Deficiency of D-beta-aminoisobutyrate: pyruvate aminotransferase in the liver of genetic high excretors of D-beta-aminoisobutyrate

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Comparison of urinary amino acids and trace elements (copper, zinc and manganese) of recent neurolathyrism patients and healthy controls from Ethiopia

OBJECTIVE

The irreversibly crippling disease neurolathyrism is caused by prolonged over-consumption of Lathyrus sativus seed. The molecular mechanism of toxicity is unclear and more biochemical information is needed.

METHODS

The urinary amino acids from 5 recent patients and 9 healthy subjects in Ethiopia were analysed by HPLC after PITC (phenyl isothiocyanate) derivatisation. The trace elements Cu, Zn and Mn of urine and seeds were determined by atomic absorption spectroscopy.

RESULTS

The free amino acids aspartic acid, glycine, beta-aminoisobutyric acid, arginine, alpha-aminoadipic acid and phenylalanine were statistically higher (p<0.05) in urine from patients than in urine from control subjects. The trace element Cu was also statistically higher (p<0.05) in patient urine.

CONCLUSION

The amino acid metabolism of the neurolathyrism patients is disturbed by over-consumption of grass pea seeds. The high concentrations of Cu found in the patient urine might indicate the involvement of trace elements in the aetiology of neurolathyrism.

L-alanine-glyoxylate aminotransferase II of rat kidney and liver mitochondria possesses cysteine S-conjugate beta-lyase activity: a contributing factor to the nephrotoxicity/hepatotoxicity of halogenated alkenes?

Several halogenated alkenes are metabolized in part to cysteine S-conjugates, which are mitochondrial toxicants of kidney and, to a lesser extent, other organs. Toxicity is due to cysteine S-conjugate beta-lyases, which convert the cysteine S-conjugate into pyruvate, ammonia and a reactive sulphur-containing fragment. A section of the human population is exposed to halogenated alkenes. To understand the health effects of such exposure, it is important to identify cysteine S-conjugate beta-lyases that contribute to mitochondrial damage. Mitochondrial aspartate aminotransferase [Cooper, Bruschi, Iriarte and Martinez-Carrion (2002) Biochem. J. 368, 253-261] and mitochondrial branched-chain aminotransferase [Cooper, Bruschi, Conway and Hutson (2003) Biochem. Pharmacol. 65, 181-192] exhibit beta-lyase activity toward S -(1,2-dichlorovinyl)-L-cysteine (the cysteine S-conjugate of trichloroethylene) and S -(1,1,2,2-tetrafluoroethyl)-L-cysteine (the cysteine S-conjugate of tetrafluoroethylene). Turnover leads to eventual inactivation of these enzymes. Here we report that mitochondrial L-alanine-glyoxylate aminotransferase II, which, in the rat, is most active in kidney, catalyses cysteine S-conjugate beta-lyase reactions with S -(1,1,2,2-tetrafluoroethyl)-L-cysteine, S -(1,2-dichlorovinyl)-L-cysteine and S -(benzothiazolyl-L-cysteine); turnover leads to inactivation. Previous workers showed that the reactive-sulphur-containing fragment released from S -(1,1,2,2-tetrafluoroethyl)-L-cysteine and S -(1,2-dichlorovinyl)-L-cysteine is toxic by acting as a thioacylating agent - particularly of lysine residues in nearby proteins. Toxicity, however, may also involve 'self-inactivation' of key enzymes. The present findings suggest that alanine-glyoxylate aminotransferase II may be an important factor in the well-established targeting of rat kidney mitochondria by toxic halogenated cysteine S-conjugates. Previous reports suggest that alanine-glyoxylate aminotransferase II is absent in some humans, but present in others. Alanine-glyoxylate aminotransferase II may contribute to the bioactivation (toxification) of halogenated cysteine S-conjugates in a subset of individuals exposed to halogenated alkenes.

Simple gas chromatographic-mass spectrometric procedure for diagnosing pyrimidine degradation defects for prevention of severe anticancer side effects

Inborn errors of pyrimidine degradation, dihydropyrimidine dehydrogenase deficiency and dihydropyrimidinase deficiency, are less rare than has generally been assumed. Many asymptomatic cases have been reported, and in patients with symptoms, the clinical abnormalities are variable and nonspecific. Withdrawal of pyrimidine analogues such as 5-fluorouracil (5FU), a commonly used anticancer drug, from the cancer chemotherapy regimens of patients with pyrimidine degradation deficiencies, however, is critical because 5FU is degraded in vivo by pyrimidine-degradative enzymes. Patients with these deficiencies suffer from severe neurotoxicity, sometimes leading to death, following administration of 5FU, and even otherwise asymptomatic homozygotes or heterozygotes may develop severe clinical symptoms upon administration of such medication. Therefore, a rapid and specific method for identifying cancer patients with these enzyme deficiencies prior to treatment with 5FU is critical. To address this problem, we established methods for highly sensitive yet specific determinations of thymine, uracil, dihydrothymine, dihydrouracil, orotate and creatinine simultaneously in 0.1-ml liquid urine or filter-paper urine. This method involves stable isotope dilution, a simplified urease treatment previously described and gas chromatography-mass spectrometry without prior fractionation. The high recovery and low C.V. values were obtained and healthy control values were also determined for these metabolites. Using artificially prepared urine specimens simulating these disorders. the chemical diagnosis can be made clearly, and no further analysis appears to be required for differential chemical diagnosis.

Inborn errors of pyrimidine degradation: clinical, biochemical and molecular aspects

The pyrimidines, uracil and thymine, are degraded in four steps. The first three steps of pyrimidine catabolism, controlled by enzyme shared by both pathways, result in the production of the neurotransmitter amino acid beta-alanine from uracil and the nonfunctional (R)-(-)-beta-aminoisobutyrate from thymine. The fourth step is controlled by several aminotransferases, which have different affinities for beta-alanine, beta-aminoisobutyrate and GABA. Defects concerning the first three steps all lead to a reduced production of beta-alanine; defects of the transaminases involving the metabolism of beta-alanine and GABA lead to accumulation of these neurotransmitter substances. In addition, other metabolites will accumulate or be reduced depending on the specific enzyme defect. Analysis of the abnormal concentrations of these metabolites in the body fluids is essential for the detection of patients with pyrimidine degradation defects. Clinically these disorders are often overlooked because symptomatology is highly aspecific. The growth in our knowledge concerning inborn errors of pyrimidine degradation has emphasized the importance of the clinical awareness of these defects as a possible cause of neurological disease and a contraindication for treatment of cancer patients with certain pyrimidine analogues. The various defects are discussed and attention is paid to clinical genetic and diagnostic aspects.

Enzymatic elimination of fluoride from alpha-fluoro-beta-alanine

Rat liver homogenates catalyzed the elimination of fluoride from (R,S)-alpha-fluoro-beta-alanine. The substrate specificity and physical properties of the defluorinating enzyme were similar to those of mitochondrial L-alanine-glyoxylate aminotransferase II (EC 2.6.1.44, AlaAT-II). Furthermore, AlaAT-II activity, measured with L-alanine and glyoxylate as substrates, copurified with the alpha-fluoro-beta-alanine-defluorinating enzyme. The NH2-terminal sequence (18 residues) of the enzyme did not show significant sequence similarity with any of the proteins currently listed in GenBank. The purified enzyme catalyzed the transamination of L-alanine (Ala) and glyoxylate (glyx) at pH 8.5 by a ping-pong mechanism with kinetic parameters of kcat = 17 sec-1, KL-Ala = 3.2 mM, and Kglyx = 0.3 mM, respectively. The kinetic parameters for the defluorination of (R)-alpha-fluoro-beta-alanine and (S)-alpha-fluoro-beta-alanine were kcat = 6.2 and 2.6 min-1, respectively, and Km = 2.7 and 0.88 mM, respectively. L-Alanine potently inhibited the defluorination reaction with an apparent Ki of 0.024 mM. (R,S)-alpha-Fluoro-beta-alanine converted the optical spectrum of the enzyme-bound cofactor from the pyridoxal form to the pyridoxamino form, which indicated that this cofactor may participate in the defluorination reaction. The product of the enzymatic reaction, malonic semialdehyde, reacted nonenzymatically with (R,S)-alpha-fluoro-beta-alanine to form an adduct that was detected spectrally. AlaAT-II was not inactivated during dehalogenation of (R,S)-alpha-fluoro-beta-alanine but was inactivated completely during dehalogenation of beta-chloro-L-alanine.

Purification, characterization and inhibition of dihydropyrimidinase from rat liver

Dihydropyrimidinase (DHPase) was purified 564-fold over the initial rat liver extract, using heat, ammonium sulfate fractionation, DEAE-Sepharose CL-6B, carboxymethyl-Sepharose CL-6B, hydroxyapatite and Sephacryl S-300 chromatography. The purified enzyme was shown to be homogeneous by gel electrophoresis both in the presence and absence of SDS. Its molecular mass, determined by gel filtration, was 215 kDa and the subunit mass was 54 kDa. DHPase catalyzed the reversible cyclization of 5,6-dihydrouracil (H2Ura) to N-carbamoyl-beta-alanine or 5,6-dihydrothymine (H2Thy) to N-carbamoyl-beta-aminoisobutyric acid. Authentic 5-bromo-5,6-dihydrouracil (BrH2Ura) and commercially available H2Thy were racemic. However, these 5-substituted 5,6-dihydropyrimidines were hydrolyzed by over 96% and 98%, respectively, by DHPase. These results suggest that dihydropyrimidinase has no stereo specificities for 5-substituents of H2Ura. The addition of H2Ura and H2Thy competitively inhibited the enzyme activity against BrH2Ura. However, the addition of N-carbamoyl-beta-alanine or N-carbamoyl-beta-amino-isobutyric acid showed hyperbolic mixed-type inhibition, when BrH2Ura was used as the substrate. The values of the dissociation constants of BrH2Ura, N-carbamoyl-beta-alanine and N-carbamoyl-beta-aminoisobutyric acid were 17 microM, 0.38 mM and 0.38 mM, respectively. DHPase from the rat liver contains 4 mol Zn2+/mol active enzyme, presumably one atom/subunit. Zn2+ also inhibited the hydrolysis of BrH2Ura by the enzyme. The Ki for Zn2+ as an inhibitor of DHPase was 23 microM, and the maximum rate of inactivation was 0.057 min-1 at 37 degrees C. H2Ura and H2Thy protected the enzyme activity from Zn2+ inactivation.

Evaluation of interconversion between (R)- and (S)-enantiomers of beta-aminoisobutyrate

The conversion of (R)- to (S)-beta-aminoisobutyrate was observed in the presence of D-3-aminoisobutyrate-pyruvate aminotransferase, aminobutyrate aminotransferase, pyruvate and L-glutamate. The reverse reaction was also found in the presence of 2-oxoglutarate and L-alanine. Neither D-3-aminoisobutyrate-pyruvate aminotransferase nor aminobutyrate aminotransferase revealed a racemase activity of the enantiomorphs.

Purification, characterization and inhibition of D-3-aminoisobutyrate aminotransferase from the rat liver

D-3-Aminoisobutyrate-pyruvate aminotransferase was purified 2000-fold from rat liver extract using heat treatment, ammonium sulfate fractionation, carboxylmethyl-Sepharose CL-6B, DEAE-Sepharose CL-6B, hydroxyapatite, Sephacryl S-200 and electrofocusing chromatographies. The purified enzyme was shown to be homogeneous by gel electrophoresis both in the presence and absence of SDS. Its molecular mass, determined by gel filtration, was 220 kDa and the subunit molecular mass was 52 kDa. The enzyme exhibited absorption maxima at 280 nm and 412 nm with a shoulder at 330 nm at neutral pH. The pH optimum for enzyme activity was 9.5 and the Km values for beta-alanine and pyruvic acid were calculated to be 0.81 mM and 0.45 mM, respectively. The purified enzyme catalyzed the transamination of omega-amino acids; beta-alanine and D-3-aminoisobutyric acid served as good amino donors, and pyruvic acid, glyoxylic acid and oxaloacetic acid were favorable amino acceptors. 6-Azauracil and 6-azathymine were found to be potent inhibitors of purified rat liver D-3-aminoisobutyrate-pyruvate aminotransferase. 6-Azauracil acted as a competitive inhibitor with respect to beta-alanine, and was an uncompetitive inhibitor with respect to pyruvic acid with a Ki of approximately 8.9 mM.

beta-Aminoisobutyric acid as a marker of thymine catabolism in malignancy

Urine from untreated patients with various tumours and controls has been examined for the excretion of beta-aminoisobutyric acid and uric acid. The patients were classified into four groups: I, beta-aminoisobutyric acid and uric acid both normal; II, beta-aminoisobutyric acid normal, uric acid elevated; III, beta-aminoisobutyric acid elevated, uric acid normal; IV, beta-aminoisobutyric acid and uric acid both elevated. Uric acid was used as an indicator for tissue-breakdown. Pseudouridine being a specific parameter for t-RNA degradation was estimated for comparison. Increased urinary concentrations of beta-aminoisobutyric acid were frequently found in tumour patients, especially in patients with leukaemia and non-Hodgkin lymphoma. Tissue breakdown being the cause of the beta-aminoisobutyric aciduria could only be considered in part of the patients. Moreover, strongly elevated ratios of beta-aminoisobutyric acid to uric acid were found. Urinary patterns of pyrimidines and purines were determined in order to exclude other abnormalities. The results are discussed in relation to thymine metabolism and renal function.

Profiling of amino acids in body fluids and tissues by means of liquid chromatography

The needs of urgent diagnoses and the needs emerging from acute forms of diseases have directed progress in amino acid profiling to modern, rapid, automated analyses that can be done at reasonable cost. The first step in this direction was the short programmes of classical ion-exchange chromatography. At the beginning of this review we attempted to survey methods of sample preparation and sample treatment, as these are frequently neglected stages where artefacts or erroneous results may arise. There are basically the following approaches in amino acid profiling by liquid chromatographic techniques. For preliminary screening of a large number of samples in clinical routine planar procedures are the methods of choice, as they allow large numbers of samples to be handled with minimum effort and at very reasonable cost. For more precise profiling, particularly where quantitative data are essential, one can choose between some of the modern procedures for separating underivatized amino acids using modern equipment for cation-exchange chromatography, by making use of a stepped series of lithium citrate buffers with ninhydrin, o-phthalaldehyde or 4-fluoro-7-nitrobenzo-2,1,3-oxadiazole detection. Ninhydrin detection is preferred in those situations where the demands on sensitivity are not high. Where, however, only small amounts of samples are available or high sensitivity is required, one of the latter two methods is preferred. The o-phthalaldehyde procedure is not suitable for the detection of secondary amines and, if these are of interest, then diazole derivatization is to be preferred. At present, however, the ninhydrin and o-phthalaldehyde detection procedures are the most popular. The other choice is to use one of the sophisticated HPLC systems equipped with fluorescence detection and to separate amino acids as derivatives. Here o-phthalaldehyde and 4-fluoro-7-nitrobenzo-2,1,3-oxadiazole derivatives offer the most versatile possibilities. Automation and computerization have penetrated both categories of liquid column separation and are applied to automated sample delivery, automated and computerized gradient formation and quantitation of the data obtained. The tables of metabolic disorders of amino acids and the roles of different amino acids in these disorders should provide preliminary information for clinical chemists.

Increased beta-aminoisobutyric acid in rat liver with 6-azauracil and its enantiomer

When 6-azauracil was subcutaneously injected, beta-aminoisobutyric acid and beta-alanine contents were increased 22 and 61-fold, respectively, in rat liver. Incorporation of [methyl-14C]thymine into beta-aminoisobutyric acid was increased to 42-fold by 6-azauracil treatment. The absolute configuration of this amino acid was proved to be the (R)-form by means of a gas-chromatographic technique. 6-Azauracil inhibited beta-alanine-pyruvate aminotransferase activity with an I50 of approx. 2.5 mM.

Excessive excretion of beta-alanine and of 3-hydroxypropionic, R- and S-3-aminoisobutyric, R- and S-3-hydroxyisobutyric and S-2-(hydroxymethyl)butyric acids probably due to a defect in the metabolism of the corresponding malonic semialdehydes

A new metabolic disorder characterised by the excessive excretion of beta-alanine, 3-hydroxypropionic acid, R- and S-3-amino- and 3-hydroxyisobutyric acids and S-2-(hydroxymethyl)butyric acid is probably due to deficient activities of malonic, methylmalonic and ethylmalonic semialdehyde dehydrogenases. These dehydrogenation reactions could be mediated by one enzyme, or by enzymes with a common subunit, and both R- and S-methylmalonic semialdehydes seem to be equally affected. The patient is now aged 4 years and has developed normally. He has a persistent gross hypermethioninaemia which is probably unrelated to the other biochemical abnormalities.

Sex difference in the accumulation of D-beta-aminoisobutyrate in organs of mouse after thymine loading

The concentration of D-beta-aminoisobutyric acid (D-BAIB) in the liver and kidney was twice as high and dropped more slowly in the female mouse than in the male after an intraperitoneal injection of thymine. The concentration of beta-alanine, formed from uracil by the same enzyme system catalyzing formation of D-BAIB from thymine, was not different in the liver and kidney of both sexes after an intraperitoneal injection of uracil. After the intraperitoneal injection of D-BAIB, the concentration of BAIB in male liver decreased faster than that in female liver. Inhibition of D-BAIB: pyruvate aminotransferase caused by injection of D-cycloserine resulted in a significant increase in the concentration of BAIB in liver of both sexes after injection of thymine, but the concentration dropped more rapidly in the male. The activity of D-BAIB: pyruvate aminotransferase was not different in the livers of male and female mice. Under the action of probenecid, an inhibitor of active transport systems, the sex difference in accumulation and disappearance of the amino acid in the liver was not observed. This suggested that the excretion of BAIB is more active in the renal tubules of the male mouse than in those of the female. However, the amount of BAIB excreted in the urine after injection of thymine was larger in the female mice than in the male mice. There may be another probenecid-sensitive enzyme for the disposal of BAIB in male mice.

Infrequency of urinary excretion of beta-aminoisobutyric acid by healthy humans

Urinary beta-aminoisobutyric acid (BAIB) levels were measured by modified ion-exchange chromatography. Daily BAIB levels are reported for a group of apparently healthy individuals, a subject with infective hepatitis and two human genetic variants who were high "excretors" of BAIB. The frequency of high "excretors" of BAIB was found using low-voltage paper electrophoresis. The frequency among 403 male European subjects was 5 (1.2%) and lower than previous estimates in the United Kingdom. The percentage recovery of fed BAIB suggests that Oriental "excretors" may have a different type of genetic lesion from non-Oriental "excretors". The clinical usefulness of urinary BAIB assays and qualitative screening is reviewed.

Urinary amino acid excretion by patients with beta-thalassemia

Twenty-four hour urinary amino acid excretion has been studied in patients with beta-thalassemia (four adult major, two adult intermedia, and three children major). beta-Aminoisobutyric acid was found to be increased (14-fold on the average) in patients with beta-thalassemia without evidence for increase in the excretion of most other amino acids. No correlation of beta-aminoisobutyric acid excretion with the hematological status of the patient was found. Elevated taurine excretion (2-fold) was also noted in the adult patients with beta-thalassemia. Greatly increased excretion of beta-aminoisobutyric acid appears to correlate with a poor prognosis and may reflect generalized tissue catabolism rather than being a specific indicator of ineffective erythropoiesis.