containing more than 80% oleic acid and only 8% linoleic acid, or conventional sunflower oil, containing only 20% oleic acid and 67% linoleic acid. LDL isolated from the plasma of animals fed the variant sunflower oil was highly enriched in oleic acid and very low in linoleic acid. These oleate-rich LDL particles were remarkably resistant to oxidative modification. Even after 16-hr exposure to copper-induced oxidation or 24-hr incubation with cultured endothelial cells, macrophage uptake of the LDL was only marginally enhanced. The results suggest that diets sufficiently enriched in oleic acid, in addition to their LDL-lowering effect, may slow the progression of atherosclerosis by generating LDL that is highly resistant to oxidative modification.

Animals↗
PubMed1990

Colocalization of 15-lipoxygenase mRNA and protein with epitopes of oxidized low density lipoprotein in macrophage-rich areas of atherosclerotic lesions.

Oxidation of low density lipoprotein (LDL) enhances its atherogenicity, and inhibition of such oxidation decreases the rate of progression of atherosclerotic lesions. The mechanism of LDL oxidation in vivo remains uncertain, but in vitro studies have suggested that cellular lipoxygenases may play a role by initiating lipid peroxidation in LDL. In situ hybridization studies using a 15-lipoxygenase riboprobe and immunostaining using antibodies against 15-lipoxygenase showed strongly positive reactivity largely confined to macrophage-rich areas of atherosclerotic lesions. Polymerase chain reaction with 15-lipoxygenase-specific oligonucleotides and restriction enzyme digestions of the amplified fragment were used to confirm the presence of 15-lipoxygenase message in the reverse-transcribed lesion mRNA. Immunostaining with antibodies reactive with oxidized LDL (but not with native LDL) indicated that the lipoxygenase colocalizes with epitopes of oxidized LDL, compatible with a role for macrophage lipoxygenase in the oxidation of LDL in vivo. Since oxidized LDL is chemotactic for blood monocytes, early lesions might progress at a markedly accelerated rate because of further recruitment of more monocytes which, in turn, would increase further the rate of oxidation of LDL. These data suggest that therapy targeted to block macrophage lipoxygenase activity might decrease the rate of development of atherosclerotic lesions.

Animals↗
PubMed1990

Phospholipase A2 activity of low density lipoprotein: evidence for an intrinsic phospholipase A2 activity of apoprotein B-100.

During oxidative modification of low density lipoprotein (LDL) there is extensive degradation of phosphatidylcholine (PtdCho) to lysophosphatidylcholine (lyso-PtdCho), with the removal of fatty acids from the 2 position. The phospholipase A2 (PLA2) activity responsible for hydrolysis is closely associated with LDL. By use of lipoxygenase-oxidized 2-[1-14C]linoleoyl PtdCho as the substrate and delipidated apoprotein B (apo-B), evidence is presented to show that (i) the activity is destroyed progressively during the oxidative modification of LDL; (ii) p-bromophenacyl bromide (pBPB), a histidine modifier that inhibits the oxidative modification of LDL, also substantially inhibits the PLA2 activity; and (iii) photooxidation of LDL in the presence of Rose Bengal completely inactivates the enzyme with concomitant loss of apo-B histidine residues. High molecular weight proteins from delipidated LDL, separated by polyacrylamide gel electrophoresis, showed PLA2 activity. It is suggested that apo-B itself may possess PLA2 activity.

Amino Acids↗
PubMed1990

Oxidative modification of LDL: comparison between cell-mediated and copper-mediated modification.

Macrophage-derived foam cells are hallmarks of early atherosclerotic lesions. Oxidatively modified LDL has been suggested to be a more atherogenic form than native LDL. Oxidized LDL--but not native LDL--is chemotactic to monocytes and is avidly degraded by macrophages, resulting in their conversion to foam cells. Incubation of LDL with any of several different types of cells, or with copper ion even in the absence of cells, results in the oxidative modification of LDL. While the cell and the copper systems generate oxidatively modified LDL with similar properties, the two systems differ in their sensitivity to inhibition by superoxide dismutase and by several lipoxygenase inhibitors. In cultured endothelial cells, inhibitors of lipoxygenase, some of them without non-specific antioxidant activity, inhibited cell-mediated modification by 50-80%. In contrast, superoxide dismutase inhibited the process by 20% or less. Moreover, we have shown that soybean lipoxygenase in a cell-free system can modify LDL directly to a form recognized and degraded specifically and rapidly by macrophages. Lipoxygenase-modified LDL is also chemotactic for human monocytes and is cleared rapidly from the circulation, properties shared by cell- or copper-modified LDL. Thus, it is suggested that cellular lipoxygenase(s) may play an important role in cell-mediated oxidative modification of LDL.

Animals↗
PubMed1989

A macrophage receptor that recognizes oxidized low density lipoprotein but not acetylated low density lipoprotein.

The formation of cholesterol-loaded macrophage foam cells in arterial tissue may occur by the uptake of modified lipoproteins via the scavenger receptor pathway. The macrophage scavenger receptor, also called the acetylated low density lipoprotein (Ac-LDL) receptor, has been reported to recognize Ac-LDL as well as oxidized LDL species such as endothelial cell-modified LDL (EC-LDL). We now report that there is another class of macrophage receptors that recognizes EC-LDL but not Ac-LDL. We performed assays of 0 degrees C binding and 37 degrees C degradation of 125I-Ac-LDL and 125I-EC-LDL by mouse peritoneal macrophages. Competition studies showed that unlabeled Ac-LDL could compete for only 25% of the binding and only 50% of the degradation of 125I-EC-LDL. Unlabeled EC-LDL, however, competed for greater than 90% of 125I-EC-LDL binding and degradation. Unlabeled Ac-LDL was greater than 90% effective against 125I-Ac-LDL; EC-LDL competed for about 80% of 125I-Ac-LDL binding and degradation. Copper-oxidized LDL behaved the same as EC-LDL in all the competition studies. Copper-mediated oxidation of Ac-LDL produced a superior competitor which could now displace 90% of 125I-EC-LDL binding. After 5 h at 37 degrees C in the presence of ligand, macrophages accumulated six times more cell-associated radioactivity from 125I-EC-LDL than from 125I-Ac-LDL, despite approximately equal amounts of degradation to trichloroacetic acid-soluble products, which may imply different intracellular processing of the two lipoproteins. Our results suggest that 1) there is more than one macrophage "scavenger receptor" for modified lipoproteins; and 2) oxidized LDL and Ac-LDL are not identical ligands with respect to macrophage recognition and uptake.

Acetylation↗
PubMed1989

Automatic landmarking of cephalograms.

This paper presents an algorithm for automatically locating certain characteristic anatomical points called landmarks on cephalograms (skull X-rays). These landmarks are used by orthodontists in diagnosis and treatment planning. The algorithm uses digital image processing and feature recognition techniques to locate the landmarks. A resolution pyramid of the digitized cephalogram is first created. The algorithm works on the smaller, lower resolution images to locate features of interest and moves to the bigger, higher resolution images if greater location accuracy is required. Prefiltering using the median filter, contrast enhancement using histogram equalization, and edge enhancement using different gradient operators are performed on the images. The algorithm uses anatomical knowledge of the human facial structure to search for features containing the landmarks. The accuracy of the algorithm in locating the landmarks is compared with values obtained from human experts. At present the algorithm attempts to locate 10 landmarks of 27 needed for a complete analysis. All 10 landmarks have been successfully located on five cephalograms of varying quality.

Algorithms↗
PubMed1989

A role for endothelial cell lipoxygenase in the oxidative modification of low density lipoprotein.

Oxidative modification of low density lipoprotein (LDL) has been implicated as a factor in the generation of macrophage-derived foam cells in vitro and in vivo. However, the exact mechanism of LDL oxidation has not been established. The present studies show that cellular lipoxygenase activity is involved in endothelial cell-induced oxidation of LDL. Inhibitors of lipoxygenase (but not inhibitors of cyclooxygenase) reduced LDL oxidation by as much as 70-85% under the conditions used. In contrast, the addition of pure (recombinant) superoxide dismutase inhibited by only approximately 25% under the same conditions. Oxidation of LDL by smooth muscle cells, on the other hand, was effectively inhibited by superoxide dismutase, as was Cu2+-catalyzed oxidation of LDL. When LDL was added to endothelial cell cultures within a dialysis bag, it did not undergo oxidative modification, suggesting that cell-LDL contact is necessary. We propose that an important element in cell-induced oxidation of LDL depends on (i) lipoxygenase oxidation of cellular lipids, followed by their exchange into LDL in the medium; (ii) direct lipoxygenase-dependent oxidation of LDL lipids during LDL-cell contact; (iii) or both.

5,8,11,14-Eicosatetraynoic Acid↗
PubMed1989

Low density lipoprotein undergoes oxidative modification in vivo.

It has been proposed that low density lipoprotein (LDL) must undergo oxidative modification before it can give rise to foam cells, the key component of the fatty streak lesion of atherosclerosis. Oxidation of LDL probably generates a broad spectrum of conjugates between fragments of oxidized fatty acids and apolipoprotein B. We now present three mutually supportive lines of evidence for oxidation of LDL in vivo: (i) Antibodies against oxidized LDL, malondialdehyde-lysine, or 4-hydroxynonenal-lysine recognize materials in the atherosclerotic lesions of LDL receptor-deficient rabbits; (ii) LDL gently extracted from lesions of these rabbits is recognized by an antiserum against malondialdehyde-conjugated LDL; (iii) autoantibodies against malondialdehyde-LDL (titers from 512 to greater than 4096) can be demonstrated in rabbit and human sera.

Alkaline Phosphatase↗
PubMed1989

Evidence for the presence of oxidatively modified low density lipoprotein in atherosclerotic lesions of rabbit and man.

Three lines of evidence are presented that low density lipoproteins gently extracted from human and rabbit atherosclerotic lesions (lesion LDL) greatly resembles LDL that has been oxidatively modified in vitro. First, lesion LDL showed many of the physical and chemical properties of oxidized LDL, properties that differ from those of plasma LDL: higher electrophoretic mobility, a higher density, higher free cholesterol content, and a higher proportion of sphingomyelin and lysophosphatidylcholine in the phospholipid fraction. A number of lower molecular weight fragments of apo B were found in lesion LDL, similar to in vitro oxidized LDL. Second, both the intact apo B and some of the apo B fragments of lesion LDL reacted in Western blots with antisera that recognize malondialdehyde-conjugated lysine and 4-hydroxynonenal lysine adducts, both of which are found in oxidized LDL; plasma LDL and LDL from normal human intima showed no such reactivity. Third, lesion LDL shared biological properties with oxidized LDL: compared with plasma LDL, lesion LDL produced much greater stimulation of cholesterol esterification and was degraded more rapidly by macrophages. Degradation of radiolabeled lesion LDL was competitively inhibited by unlabeled lesion LDL, by LDL oxidized with copper, by polyinosinic acid and by malondialdehyde-LDL, but not by native LDL, indicating uptake by the scavenger receptor(s). Finally, lesion LDL (but not normal intimal LDL or plasma LDL) was chemotactic for monocytes, as is oxidized LDL. These studies provide strong evidence that atherosclerotic lesions, both in man and in rabbit, contain oxidatively modified LDL.

Adult↗
PubMed1988

In vivo inhibition of foam cell development by probucol in Watanabe rabbits.

Previous studies from this laboratory have shown that oxidative modification of low-density lipoprotein (LDL) causes it to be recognized by the scavenger receptor of the macrophage. Consequently, the rate of degradation of oxidized LDL by macrophages can be 3 to 10 times that of native LDL. Antioxidants, such as probucol, are highly effective in preventing the oxidative modification of LDL. Our recent studies show that probucol treatment of LDL receptor-deficient Watanabe heritable hyperlipidemic (WHHL) rabbits selectively inhibits the degradation of LDL in fatty streak lesions (which are rich in macrophage-derived foam cells) without inhibiting degradation in nonlesioned areas (where degradation is predominantly in smooth muscle cells, which do not express the scavenger receptor). Furthermore, the rate of progression of lesions in probucol-treated animals was significantly slower than in a lovastatin-treated group maintained at equal total plasma cholesterol levels. These results strongly suggest that probucol, through an antioxidant activity not necessarily related to its ability to lower plasma cholesterol levels, can slow the progression of the foam-cell-rich fatty streak lesion of atherosclerosis.

Animals↗
PubMed1988

Lysophosphatidylcholine: a chemotactic factor for human monocytes and its potential role in atherogenesis.

Native low density lipoprotein (LDL) does not affect monocyte/macrophage motility. On the other hand, oxidatively modified LDL inhibits the motility of resident peritoneal macrophages yet acts as a chemotactic factor for circulating human monocytes. We now show that lysophosphatidylcholine (lyso-PtdCho), which is generated by a phospholipase A2 activity during LDL oxidation, is a potent chemotactic factor for monocytes. It is not chemotactic for neutrophils or for resident macrophages. Platelet-activating factor, after treatment with phospholipase A2, becomes chemotactic for monocytes, whereas the intact factor is not. Synthetic 1-palmitoyl-lyso-PtdCho showed chemotactic activity comparable to that of the lyso-PtdCho fraction derived from oxidized LDL. The results suggest that lyso-PtdCho in oxidized LDL may favor recruitment of monocytes into the arterial wall during the early stages of atherogenesis. Generation of lyso-PtdCho, either from LDL itself or from membrane phospholipids of damaged cells, could play a more general role in inflammatory processes throughout the body.

Animals↗
PubMed1988

Ischemic acute renal failure and antioxidant therapy in the rat. The relation between glomerular and tubular dysfunction.

The effects of antioxidant therapy with probucol were evaluated in rats subjected to 1 h renal ischemia and to 24 h reperfusion. Probucol exerted significant antioxidant effects in renal cortical tubules in vitro when exposed to a catalase-resistant oxidant. At 24 h probucol treatment (IP) improved single nephron glomerular filtration rate (SNGFR) (28.1 +/- 3.3 nl/min) in comparison to untreated ischemic (I) rats (15.2 +/- 3.0), primarily as a result of improving SNGFR in a population of low SNGFR, low flow and/or obstructed nephrons. However, absolute proximal reabsorption remained abnormally low in IP rats at 24 h (5.9 +/- 0.8 nl/min), and cell necrosis was greater than in I rats. Kidney GFR remained low in IP rats due to extensive tubular backleak of inulin measured by microinjection studies. Evaluations after 2 h of reperfusion revealed a higher SNGFR in IP (36 +/- 3.1 nl/min) than I rats (20.8 +/- 2.7 nl/min). Absolute proximal reabsorption was essentially normal (11.6 +/- 1.3 nl/min) in IP rats, which was higher than IP rats at 24 h and the concurrent I rats. Administration of the lipophilic antioxidant, probucol, increased SNGFR and proximal tubular reabsorption within 2 h after ischemic renal failure. Although SNGFR remained higher than I rats at 24 h, absolute reabsorption fell below normal levels and tubular necrosis was more extensive in IP rats. Early improvement in nephron filtration with antioxidants may increase load dependent metabolic demand upon tubules and increase the extent of damage and transport dysfunction.

Acute Kidney Injury↗
PubMed1988

Enzymatic modification of low density lipoprotein by purified lipoxygenase plus phospholipase A2 mimics cell-mediated oxidative modification.

Low density lipoprotein (LDL) can be oxidatively modified by cultured endothelial cells or by cupric ions, resulting in increased macrophage uptake of the lipoprotein. This process could be relevant to the formation of macrophage-derived foam cells in the early atherosclerotic lesion. The mechanism of endothelial cell modification of LDL is unknown. In the present work we show that incubation of LDL with purified soybean lipoxygenase, in the presence of pure phospholipase A2, can mimic endothelial cell-induced oxidative modification. Typically, incubation with lipoxygenase plus phospholipase A2 caused: 1) generation of about 15 nmol of thiobarbituric acid-reactive substances per mg of LDL protein; 2) a 4- to 7-fold increase in the rate of subsequent macrophage degradation of the LDL; 3) a 10-fold decrease in recognition by fibroblasts; 4) a marked increase in electrophoretic mobility in agarose gels; and, 5) disappearance of intact apoprotein B on SDS polyacrylamide gels. Degradation of the enzymatically modified LDL by macrophages was competitively inhibited by endothelial cell-modified LDL and by polyinosinic acid, but only partially suppressed by acetylated LDL. The lipoxygenase plus phospholipase A2-induced modification of LDL is not necessarily identical to endothelial cell modification, but it is a useful model for studying the mechanism of oxidative modification of LDL. This work also represents the first example of oxidative modification of LDL by specific enzymes leading to enhanced recognition by macrophages.

Animals↗