Search PubMed⌕ Search

Biomedical subjects

H Brunengraber

Publications and source records attributed to H Brunengraber.

At least 91 records · Page 5Linked to original sources
PubMed1984

The shunt pathway of mevalonate metabolism in the isolated perfused rat liver.

The shunt pathway of mevalonate metabolism (Edmond, J., and Popják, G. (1974) J. Biol. Chem. 249, 66-71) has been studied in isolated livers from fed rats perfused with physiological concentrations of variously labeled [14C]mevalonates. The measured rates of 14CO2 production were converted to rates of mitochondrial acetyl-CoA production from mevalonate by methods which take into account underestimations of metabolic rates derived from 14CO2 production. Our data confirm that the shunt pathway leads to mitochondrial acetyl-CoA. The apparent negligible rate of mevalonate shunting in liver, previously reported by others, stems from the very low contribution (congruent to 0.1%) of plasma mevalonate to total mevalonate metabolism in the liver. This contribution was assessed from the relative incorporations of 3H2O and [5-14C]mevalonate into sterols. In livers from fed rats, the shunt diverts about 5% of the production of mevalonate. The total rate of mevalonate shunting in the liver is about 200 times greater than in two kidneys. The liver is therefore the main site of mevalonate shunting in the rat.

Acetyl Coenzyme A↗
PubMed1984

The shunt pathway of mevalonate metabolism in the isolated perfused rat kidney.

The shunt pathway of mevalonate metabolism (Edmond, J., and Popják, G. (1974) J. Biol. Chem. 249, 66-71) has been studied in isolated kidneys from rats perfused with physiological concentrations of variously labeled [14C]- and [3H]mevalonates. The rate of operation of the shunt pathway was quantified by the production of either 14CO2 or 3H2O from the tracers. The measured rates of 14CO2 production from [14C] mevalonate were converted to rates of mitochondrial acetyl-CoA production by methods which take into account underestimations of metabolic rates derived from 14CO2 production. We have shown that the sex difference in renal shunting of mevalonate (Wiley, M. H., Howton, M. M., and Siperstein, M. D. (1979) J. Biol. Chem. 254, 837-842) occurs at physiological levels of substrate. The shunt pathway diverts up to 17% of the flux of mevalonate entering the cholesterol synthesis pathway in the kidney. It may, therefore, play a role in the long term regulation of cholesterol synthesis in this organ, as had been hypothesized by Edmond and Popják.

Acetyl Coenzyme A↗
PubMed1984

Inhibition of N-nitrosodimethylamine metabolism by ethanol and other inhibitors in the isolated perfused rat liver.

The effects of several potential inhibitors of N-nitrosodimethylamine (NDMA) metabolism have been studied in isolated perfused rat livers. Low concentrations of ethanol (less than 0.5 mM) were found to strongly inhibit hepatic metabolism of NDMA. This effect does not require the metabolism of ethanol. The metabolism and toxicity of NDMA may therefore be influenced by the intake of alcohol as well as by ethanol produced endogenously by the intestinal flora. Other compounds found to inhibit NDMA metabolism include n-propanol, tert-butanol, 4-methylpyrazole, amino-triazole and acetaldehyde.

1-Propanol↗
PubMed1984

An isolated rat liver model for the evaluation of thermal techniques to quantify perfusion.

An isolated, thermally regulated, perfused rat liver model system is presented. The model was developed to evaluate thermal methods to quantify perfusion in small volumes of tissue. The surgically isolated rat liver is perfused with an isothermal oxygenated Krebs-Ringer bicarbonate buffer solution via the cannulated portal vein. A constant-pressure head variable-resistance scheme is utilized to control the total flow to the liver. Total flow is quantified by hepatic vein collection. The spatial distribution of perfusion within the liver is determined using two independent methods. In the first method, radio-labelled microspheres are injected into the portal vein, and the regional flow distribution is determined from the relative radioactivity of each section of tissue. In the second method, the tissue is thermally perturbed, and the time constant of the tissue temperature recovery is measured. The regional distribution is determined from the relative time constants of each section of tissue. Both methods require the measurement of total liver flow to determine the absolute perfusion at each point. Results obtained by the two methods were well correlated (0.973). The rat liver system offers a stable, controllable, and measurable perfusion model for the evaluation of new perfusion measurement techniques.

Animals↗
PubMed1983

Assessment of the flux of mitochondrial acetyl-CoA in liver and kidney by using the differential production of 14CO2 from tracers of (1-14C)- and (2-14C)-labelled 4-methyl-2-oxovalerate.

A procedure is described to convert rates of (14)CO(2) production into rates of mitochondrial acetyl-CoA production from a (14)C-labelled substrate. The principle is illustrated in perfused rat liver and kidney by the differential yield of (14)CO(2) from 4-methyl-2-oxo[1-(14)C]valerate and 4-methyl-2-oxo[2-(14)C]valerate.

Acetyl Coenzyme A↗
PubMed1983

Rates of catabolism calculated from 14CO2 production: artifacts and realities.

We present evidence that the metabolism of acetate and ethanol by the liver markedly decreases the yield of label from mitochondrial [14C]acetyl-CoA to 14CO2. The production of [14C]acetyl-CoA from a [14C]labeled substrate can be calculated from the production of 14CO2 if one assesses (1) 14CO2 reincorporation and (2) the yield of label from [14C]acetyl-CoA to CO2.

Acetates↗
PubMed1983

Pharmacokinetic model for N-nitrosodimethylamine based on Michaelis-Menten constants determined with the isolated perfused rat liver.

A pharmacokinetic model was constructed to describe the absorption, distribution, and metabolic clearance of N-nitrosodimethylamine. The model is composed of two compartments, total body water and the liver, which are linked by blood flow. Metabolic clearance is presumed to occur only in the liver. Liver clearance kinetics was determined with isolated perfused livers. Clearance appeared to obey Michaelis-Menten kinetics with Km = 8.3 +/- 4.8 microM and Vmax = 0.15 +/- 0.02 mumol/min . liver. The observed value for Km is about 1 order of magnitude lower than any observed when clearance is determined using liver microsome preparations. The model is used to calculate whole-body clearance of N-nitrosodimethylamine and relative tissue exposure as a function of the route of administration. The calculations are compared with previously published experimental data, and it is shown that the accuracy of the model for low doses is a result of the novel value observed for Km in the perfused liver.

Animals↗
PubMed1982

Underestimation of metabolic rates owing to reincorporation of 14CO2 in the perfused rat liver.

(14)CO(2) production by perfused rat livers was simulated by infusing NaH(14)CO(3) into the perfusate. Recovery of label as (14)CO(2) gas + perfusate bicarbonate was 45-85%. Rates of (14)CO(2) exchange in the liver are 3-70 times greater than net rates of CO(2) production. Therefore (14)CO(2) reincorporation can lead to significant underestimations of rates of oxidation of (14)C-labelled substrates in liver.

Animals↗
PubMed1982

Lipogenesis from ketone bodies in the isolated perfused rat liver. Evidence for the cytosolic activation of acetoacetate.

The production of ketone bodies by the isolated perfused rat liver has been measured by the dilution of the specific activity of tracer amounts of beta-hydroxy[3-14C]butyrate and by accumulation in the perfusate. The latter method has been found to underestimate ketogenesis by 12 to 44% because it does not take into account acetoacetate utilization by the liver. Incorporation of ketone bodies into fatty acids and 3-beta-hydroxysterols was compared to total lipid synthesis measured by incorporation of tritium from tritiated water. A preferential labeling of 3-beta-hydroxysterols over fatty acids was observed, which is consistent with the activation of acetoacetate in the cytosol by acetoacetyl-CoA synthetase. Ketone bodies contribute 19 to 80% of the carbon incorporated into sterols and up to 22% of the carbon incorporated into fatty acids, depending upon the metabolic status of the liver. The activity of acetoacetyl-CoA synthetase is more than sufficient to account for the rate of ketone body utilization. Conditions that decrease the citrate cleavage pathway of acetyl group translocation through the mitochondrial membrane are associated with an increase in carbon flux through acetoacetyl-CoA synthetase. Formation of acetoacetate in the mitochondria and its utilization in the cytosol thus appear to be a secondary pathway of acetyl group translocation operating concurrently with the predominant citrate cleavage pathway.

Acetoacetates↗
PubMed1982

Metabolism of plasma mevalonate in rats and humans.

A circadian rhythm in plasma mevalonate was identified in human subjects. This variation, over a 5-fold range, is paralleled by a rhythm in urinary excretion. No such diurnal change in plasma mevalonate was observed in schedule-fed, light-cycled rats, despite the presence of a pronounced rhythm in liver HMG-Coa reductase and sterol synthesis. A linear correlation was found between liver HMG-CoA reductase activity and the rate of hepatic sterol synthesis. Sterol synthesis accounted for 59% of the HMG-CoA reductase activity. A 4-fold increase in plasma mevalonate following bilateral nephrectomy did not feed back on liver HMG-CoA reductase. Turnover rates for circulating R- and S-mevalonate were determined by the kinetics of tritiated tracers. S-Mevalonate exhibited first-order kinetics with a T 1/2 of 19 to 23 min, while R-mevalonate kinetics could be resolved into two phases with half-lives of 9 and 42 min. The renal uptake of circulating mevalonate was measured by the initial rate of increase in plasma mevalonate immediately following bilateral nephrectomy; this was confirmed by determination of the renal arterio-venous difference. This value ranges between 500 and 600 pmol/min for a 250-g rat.

Adult↗
PubMed1982

Origin of biliary cholesterol and lecithin in the rat: contribution of new synthesis and preformed hepatic stores.

The contribution of de novo synthesis to the secretion of cholesterol and lecithin in bile was assessed in isolated rat livers, perfused with a lipid-free medium. Cholesterol and lecithin synthesis were measured by the incorporation of tritiated water and [14C]-choline, respectively. Taurocholate stimulated the secretion of biliary lipids to the same extent in perfused livers and in live rats. During the first hour of perfusion, and when hepatic synthesis was active, newly synthesized cholesterol accounted for about 10% of biliary cholesterol and newly synthesized lecithin for 3% of biliary lecithin. Fasting reduced the contribution of newly synthesized cholesterol in bile to less than 1% but did not change the rate of biliary cholesterol secretion. After 2 hours of perfusion, newly synthesized biliary cholesterol accounted for only 4% of total hepatic sterol synthesis. Biliary lecithin, synthesized hepatic lecithin. We conclude that new synthesis makes only a small contribution to biliary cholesterol and lecithin secretion, and that, in the absence of perfusate lipids, both biliary cholesterol and lecithin must be predominantly mobilized from a preformed hepatic pool.

Animals↗
PubMed1981

Hydroxycitrate.

Explore the source record for details and available documents.

ATP Citrate (pro-S)-Lyase↗
PubMed1981

Urinary clearance and metabolism of mevalonate by the isolataed perfused rat kidney.

The urinary excretion and the incorporation into lipids of R[3-14C]mevalonate was investigated in isolated rat kidneys perfused with physiological concentration sof the substrate (80-500 pmol/ml). The clearance of R[3-14C]mevalonate and of the unnatural enantiomer S[5-14C]mevalonate were compared to the glomerular filtration ratea measured by the clearance of inulin. Evidence is presented that half of R-mevalonate filtered in the glomerulus is reabsorbed in the tubule whereas S-mevalonate is not reabsorbed. The kidney tubule appears to discriminate between the R and S forms of the mevalonate salt. Urinary excretion and incorporation into lipids accounted for 22% and 46%, respectively, of the uptake of R[3-14C]mevalonate from the perfusate. The label of R[3-14C]mevalonate recovered in lipids was distributed among saponifiable (15%), digitonin-precipitable sterols (18%) and squalene + prenols (67%). Sterol synthesis in the kidney appears to be controlled, at least in part, by the level of circulatinga R-mevalonate.

Animals↗
PubMed1980

The source of acetyl coenzyme A for acetylcholine synthesis in the perfused rat phrenic nerve-hemidiaphragm.

Acetylcholine release by the phrenic nerve was measured in the isolated, perfused rat hemidiaphragm. In controls, the rate of release of acetylcholine increases with time; however, when (--)-hydroxycitrate, an inhibitor of ATP-citrate lyase, is added to the perfusate, the release of acetylcholine stabilizes at a level 40% below the final control value. In this preparation, the capacity of the citrate cleavage pathway to transfer acetyl coenzyme A from the nerve cell mitochondria to the cytosol increases with time; this is not the case for other transport processes.

ATP Citrate (pro-S)-Lyase↗
PubMed1980

Contributions of cytosolic and mitochondrial acetyl-CoA syntheses to the activation of lipogenic acetate in rat liver.

Acetate derived from ethanol oxidation is activated by cytosolic and mitochondrial acetyl-CoA synthetases before contributing to the extra-mitochondrial processes of fatty acid and 3-beta-hydroxysterol synthesis. Mitochondrially-generated acetyl-CoA is transferred to the cytosol via citrate and ATP-citrate lyase; this transfer is blocked by (-)-hydroxycitrate. Rats were injected IV with 3.3 mmol/kg of [2-3H,2-14C] acetate and IP with either 0.5 mmol/kg hydroxycitrate or saline. After one hour, the rats were killed and the incorporation of label was measured in liver fatty acids and 3-beta-hydroxysterols. The 3H/14C ratio was increased by 12 and 13% in the fatty acids and 3-beta-hydroxysterols of the hydroxycitrate-treated group. The lower ratio in the fatty acids and 3-beta-hydroxysterols derived from mitochondrially-generated acetyl-CoA is ascribed to a loss of 3H in the citrate synthase reaction. The data showed that (1) fatty acids and 3-beta-hydroxysterols syntheses use the same pool of cytosolic acetyl-CoA; and (2) in the absence of an isotope effect in the citrate synthase reaction, mitochondrially-generated acetyl-CoA contributes about 36% to lipogenesis from acetate.

Acetate-CoA Ligase↗