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Biomedical subjects

A C Chan

Publications and source records attributed to A C Chan.

At least 217 records · Page 12Linked to original sources
PubMed1989

Phosphatidylcholine metabolism in isolated rat heart: modulation by ethanol and vitamin E.

Prolonged ethanol administration has been reported to cause defects in cardiac performance and abnormal cardiac lipid contents. However, little is known regarding the short-term administration of ethanol to the perfused heart and its effect on cardiac phospholipid metabolism. In this study, the isolated Langendorff heart perfusion was used as a model to study the effects of ethanol and a combination of ethanol and vitamin E (DL-alpha-tocopherol) on phospholipid metabolism. When perfused with 1% ethanol for 4 h, the major cardiac phospholipids were not altered but a 60% increase in lysophosphatidylcholine level was observed. Studies on the lysophosphatidylcholine metabolic enzymes revealed that phospholipase A (both phospholipase A1 and A2) activity was enhanced in the ethanol-perfused heart, but lysophospholipase and acyltransferase activities were unaffected by ethanol treatment. When the heart was perfused with 1% ethanol in the presence of 50-100 microM vitamin E, the ethanol-induced lysophosphatidylcholine accumulation was completely abolished. This was largely attributed to the attenuation of phospholipase A activities by vitamin E. In order to delineate the opposing effects of ethanol and vitamin E on phospholipid metabolism in the heart, phospholipase A activities in the subcellular fractions were determined in the presence of 0.5-2.0% ethanol or a combination of 1% ethanol and 0-100 microM vitamin E. Ethanol alone exhibited a biphasic effect on phospholipase A activity with maximum stimulation of enzyme activities at 1% concentration. When phospholipase A was assayed in 1% ethanol and vitamin E (25-100 microM), its activity was inhibited by vitamin E in a dose-dependent manner. The mechanism by which ethanol enhanced phospholipase A activities was further investigated with a partially purified enzyme from the rat heart cytosol. Kinetic studies with different concentrations of phosphatidylcholine revealed that at low substrate concentrations, ethanol was inhibitory to the reaction, whereas at high substrate concentrations, the reaction was enhanced by ethanol. Vitamin E (50 microM) completely abolished the ethanol-induced enhancement of enzyme activity in a noncompetitive manner. Since lysophosphatidylcholine is cytolytic at high concentration and its accumulation in the heart has been postulated as a biochemical cause of cardiac dysfunction, the level of the lysolipid in the heart must be under rigid control. Our result suggest that the modulation of cardiac phospholipase A activity is an important mechanism for the the regulation of lysophosphatidylcholine levels in the rat heart.

Acyltransferases↗
PubMed1989

Effects of dietary vitamin E on the biosynthesis of 5-lipoxygenase products by rat polymorphonuclear leukocytes (PMNL).

Activation of polymorphonuclear neutrophils (PMNL) leads to the release of arachidonate from cellular phospholipids via a phospholipase A2, and conversion of products of the 5-lipoxygenase pathway. Evidence to date indicates the dietary vitamin E ((R,R,R)-alpha-tocopherol) can influence both cyclooxygenase and phospholipase A2 activities and that the effect of this vitamin is cell/tissue specific. The present study was undertaken in order to examine the effects of varying dietary tocopherol on PMNL tocopherol content and 5-lipoxygenase product profile using the ionophore A23187 as stimulant in the presence and absence of exogenous arachidonate. Feeding semi-purified diets containing 0, 30 or 3000 ppm of (R,R,R)-alpha-tocopherol acetate to weanling rats for 17 weeks resulted in a dose-related enrichment of PMNL tocopherol. Stimulation of PMNL elicited a significant and rapid loss of tocopherol. When PMNL were stimulated with A23187 alone, the synthesis of 5-HETE, LTB4 and 19-hydroxy-LTB4 was decreased in proportion to increasing dietary tocopherol concentrations. However, when exogenous arachidonate was provided with A23187, intermediate amounts of dietary tocopherol (30 ppm) still suppressed the formation of 5-lipoxygenase products, but high doses (3000 ppm) did not have any additional inhibitory effect. This differential response to high concentrations of vitamin E in the presence and absence of exogenous arachidonate highly suggest that at these concentrations, tocopherol may act principally at the level of substrate release whereas at lower concentrations, 5-lipoxygenase is inhibited. Data from this study demonstrated that attenuation of the formation of 5-lipoxygenase products in PMNL can be achieved by dietary vitamin E enrichment.

Animals↗
PubMed1989

Single dose prophylactic antibiotics in caesarean sections.

A prospective, double-blind randomized trial was carried out at the Department of Obstetrics and Gynaecology, the Chinese University of Hong Kong, involving 4 groups of patients receiving a single intravenous dose of placebo, ampicillin, ampicillin and metronidazole, and ampicillin and sulbactam before operation. No difference in postoperative morbidity was present between the 4 groups of patients.

Adult↗
PubMed1988

Effect of vitamin E enrichment on arachidonic acid release and cellular phospholipids in cultured human endothelial cells.

The effect of (R,R,R)-alpha-tocopherol on agonist-stimulated arachidonate release and cellular lipids was investigated in cultured human umbilical cord endothelial cells. Endothelial cells in culture incorporate added tocopherol in a dose-dependent manner at both physiological (23.2 microM) or pharmacological (92.8 microM) concentrations which were well tolerated by the cells, as judged by unaltered cell number and viability. Two experiments were conducted in which cells were either incubated with (R,R,R)-alpha-tocopherol followed by labelling with [1-14C]arachidonic acid or they were labelled with arachidonate followed by incubation with tocopherol. Irrespective of the sequence of incubation with arachidonate and tocopherol, (R,R,R)-alpha-tocopherol-enriched cells released significantly more labelled arachidonate when stimulated with thrombin (2.5 U/ml) or ionophore A23187 (1 microM) for 10 min. The magnitude of [1-14C]arachidonate release was higher from ionophore A23187 stimulation than from thrombin stimulation, but the trend of increased arachidonate release in tocopherol-enriched cells was the same. Results from these studies demonstrate that (R,R,R)-alpha-tocopherol can stimulate arachidonate release in human endothelial cells. This observation is in direct contrast to the role of tocopherol, which has been shown to inhibit platelet and cardiac phospholipase A2 activity in rats, and to reduce thrombin-stimulated thromboxane release in rat platelets.

Arachidonic Acid↗
PubMed1988

Origin of the fourth component of complement related Chido and Rodgers blood group antigens.

We have reviewed the relationship between C4 and its related blood group and discussed the mechanisms whereby a fragment of C4 could become attached to erythrocytes (E). We hypothesize that there is chronic fluid-phase activation of C4 by either C1 to form C4b or spontaneous cleavage of the thioester to form iC4. These activated molecules bind to E. Proteolytic degradation of the bound C4b or iC4 would leave a covalently attached fragment of C4 on E and thereby give rise to the Ch and Rg blood group antigens. This system is of further immunopathologic interest since this 'normal' activation or turnover of C4 is closely regulated. In patients deficient in regulatory proteins, this spontaneous or normal turnover of C4 and C3 may initiate a pathologic condition.

Animals↗
PubMed1988

Effect of dietary vitamin E on platelet tocopherol values and 12-lipoxygenase activity.

This study was designed to evaluate the effects of different amounts of dietary vitamin E on platelet tocopherol levels and 12-lipoxygenase activity when exogenous arachidonic acid was used as substrate. Weanling male Sprague-Dawley rats were fed diets containing 0, 50, and 5000 ppm of D-alpha-tocopherol acetate for 4 months. Platelet tocopherol was increased with increasing concentrations of dietary vitamin E; however, the conversion of exogenously added arachidonate by platelet to 12-HETE (12-hydroxyeicosatetraenoic acid) and thromboxane B2 from these three dietary groups was essentially the same. This study provides direct evidence that platelet 12-lipoxygenase activity is independent of its vitamin E content when exogenously added arachidonate was used as substrate.

Animals↗
PubMed1987

Regulation by vitamin E of phosphatidylcholine metabolism in rat heart.

Lysophosphatidylcholine is the major lysophospholipid in mammalian tissues and has been shown to be cytolytic at high concentrations. In the present study we demonstrated that the level of lysophosphatidylcholine was significantly increased in the heart of rats fed with a vitamin E-deficient diet. Moreover, the cardiac lysophosphatidylcholine level was decreased in rats fed with a high vitamin E diet. The alterations in cardiac lysophosphatidylcholine level by dietary vitamin E were attributed to the changes in the activity of cardiac phospholipase A. Dietary vitamin E affected both phospholipase A1 and A2 in the same manner, but had no effect on the other major enzymes which are responsible for the metabolism of lysophosphatidylcholine. Kinetic studies revealed that the inhibition of enzyme activity by vitamin E was essentially non-competitive. The accumulation of lysophosphatidylcholine in the rat heart may be one of the underlying biochemical causes of the observed cardiac dysfunctions produced during vitamin E deficiency.

1-Acylglycerophosphocholine O-Acyltransferase↗
PubMed1986

Sequence analysis of the COOH terminus of the alpha-chain of the fourth component of human complement. Identification of the site of its extracellular cleavage.

The predominant form of the fourth component of complement in human and murine plasma (C4p) has an Mr approximately 5,000 less than C4 secreted by hepatocytes or macrophages (C4s). Here we demonstrate that the difference in Mr results from excision of a peptide from the COOH terminus of the alpha-chain of C4s. The site of truncation of the alpha-chain of C4 in plasma was established by sequence analysis of the COOH-terminal cyanogen bromide fragment isolated by high performance liquid chromatography. Sequential Edman degradation, digestion with carboxypeptidase Y, analysis of amino acid composition, and partial sequence analysis of an overlapping tryptic peptide established the sequence of this peptide to be Glu-Ala-Asn-Glu-Asp-Tyr-Glu-Asp-Tyr-Glu-Tyr-Asp-Glu-Leu-Pro-Ala. Comparison of our results with reported sequences establishes the COOH-terminal alanine to be residue 748 in the alpha-chain. This identifies the site at which C4s is cleaved by an extracellular protease and suggests that the protease has an elastase-like activity.

Amino Acid Sequence↗
PubMed1986

Vitamin E inhibits platelet phospholipase A2.

One of the most important functions of phospholipase A2 is the release of arachidonic acid from membrane phospholipids for the synthesis of biologically active eicosanoids. We have demonstrated in our laboratory that vitamin E inhibits platelet phospholipase A2 in a dose-dependent manner. Rats fed a 100 ppm or a 1000 ppm vitamin E diet exhibit diminished phospholipase A2 activity compared to those fed a vitamin E-free diet. Addition of vitamin E to a sonicated platelet suspension resulted in further suppression of the phospholipase A2 activity in all groups of rats. In order to gain insight into the mechanism of vitamin E inhibition of platelet phospholipase A2, we partially purified this enzyme by gel filtration chromatography. Enzyme activity was localized in the soluble supernatant fraction of a high-speed spin. This partially purified rat platelet phospholipase A2 had an absolute requirement for Ca2+ and was inhibited by various forms of tocopherol. Tocol inhibited the enzyme to a greater extent than either D- or DL-alpha-tocopherol, while there was little or no effect from DL-alpha-tocopherol acetate. These results emphasize the importance of the hydroxyl moiety on the chromanol of the vitamin E molecule for its inhibitory action, compared to that of the methyl groups which are absent in tocol. This inhibitory action of vitamin E on platelet phospholipase A2 suggests a crucial function for vitamin E in regulating arachidonate release from the membrane phospholipids and its subsequent metabolism.

Animals↗
PubMed1986

Transitory stimulation of human platelet 12-lipooxygenase by vitamin E supplementation.

Human platelet lipooxygenase converts arachidonic acid to 12-hydroperoxy-eicosatetraenoic acid (12-HPETE), which is rapidly reduced by peroxidases to 12-hydroxy-eicosatetraenoic acid (12-HETE). This study was conducted to examine the effect of vitamin E supplementation on platelet 12-lipooxygenase activity. Sixteen healthy subjects were supplemented with 400 IU/day of either D-alpha-tocopherol (268 mg) or DL-alpha-tocopherol (364 mg) for 4 wk. Supplements elicited a transitory increase of lipooxygenase activity but a suppression of peroxidase activity, as indicated by increased 12-HETE production and 12-HPETE accumulation. Plasma-tocopherol concentration was double the presupplement value and remained stable during supplementation. Neither age, sex, nor isomeric form of tocopherol supplement significantly influenced the pattern of response. Results show that vitamin E exerts a differential effect on platelet lipooxygenase and peroxidase activities.

12-Hydroxy-5,8,10,14-eicosatetraenoic Acid↗
PubMed1986

Effects of fatty acids on phosphatidylcholine biosynthesis in isolated hamster heart.

The effects of stearic, oleic, and arachidonic acids on phosphatidylcholine biosynthesis in the hamster heart were investigated. When hamster hearts were perfused with labelled choline in the presence of fatty acids, biosynthesis of phosphatidylcholine was stimulated only by stearic acid. Stearic acid was found to accumulate in unesterified (free) form in the hamster heart after perfusion. The stimulation by stearic acid was mediated in vivo by an enhancement of CTP:phosphocholine cytidylyltransferase activity in the microsomal fraction of the hamster heart and the enzyme activity in the cytosolic fraction was not affected. In contrast with the observations in rat hepatocytes, cytidylyltransferase from the hamster heart was not stimulated directly by stearic acid. The selective activation of the microsomal enzyme when the heart was perfused with stearic acid suggests that activation of the enzyme was mediated via the modification of the membrane by stearic acid.

Animals↗
PubMed1986

Competition of n-3 and n-6 polyunsaturated fatty acids in the isolated perfused rat heart.

When perfused with exogenous arachidonic acid (AA) or eicosapentaenoic acid (EPA), the rat heart incorporated these fatty acids into phospholipids, chiefly as phosphatidylcholine. The pattern of fatty acid incorporation at any given concentration of fatty acid in the perfusate was not different between n-3 and n-6 polyenoates. When rat hearts were perfused with the same amounts but different mixtures of EPA and AA, the incorporation of EPA showed a marked increase proportional to the EPA/AA ratio present in the perfusate. Results indicated that cardiac muscle phospholipids incorporate n-3 and n-6 polyenoates equally effectively and hence enrichment of n-3 polyenoate can displace AA in the cardiac phospholipid pool.

Acylation↗
PubMed1985

Increased arachidonate incorporation in perfused heart phospholipids from vitamin E-deficient rats.

When perfused with exogenous arachidonic acid (AA), the rat heart incorporates this eicosanoid precursor into the phospholipids. The incorporation of AA into phosphatidylcholine was 6-fold higher than the incorporation of AA into phosphatidylethanolamine. When vitamin E-deficient rat hearts were perfused with AA, there was a marked increase in incorporation of AA into both phosphatidylcholine and phosphatidylethanolamine compared to rats fed with a vitamin E supplemented diet. The result of this study suggests that vitamin E has a regulatory role in phospholipid biosynthesis in the mammalian heart.

Animals↗
PubMed1985

Characterization of human placental blood vessel phospholipase A2, demonstration of substrate selectivity for arachidonyl-phosphatidylcholine.

The hydrolysis of acyl esters in phosphatidylcholine by phospholipase A2 (PLA2) for human placental blood vessel was investigated. The enzyme displayed an alkaline pH optimum and an absolute requirement of Ca2+ for activity. In contrast to rat tissues, the human placental blood vessel PLA2 showed a selective preference for arachidonate over linoleate acyl group at the sn-2 position of phosphatidylcholine.

Arachidonic Acids↗
PubMed1985

Oligosaccharide structure of human C4.

The oligosaccharide structure of human C4 was studied by using C4 purified from plasma and C4 secreted by human hepatoma-derived cell line, HepG2. The alpha- and beta-chains of human C4 are glycosylated, whereas the gamma-chain is devoid of carbohydrate. The alpha-chain has three complex fucosylated oligosaccharides of the biantennary type, one each on the alpha 2, alpha 3, and alpha 4 fragments. The beta-chain has a single high mannose oligosaccharide primarily of the Man9GlcNAc2 type. The approximately 2000 Mr difference between the alpha-chains of the two C4 gene products (C4A and C4B) was localized to the alpha 2 fragment and is not due to carbohydrate. Sulfation of the C4 alpha-chain was localized to the alpha 4 fragment of the alpha-chain. Hence, the Mr difference between the two gene products is likely to reside in amino acid differences. The oligosaccharide structure of three incompletely processed C4 molecules was also analyzed. These molecules have the oligosaccharide composition of the appropriate individual subunits. Therefore, intracellular proteolytic processing to the multi-chain form of C4 is not required for proper oligosaccharide processing.

Acetylglucosaminidase↗
PubMed1984

The C4 and Slp genes of the complement region of the murine H-2 major histocompatibility complex.

Recent analyses, at the protein and DNA levels of structure, of the murine complement components C4 and the closely related sex-limited protein, Slp have led to new insights into the H-2/S region-linked C4 and Slp genes and their products. The primary products are 200 000 Da precursors which are cleaved, intracellularly and extracellularly, into the the mature alpha-beta-gamma-subunit molecules of plasma. Precursor order of subunits is beta-alpha-gamma; a complementary DNA clone spanning the alpha-gamma junction has been extensively analysed. The C-terminal of the alpha-chain is of particular interest because of post-secretion processing which differentiates 'secreted' and 'plasma' forms of C4, both apparently functional, and because allelic variants of C4 and the Slp protein, which differ substantially in molecular masses, owe their differences principally to different levels of glycosylation of the alpha-chain. Allelic variations in rate of C4 synthesis (C4-high compared with C4-low) have been analysed in cultures of hepatocytes and macrophages. Three distinct modes of genetic regulation of the expression of the Slp protein have been identified.

Alleles↗
PubMed1984

Functional studies on the secreted form of human C4 (C4s), two incompletely processed two-subunit C4 molecules (beta - alpha + gamma and beta + alpha - gamma), and pro-C4.

The functional properties of the secreted form of C4 (C4s), which has a Mr approximately 5000 greater than the predominant C4 molecule found in plasma (C4p), two incompletely processed two-chain C4 molecules (beta - alpha + gamma and beta + alpha - gamma), and the extracellular C4 precursor (designated pro-C4(E)] were evaluated. All four molecules are secreted in parallel by a human hepatoma-derived cell line (Hep G2). Secretion of hemolytically active C4 is linear up to approximately 12 hr, peaks at 24 hr, and then progressively decreases over the next 48 hr. This loss of C4s functional activity parallels the proteolytic conversion of C4s to C4bs. To compare the hemolytic efficiencies of C4s and C4p, a solid-phase competitive radioimmunoassay was developed to permit measurement of the small quantities of C4 antigen in these cultures. The hemolytic efficiencies of C4s and C4p were similar. These results indicate that extracellular processing of C4s to C4p does not modulate the hemolytic activity of the molecule. Consistent with their ability to bind methylamine, both the alpha s-chain and the alpha - gamma subunit undergo denaturation-induced autolysis. The extracellular and intracellular pro-C4 molecules are also sensitive to autolytic cleavage. Interestingly, the beta - alpha subunit is resistant to autolysis. In experiments in which C4s and C4p were cleaved by C1-s to C4bs, C4(beta - alpha + gamma), C4(beta + alpha - gamma), and pro-C4(E) were resistant to C1-s cleavage and thus hemolytically inactive relative to C4s. These data indicate that processing of C4 to a three-chain structure is required to provide the proper conformation for efficient activation by C1.

Animals↗