Sialic acid metabolism in rat liver: effect of carbon tetrachloride

Sialic acid metabolism was investigated in the livers of control rats and of rats treated with a single oral dose (1.5 ml/kg body weight) of carbon tetrachloride. The main change observed during the necrotic stage of CCl4 poisoning (18 h after treatment) was a highly significant reduction in sialyltransferase activity. Slight reciprocal changes in neuraminidase activities, i.e., a small decrease in cytosolic neuraminidase and a small increase in the membrane bound enzyme were also observed. At 72 h after CCl4 treatment, during the stage of liver regeneration, the main change was a marked elevation in membrane-bound neuraminidase (two fold above control values). Moderate increases in the specific activities of CMP-N-acetylneuraminic acid synthetase and sialyltransferase were also observed. A considerable decrease in the sialic acid content of the isolated smooth endoplasmic reticulum (one half of control values) was detected at 72 h after CCl4 administration. The sialic acid content of the rough endoplasmic reticulum, on the other hand, remained at control levels.     

Enhancement of hydroperoxide-dependent lipid peroxidation in rat liver microsomes by ascorbic acid

Simultaneous addition of ascorbic acid and organic hydroperoxides to rat liver microsomes resulted in enhanced lipid peroxidation (approximately threefold) relative to incubation of organic hydroperoxides with microsomes alone. No lipid peroxidation was evident in incubations of ascorbate alone with microsomes. The stimulatory effect of ascorbate on linoleic acid hydroperoxide (LAHP)-dependent peroxidation was evident at all times whereas stimulation of cumene hydroperoxide (CHP)-dependent peroxidation occurred after a lag phase of up to 20 min. EDTA did not inhibit CHP-dependent lipid peroxidation but completely abolished ascorbate enhancement of lipid peroxidation. Likewise, EDTA did not significantly inhibit peroxidation by LAHP but dramatically reduced ascorbate enhancement of lipid peroxidation. The results reveal a synergistic prooxidant effect of ascorbic acid on hydroperoxide-dependent lipid peroxidation. The inhibitory effect of EDTA on enhanced peroxidation suggests a possible role for endogenous metals mobilized by hydroperoxide-dependent oxidations of microsomal components.     

The effect of molybdenum on ascorbic acid metabolism in rats

1. The effect of dietary molybdenum on the growth rate and also on ascorbic acid metabolism in rats was studied. An excess of dietary molybdenum resulted in growth retardation and loss of weight. Tolerance to molybdenum was affected by the nature of the molybdenum salt administered. 2. Molybdenum ingestion altered certain aspects of ascorbic acid metabolism in rats. The conversion of d-glucuronolactone into l-ascorbic acid in vitro and the oxidative breakdown of l-ascorbic acid by liver enzymes decreased with high molybdenum intakes. The activity of liver uronolactonase was slightly inhibited. The activities of l-gulonate dehydrogenase and l-gulonate decarboxylase were not affected appreciably. 3. Molybdenum supplementation of the control diet resulted in an increase in ascorbic acid content of spleen and adrenal gland, and in a marked decrease in the urinary excretion of ascorbic acid and glucuronic acid. The implications of these findings are discussed.     

Protective effect of ascorbic acid against oxidative stress induced by inorganic arsenic in liver and kidney of rat

Ascorbic acid treatment in arsenic trioxide treated rats increased arsenic excretion, inhibited lipid peroxidation, improved GSH status, regulated GSSG turnover and also restored glutathione-S-transferases activity in liver and kidney. Suitable mechanisms leading to ascorbic acid protection have been discussed. Upregulation of GSH dependent enzymes was found to be necessary for a protective effect. Protection is finally attributed to higher GSH levels observed in the liver and kidney of ascorbic acid and inorganic arsenic treated rats. It is also concluded that ascorbic acid protection is influenced by gender dependent factors. Arsenic poisoning is a global problem now. Gender differences need to be considered while applying therapeutic measures.     

Alterations by perfluorooctanoic acid of glycerolipid metabolism in rat liver

The effects of perfluorooctanoic acid (PFOA) feeding on hepatic levels of glycerolipids and the underlying mechanism were investigated. Feeding of rats with 0.01% of PFOA in the diet for 1 week caused an increase in the contents of phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn), phosphatidylinositol (PtdIns), phosphatidylserine (PtdSer) and triglyceride (TG), which were 2.2, 2.4, 2.4, 1.6 and 5.2 times over control, respectively, on the basis of whole liver. The activities of glycerol-3-phosphate acyltransferase, diacylglycerol kinase and PtdSer decarboxylase were significantly increased upon PFOA feeding, whereas the activities of CTP:phosphoethanolamine cytidylyltransferase and PtdEtn N-methyltransferase were decreased. On the other hand, the activity of CTP:phosphocholine cytidylyltransferase was not increased by PFOA. Upon PFOA feeding, hepatic level of 16:0-18:1 PtdCho was markedly increased and, by contrast, the levels of molecular species of PtdCho which contain 18:2 were decreased, resulting in the reduced concentration of molecular species of serum PtdCho containing 18:2. The increase in the level of hepatic 16:0-18:1 PtdCho seemed to be due to 3-fold increase in the activities of both delta9 desaturase and 1-acylglycerophosphocholine (1-acyl-GPC) acyltransferase. The mechanism by which PFOA causes the accumulation of glycerolipids in liver was discussed.     

Inhibition of key enzymes of carbohydrate metabolism in regenerating mouse liver by ascorbic acid.

The effect of ascorbic acid on the key enzymes of carbohydrate metabolism e.g. hexokinases, phosphofructokinase, pyruvate kinase, lactate dehydrogenase, glucose-6-phosphate dehydrogenase and malic enzyme was determined in regenerating mouse liver. All the enzymes showed a significant increase in the activity during regeneration. Ascorbic acid reduced the activities of the enzymes in regenerating liver. A decrease in liver weight in ascorbic acid treated animals may be correlated with its effect on these enzymes as glycolytic pathway is the main source of energy required by the dividing cells.     

Regeneration of ascorbic acid by rat colon

Plants and animals alike use ascorbic acid in a variety of reactions that result in net generation of dehydro-L-ascorbic acid. The ability to reduce dehydro-L-ascorbic acid back to ascorbic acid would conserve "total ascorbate" and would help to maintain the toxic oxidized form of the molecule at a low level. This study evaluated the rate of dehydro-L-ascorbic acid reduction either by following the rate of NADPH consumption or by analysis of the amount of 14C-labeled dehydro-L-ascorbic acid converted to ascorbic acid. A large percentage of the NADPH consumed by a semipurified preparation of rat colonic mucosa in vitro was dependent on the presence of dehydro-L-ascorbic acid. The tissue factor active in regenerating ascorbic acid is intermediate in size between cytochrome c and blue dextran. The present results indicate that the mucosa reduced dehydro-L-ascorbic acid by a cytosolic enzyme that uses NADPH as a hydrogen donor. Subsequent to precipitation by ammonium sulfate, the 55-70% fraction contains most of the reductase activity while consisting of only 17% of the cellular soluble protein.     

Changes in catecholamine metabolism by ascorbic acid deficiency in spontaneously hypertensive rats unable to synthesize ascorbic acid

We have previously reported the establishment of a novel rat strain, SHR-od, with both spontaneous hypertension and a defect of ascorbic acid biosynthesis. Blood pressure in mature SHR-od fed an ascorbic acid-supplemented diet is over 190-200 mmHg, while it decreased to around 120 mmHg at 4-5 weeks after the cessation of ascorbic acid supplementation. With regard to possible mechanisms of blood pressure lowering, we focused on catecholamine synthesis in adrenal glands, since catecholamine is a major factor for blood pressure regulation and ascorbic acid is a co-factor of dopamine beta-hydroxylase (DBH) in catecholamine biosynthesis. Male SHR-od (25-week-old) and normotensive ODS rats with a defect in ascorbic acid biosynthesis (25-week-old) were fed a Funabashi-SP diet with or without ascorbic acid (300 mg/kg diet) for 28 days or 35 days. In SHR-od, systolic blood pressure (191 +/- 6 mmHg) began to decrease from day 21 in the ascorbic acid-deficient group, whereas no significant difference was found in ODS rats. In spite of significant lowering of blood pressure, no significant differences were found in catecholamine levels in serum, adrenal glands and brain on day 28. On day 35, however, urinary excretion of norepinephrine and epinephrine in the ascorbic acid-deficient SHR-od were higher at 490% (P < 0.05) and 460% (P < 0.05) of the respective control. Serum catecholamine concentrations and the adrenal catecholamine content tended to be higher in the ascorbic acid-deficient SHR-od than the control of SHR-od and reached to similar level in ODS rats. The administration of ascorbic acid (intraperitoneal injection, 60 mg ascorbic acid/kg body weight, once a day) to the ascorbic acid-deficient SHR-od restored blood pressure to the range 180-190 mmHg within two days. These findings indicate that ascorbic acid deficiency affects catecholamine metabolism in the adrenal glands of SHR-od in response to blood pressure lowering, suggesting catecholamines are not involved in the mechanism for the remarkable reduction in blood pressure in response to ascorbic acid deficiency.     

Fatty acid metabolism in the perfused rat liver

1. The formation of acetoacetate, beta-hydroxybutyrate and glucose was measured in the isolated perfused rat liver after addition of fatty acids. 2. The rates of ketone-body formation from ten fatty acids were approximately equal and independent of chain length (90-132mumol/h per g), with the exception of pentanoate, which reacted at one-third of this rate. The [beta-hydroxybutyrate]/[acetoacetate] ratio in the perfusion medium was increased by long-chain fatty acids. 3. Glucose was formed from all odd-numbered fatty acids tested. 4. The rate of ketone-body formation in the livers of rats kept on a high-fat diet was up to 50% higher than in the livers of rats starved for 48h. In the livers of fat-fed rats almost all the O(2) consumed was accounted for by the formation of ketone bodies. 5. The ketone-body concentration in the blood of fat-fed rats rose to 4-5mm and the [beta-hydroxybutyrate]/[acetoacetate] ratio rose to 11.5. 6. When the activity of the microsomal mixed-function oxidase system, which can bring about omega-oxidation of fatty acids, was induced by treatment of the rat with phenobarbitone, there was no change in the ketone-body production from fatty acids, nor was there a production of glucose from even-numbered fatty acids. The latter would be expected if omega-oxidation occurred. Thus omega-oxidation did not play a significant role in the metabolism of fatty acids. 7. Arachidonate was almost quantitatively converted into ketone bodies and yielded no glucose, demonstrating that gluconeogenesis from poly-unsaturated fatty acids with an even number of carbon atoms does not occur. 8. The rates of ketogenesis from unsaturated fatty acids (sorbate, undecylenate, crotonate, vinylacetate) were similar to those from the corresponding saturated fatty acids. 9. Addition of oleate together with shorter-chain fatty acids gave only a slightly higher rate of ketone-body formation than oleate alone. 10. Glucose, lactate, fructose, glycerol and other known antiketogenic substances strongly inhibited endogenous ketogenesis but had no effects on the rate of ketone-body formation in the presence of 2mm-oleate. Thus the concentrations of free fatty acids and of other oxidizable substances in the liver are key factors determining the rate of ketogenesis.