Pathways of acetone's metabolism in the rat

Tertiary-Butanol: a toxicological review

Tert-Butanol is an important intermediate in industrial chemical synthesis, particularly of fuel oxygenates. Human exposure to tert-butanol may occur following fuel oxygenate metabolism or biodegradation. It is poorly absorbed through skin, but is rapidly absorbed upon inhalation or ingestion and distributed to tissues throughout the body. Elimination from blood is slower and the half-life increases with dose. It is largely metabolised by oxidation via 2-methyl-1,2-propanediol to 2-hydroxyisobutyrate, the dominant urinary metabolites. Conjugations also occur and acetone may be found in urine at high doses. The single-dose systemic toxicity of tert-butanol is low, but it is irritant to skin and eyes; high oral doses produce ataxia and hypoactivity and repeated exposure can induce dependence. Tert-Butanol is not definable as a genotoxin and has no effects specific for reproduction or development; developmental delay occurred only with marked maternal toxicity. Target organs for toxicity clearly identified are kidney in male rats and urinary bladder, particularly in males, of both rats and mice. Increased tumour incidences observed were renal tubule cell adenomas in male rats and thyroid follicular cell adenomas in female mice and, non-significantly, at an intermediate dose in male mice. The renal adenomas were associated with alpha(2u)-globulin nephropathy and, to a lesser extent, exacerbation of chronic progressive nephropathy. Neither of these modes of action can function in humans. The thyroid tumour response could be strain-specific. No thyroid toxicity was observed and a study of hepatic gene expression and enzyme induction and thyroid hormone status has suggested a possible mode of action.

Direct evidence of iNOS-mediated in vivo free radical production and protein oxidation in acetone-induced ketosis

Diabetic patients frequently encounter ketosis that is characterized by the breakdown of lipids with the consequent accumulation of ketone bodies. Several studies have demonstrated that reactive species are likely to induce tissue damage in diabetes, but the role of the ketone bodies in the process has not been fully investigated. In this study, electron paramagnetic resonance (EPR) spectroscopy combined with novel spin-trapping and immunological techniques has been used to investigate in vivo free radical formation in a murine model of acetone-induced ketosis. A six-line EPR spectrum consistent with the alpha-(4-pyridyl-1-oxide)-N-t-butylnitrone radical adduct of a carbon-centered lipid-derived radical was detected in the liver extracts. To investigate the possible enzymatic source of these radicals, inducible nitric oxide synthase (iNOS) and NADPH oxidase knockout mice were used. Free radical production was unchanged in the NADPH oxidase knockout but much decreased in the iNOS knockout mice, suggesting a role for iNOS in free radical production. Longer-term exposure to acetone revealed iNOS overexpression in the liver together with protein radical formation, which was detected by confocal microscopy and a novel immunospin-trapping method. Immunohistochemical analysis revealed enhanced lipid peroxidation and protein oxidation as a consequence of persistent free radical generation after 21 days of acetone treatment in control and NADPH oxidase knockout but not in iNOS knockout mice. Taken together, our data demonstrate that acetone administration, a model of ketosis, can lead to protein oxidation and lipid peroxidation through a free radical-dependent mechanism driven mainly by iNOS overexpression.

On the mammalian acetone metabolism: from chemistry to clinical implications

Despite the description of the ways of acetone metabolism, its real role(s) is (are) still unknown in metabolic network. In this article, a trial is made to ascertain a comprehensive overview of acetone research extending discussion from chemistry to clinical implications. Mammals are quite similar regarding their acetone metabolism, even if species differences can also be observed. By reviewing experimental data, it seems that plasma concentration of acetone in different species is in the order of 10 microm range and the concentration-dependent acetone metabolism is common to all mammals. At low concentrations of plasma acetone, the C3 pathways are operative, while at higher concentrations, the metabolism through acetate becomes dominant. Glucose formation from acetone may also contribute to the maintenance of a constant blood glucose level, but it seems to be only a minor source for that. From energetical point of view, an interorgan cooperation is suggested because transportable C3 fragments produced in the liver can serve as alternative sources of energy for the peripheral tissues in the short of circulating glucose. The degradation of acetoacetate to acetone contributes to the maintenance of pH buffering capacity, as well. Special attention is paid to the discussion of acetone production in diseases amongst which endogenous and exogenous acetonemiae have been defined. Acetonemiae of endogenous origin are due to the increased rate of acetone production followed by an increase of degrading capacity as cytochrome p450IIE1 (CYPIIE1) isozymes become induced. Exogenous acetonemiae usually resulted from intoxications caused by either acetone itself or other exogenous compounds (ethanol, isopropyl alcohol). It is highlighted that, on the one hand, isopropanol is also a normal constituent of metabolism and, on the other hand, the flat opinion that the elevation of its plasma level is a sign of alcoholism cannot further be held. The possible future directions of research upon acetone are depicted by emphasizing the need for the clear-cut identification of mammalian acetoacetate decarboxylase, and the investigation of race differences and genetic background of acetone metabolism.

Oxidative stress, microsomal and peroxisomal fatty acid oxidation in the liver of rats treated with acetone

Parameters of oxidative stress, microsomal cytochrome P450 activity and peroxisomal fatty acid oxidation were studied in liver of rats following acetone (1% v/v) consumption for 7 days. Acetone treatment increased the activity of catalase and decreased the activities of superoxide dismutase (SOD) and glutathione peroxidase (GTPx), but did not significantly modify the liver content of malondialdehyde (MDA) and reduced glutathione. Also, acetone increased the total content of cytochrome P450, the microsomal lauric acid hydroxylation, aminopyrine N-demethylation and the peroxisomal beta-oxidation of palmitoyl CoA. These effects were similar to those found previously in starved and ethanol-treated rats, supporting the hypothesis that ketone bodies would be the common inducer of microsomal and peroxisomal fatty acid oxidation in these metabolic states.

Modulation of peroxisomal and microsomal fatty acid oxidation by acetone. A comparative study between liver and kidney

The effect of acetone consumption on some microsomal and peroxisomal activities was studied in rat kidney and these results were compared with data from former investigations in liver. Acetone increased the microsomal lauric acid hydroxylation, the aminopyrine N-demethylation catalyzed by cytochrome P450 and the microsomal UDP-glucuronyltransferase activity. Also, acetone increased the peroxisomal beta-oxidation of palmitoyl CoA and catalase activities in kidney. These studies suggest that acetone is a common inducer of the microsomal and peroxisomal fatty acid oxidation, as previously shown in both starved and ethanol treated rats. Our results support the hypothesis that microsomal fatty acid omega-hydroxylation results in the generation of substrates being supplied for peroxisomal beta-oxidation. We propose that the final purpose of these linked fatty acid oxidations could be the catabolism of fatty acids or the generation of a substrate for the synthesis of glucose from fatty acids. This pathway would be triggered by acetone treatment in a similar way in liver and kidney.

Estimates of Krebs cycle activity and contributions of gluconeogenesis to hepatic glucose production in fasting healthy subjects and IDDM patients

Normal subjects, fasted 60 h, and patients with insulin-dependent diabetes mellitus (IDDM), withdrawn from insulin and fasted overnight, were given phenylacetate orally and intravenously infused with [3-14C]lactate and 13C-bicarbonate. Rates of hepatic gluconeogenesis relative to Krebs cycle rates were estimated from the 14C distribution in glutamate from urinary phenylacetylglutamine. Assuming the 13C enrichment of breath CO2 was that of the CO2 fixed by pyruvate, the enrichment to be expected in blood glucose, if all hepatic glucose production had been by gluconeogenesis, was then estimated. That estimate was compared with the actual enrichment in blood glucose, yielding the fraction of glucose production due to gluconeogenesis. Relative rates were similar in the 60-h fasted healthy subjects and the diabetic patients. Conversion of oxaloacetate to phosphoenolpyruvate was two to eight times Krebs cycle flux and decarboxylation of pyruvate to acetyl-CoA, oxidized in the cycle, was less than one-30th the fixation by pyruvate of CO2. Thus, in estimating the contribution of a gluconeogenic substrate to glucose production by measuring the incorporation of label from the labelled substrate into glucose, dilution of label at the level of oxaloacetate is relatively small. Pyruvate cycling was as much as one-half the rate of conversion of pyruvate to oxaloacetate. Glucose and glutamate carbons were derived from oxaloacetate formed by similar pathways if not from a common pool. In the 60-h fasted subjects, over 80% of glucose production was via gluconeogenesis. In the diabetic subjects the percentages averaged about 45%.

Net glucose production from acetone in isolated murine hepatocytes. The effect of different pretreatments of mice

1. To evaluate the condition under which net glucose production from acetone, added as sole substrate, occurs different pretreatments of mice, in combination with starvation, were used; (i) acetone pretreatment (acetone is a known inducer of cytochrome P-450 isozymes involved in this pathway), (ii) fructose pretreatment (to induce NADPH+H+ generating enzymes) or (iii) their combination. 2. There was net glucose formation from acetone only in that case, when the cells were prepared from 48 hr fasted animals pretreated with both acetone and fructose. However, using 2-14C-acetone, incorporation of 14C-carbon into glucose could be detected in all the cases and, at the same time, acetone was without any effect on protein synthesis. 3. The addition of acetone increased gluconeogenesis from alanine in almost all the cases. The only exception from this general rule was that the case, when hepatocytes were prepared from acetone pretreated 48 hr starved mice where, instead of the elevation of glucose formation, a decrease of that was caused by acetone. 4. Acetone decreased 14C-carbon incorporation into glucose from 14C-(U)-alanine added at saturating concentration in hepatocytes prepared from starved mice. 5. Similarly to acetone there was no net glucose formation from acetone either when added alone, however, it enhanced gluconeogenesis from alanine at non-saturating concentrations of the amino acid. 6. Methylglyoxal proved gluconeogenic in all the cases. 7. It is concluded that net glucose formation from acetone as sole substrate occurs only under those conditions which are far from a physiological situation, however, when gluconeogenesis from another substrate takes place, acetone can contribute to net glucose formation in hepatocytes prepared from fasted mice.

Effect of glyburide on hepatic glucose metabolism

Glyburide, along with the other second-generation oral hypoglycemic agent glipizide, has been used as adjunctive therapy for the treatment of non-insulin-dependent diabetes mellitus. After glyburide therapy, basal glycemia and glucose response to a meal are greatly improved. The mechanism for the glucose-lowering effect of the drug remains controversial. Glyburide is generally thought to exert two major actions: stimulation of pancreatic insulin secretion and enhancement of insulin action in hepatic and extrahepatic tissues. Studies in patients with non-insulin-dependent diabetes mellitus indicate that the action of glyburide on the liver plays a central role in decreasing glucose. With short-term therapy, glyburide decreases hepatic glucose production by elevating pancreatic insulin secretion. However, this increase in pancreatic insulin secretion is not sustained as therapy is continued, suggesting that glyburide then acts directly on the liver. The mechanism for the improvement in hepatic glucose metabolism after long-term treatment is not known. In vitro, glyburide has been shown to inhibit gluconeogenesis as well as glycogenolysis and to enhance hepatic glucose uptake, thus providing possible explanations for the action of the drug on the liver.

Metabolism of S-3-hydroxybutyrate in the perfused rat liver

The metabolism of millimolar concentrations of S-3-hydroxybutyrate (the unnatural enantiomer) has been studied in perfused livers from fed and starved rats. Protocols were designed to test whether S-3-hydroxybutyrate is metabolized in the cytosol or in the mitochondria via a racemase, a dehydrogenase, or a ligase. Our data show that only a minor fraction of S-3-hydroxybutyrate metabolism could occur via L-3-hydroxyacid dehydrogenase. Most of the metabolism of S-3-hydroxybutyrate proceeds via mitochondrial activation. In rat liver, S-3-hydroxybutyrate is converted to physiological ketone bodies (i.e., R-3-hydroxybutyrate, acetoacetate, acetone), lipids, and CO2. Carbons from S-3-hydroxybutyrate are transferred from the mitochondria to the cytosol mostly via citrate and the citrate cleavage pathway.

The metabolism of acetone in the pregnant rat

The administration of a single dose of acetone (100 mg/kg bw) to virgin and 21-day pregnant rats resulted in the appearance of relatively high concentrations of 1,2-propanediol, acetol and methylglyoxal in plasma and liver. In the fetuses no methylglyoxal was detectable. The acetone metabolism curves tend to indicate that the capacity for acetone disposal may be enhanced in the 48 hr-fasted pregnant rat, thus enabling the animal to re-use acetone metabolically, possibly for accelerated gluconeogenesis.