Gas chromatography-mass spectrometry of carboxylic acids in tissues as their tert.-butyldimethylsilyl derivatives

Analysis of tert.-butyldimethylsilyl [1-13C]palmitic acid in stool samples by gas chromatography-mass spectrometry with electron impact ionisation: comparison with combustion isotope-ratio mass spectrometry

The use of 13C-labelled compounds to study lipid metabolism is increasing. Typically less than 40% of the orally administered label is recovered in breath CO2. The remainder must be either absorbed and not oxidised or not absorbed and remain in the faeces. Two methods of determining how much tracer passes through the body, and is present in the stool, were compared. Compound specific analysis of tert.-butyldimethylsilyl [13C]hexadecanoic acid by gas chromatography-mass spectrometry (GC-MS) with electron impact ionisation was compared with bulk analysis of whole stool and lipid extract by continuous flow isotope ratio mass spectrometry (CF-IRMS) with a combustion interface. The mean difference between the IRMS and GC-MS methods was -0.02 mmol 13C d(-1) with a mean excretion of 14.2 mmol 13C d(-1). Combustion IRMS is both simpler and cheaper, when the objective is to determine how much administered dose appears in stool, and information about the form of the label is not required.

Coupling of ethanol metabolism to lipid biosynthesis: labelling of the glycerol moieties of sn-glycerol-3-phosphate, a phosphatidic acid and a phosphatidylcholine in liver of rats given [1,1-2H2]ethanol

The mechanism behind ethanol-induced fatty liver was investigated by administration of [1,1-2H2]ethanol to rats and analysis of intermediates in lipid biosynthesis. Phosphatidic acid and phosphatidylcholine were isolated by chromatography on a lipophilic anion exchanger and molecular species were isolated by high-performance liquid chromatography in a non-aqueous system. The glycerol moieties of palmitoyl-linoleoylphosphatidic acid, the corresponding phosphatidylcholine and free sn-glycerol-3-phosphate were analysed by GC/MS of methyl ester t-butyldimethylsilyl derivatives. The deuterium labelling in the glycerol moiety of the phosphatidic acid was 2-3-times higher than in free sn-glycerol-3-phosphate, indicating that a specific pool of sn-glycerol-3-phosphate was used for the synthesis of phosphatidic acid in liver. The results indicate that NADH formed during ethanol oxidation is used in the formation of a pool of sn-glycerol-3-phosphate that gives rise to triacylglycerol and possibly fatty liver.

Transfer of deuterium from [1,1-2H2]ethanol to steroids and organic acids in the rat testis

Rats were given [1,1-2H2]ethanol in a single dose, and the 2H content was determined in testicular steroids and in organic acids of low molecular mass in the testis, liver and blood. The acids were quantified by capillary gas chromatography/mass spectrometry of t-butyldimethylsilyl derivatives with [2H4]lactate as internal standard. In addition to lactate, pyruvate, 3-hydroxybutyrate and acids of the tricarboxylic acid cycle, the testis was shown to contain 2-hydroxybutyrate, 2-hydroxy-2-methylbutyrate, 2-hydroxyisohexanoate and glycerate. No 2H was found in pregnenolone, 5-androstene-3 beta,17 beta-diol or testosterone, whereas the abundance of monodeuterated molecules of 5 alpha-androstane-3 alpha,17 beta-diol and its 3 beta-isomer were 7.6% and 11.2% respectively. The abundance of monodeuterated lactate was 7.0% in the testis and 5.3% in the blood. The other acids were less labelled but 3-hydroxybutyrate had a higher 2H content in the testis (3.1%) than in the liver. These results support the contention that ethanol is oxidized in an alcohol dehydrogenase-catalysed reaction in testis in vivo and that the acute inhibition of the testosterone production is due at least partly to a redox effect. The labelling and increased concentration of 3-hydroxybutyrate in the testis indicate that a change in the mitochondrial redox state might be involved.

Oxidoreduction of butanol in deermice (Peromyscus maniculatus) lacking hepatic cytosolic alcohol dehydrogenase

In view of conflicting information in the literature regarding enzyme systems responsible for alcohol oxidation in deermice previously reported to lack hepatic alcohol dehydrogenase (ADH) activity, the reversibility of butanol oxidation was studied in vivo and in liver-perfusion systems. Mixtures of [1,1-2H2]ethanol and butanol were given intraperitoneally to deermice lacking (ADH-) or possessing (ADH+) ADH activity, followed by analysis of alcohols in blood by GC/MS. 2H exchange between the two alcohols was seen in all experiments. In ADH- deermice, the 2H excess of butanol increased steadily and reached 18 +/- 5% after 2.5 h. In ADH+ deermice, butanol was rapidly eliminated and the 2H excess was about 7% after 0.5 h. In similar experiments with rats, the 2H excess was about 40% for 2 h. Perfusions of livers from ADH- deermice with mixtures of unlabelled and 1-[2H]butanol showed significant but slow intermolecular hydrogen transfer at C1, indicating oxidoreduction catalyzed by a dehydrogenase. Slow reduction of butanal was observed in mitochondria from ADH- deermice. ADH activity with a pH optimum of 10 and Km for ethanol of 6 mM was detected in the inner mitochondrial membranes from rats and deermice. However, low rates of oxidation observed in experiments carried out with perfused livers and in vitro suggest that this enzyme system does not contribute significantly to alcohol oxidation in vivo. Thus, perfused liver from ADH- deermice appears to be a useful system for studies of ADH-independent oxidation of alcohols. The 2H exchange between the alcohols seen in vivo indicates that both ethanol and butanol are substrates for a common extrahepatic dehydrogenase in ADH- deermice.

Analysis of aldehydic lipid peroxidation products in rat liver and hepatocytes by gas chromatography and mass spectrometry of the oxime-tert-butyldimethylsilyl derivatives

A method for analysis of aliphatic aldehydes in biological samples is described. Cyclohexanone is added as internal standard and the samples are treated with hydroxylamine and perchloric acid. The oximes are extracted and converted to the oxime-tert-butyldimethylsilyl derivatives, which are quantitated by capillary gas chromatography and identified by mass spectrometry. The characteristic M-57 fragment ions in the mass spectra enabled a rapid identification of the derivatives of the aldehydes, alkanals, alk-2-enals, alka-2,4-dienals, and 4-hydroxyalk-2-enals, which in addition gave rise to characteristic double peaks in the gas chromatographic analysis. The method was applied to analysis of autoxidized arachidonic acid, ADP-Fe3(+)-treated rat hepatocytes, and rat liver given a single dose of ethanol, 5 g/kg. The amounts of hexanal and 4-hydroxynon-2-enal were not increased 6 h after the administration of ethanol.

Compartmentation of acetyl CoA studied by analysis of tricarboxylic acid cycle acids and 3-hydroxybutyrate in bile of rats given [2,2,2-2H3]ethanol

Acetate, 3-hydroxybutyrate, pyruvate, lactate, citrate, 2-oxoglutarate, succinate, fumarate and malate were analysed in rat bile by gas chromatography and gas chromatography/mass spectrometry of their O-melthyloxime-t-butyldimethylsilyl derivatives. The concentration of acetate increased to about 1.8 mmol/l after administration of [2,2,2-2H3]ethanol. Acetate was formed from ethanol to an extent of about 82% and retained all of the 2H at C-2, whereas 15% of the 2H had been lost in the tricarboxylic acid cycle intermediates and 24% in 3-hydroxybutyrate. Thus the exchange of 2H for 1H takes place after formation of acetyl CoA. For citrate and 3-hydroxybutyrate, 41% and 11% respectively was formed from [2,2,2-2H3]ethanol. These results indicate that different pools of acetyl CoA are used for the synthesis of ketone bodies and citrate, with the latter being derived from ethanol to a much larger extent. Smaller fractions of 2-oxoglutarate (16%) and succinate (5%) were derived from [2,2,2--2H3]ethanol, indicating significant contributions from amino acids.

Simultaneous determination of C2-C22 non-esterified fatty acids and other metabolically relevant carboxylic acids in biological material by gas chromatography of their benzyl esters

A method for the simultaneous determination of non-esterified short-, medium- and long-chain fatty acids and other types of metabolically relevant carboxylic acids such as hydroxy, keto, aromatic and dicarboxylic acids in biological material by capillary gas chromatography of benzyl ester derivatives is described. Sample preparation avoiding incomplete isolation of carboxylic acids consisted of deproteinization and extraction with ethanol, fixation of carboxylic acids as carboxylates, removal of interfering compounds such as neutral lipids by hexane extraction and amino acids, acyl carnitines and other cations by cation-exchange chromatography, derivatization of keto groups of ketocarboxylic acids into O-methyl oximes and benzyl ester formation by reaction of the potassium carboxylates with benzyl bromide via crown ether catalysis. The sample preparation conditions were investigated, showing the usefulness of this method for quantitative determinations. Chromatograms obtained from human serum, human urine and rat heart ventricle and concentrations of carboxylic acids in these specimens are presented.

Effect of ethanol on the redox state of the coenzyme bound to alcohol dehydrogenase studied in isolated hepatocytes

Hepatocytes were isolated from fed female rats and incubated with a redox indicator system consisting of cyclohexanone and unlabelled or perdeuterated cyclohexanol. The concentrations and deuterium contents of these were measured by g.l.c. and g.l.c.-m.s. of oxime t-butyldimethylsilyl derivatives. The equilibrium composition represented the redox state of the coenzyme bound to alcohol dehydrogenase, since 4-methylpyrazole inhibited the interconversion. Reduction appeared to be catalysed to a small extent also by an NADPH-dependent aldehyde reductase. The NADH/NAD+ ratio on alcohol dehydrogenase was 3 orders of magnitude higher in the presence of ethanol than in its absence. This redox shift has the degree expected from reported kinetic constants. The shift was due both to a decreased rate of oxidation and to an increased rate of reduction in the indicator system. The results indicate that the redox effect of ethanol on the free NAD system is due to efficient removal of acetaldehyde from a near-equilibrium system consisting of ethanol, acetaldehyde and bound coenzymes, together with dissociation of NADH from the enzyme. The effect on the redox state of the bound coenzyme was less marked when the ethanol was deuterated at C-1, indicating an isotope effect. The 2H excess in the cyclohexanol formed was about 70% of that in the [1,1-2H2]ethanol. This dilution, which is caused by binding of free NADH to the enzyme, indicates that reoxidation of cytosolic NADH partly limits the rate of ethanol oxidation.

Hydroxylation of acetone by ethanol- and acetone-inducible cytochrome P-450 in liver microsomes and reconstituted membranes

Acetone oxidation in rat liver microsomes was induced 5- or 8-fold by the treatment of the animals with ethanol or acetone, respectively. The apparent Km of the reaction was 0.9 mM, a value lower than the concentration reported for plasma acetone under starvation conditions. The major acetone metabolite was identified as acetol by GC-MS. Acetone oxidation in microsomes was inhibited by typical P-450 inhibitors as well as by compounds (e.g. imidazole) known to interact with the ethanol-inducible P-450 form. Antibodies against this P-450 isozyme were inhibitory for the reaction in rabbit liver microsomes and this isozyme was the only one that showed acetone hydroxylation activity in reconstituted membranes. Imidazole inhibited the conversion of [14C]acetone into low-Mr compounds (e.g. glucose) in vivo. It is suggested that the ethanol- and acetone-inducible P-450 make use of acetone as an endogenous substrate in the utilization of the compound for, e.g. glucose production under conditions of starvation and diabetic ketoacidosis.