Purification and characterization of D-3-aminoisobutyrate-pyruvate aminotransferase from rat liver

1H NMR-Based Metabolomics and Lipid Analyses Revealed the Effect of Dietary Replacement of Microbial Extracts or Mussel Meal with Fish Meal to Arctic Charr (Salvelinus alpinus)

The effects of replacing 40% of dietary fish meal (FM) in a reference diet (REF) with either mussel meal (MM), zygomycete fungi (ZYG), extracted baker’s yeast (EY), or non-extracted baker’s yeast (NY) on the lipid and metabolic profile of Arctic charr (Salvelinus alpinus) were investigated. After a 14-week feeding trial, liver and muscle tissues were collected for lipid (lipid content, lipid class, fatty acid composition) and 1H NMR-based metabolomics analyses (aqueous and chloroform phases). Lipid analyses showed that fish fed ZYG diet had lower liver lipid content and thereby 10% higher level of docosahexaenoic acid compared with REF. Metabolomics analyses showed that on the one hand fish fed NY diet affected liver metabolites (2–3 fold higher concentrations of e.g., n,n-dimethylglycine and betaine) compared with REF, while, on the other hand, the muscle metabolic fingerprint was mainly affected by EY. In general, affected metabolites (e.g., alanine, anserine, betaine, hydroxyproline, isoleucine, malonate, n,n-dimethylglycine, proline, succinate, and valine) in fish fed test diets suggested that the test meal ingredients caused mainly a response in muscle metabolism. Fish metabolism was least affected by MM, which suggests that it may be suitable to replace fish meal in Arctic charr diets.

Distribution of D-3-aminoisobutyrate-pyruvate aminotransferase in the rat brain

Background

D-3-aminoisobutyrate, an intermediary product of thymine, is converted to 2-methyl-3-oxopropanoate using pyruvate as an amino acceptor by D-3-aminoisobutyrate-pyruvate aminotransferase (D-AIB AT; EC 2.6.1.40). A large amount of D-AIB AT is distributed in the kidney and liver; however, small amounts are found in the brain. Recently, D-AIB AT was reported to metabolize asymmetric dimethylarginine (ADMA) in vivo and was suggested to be an important enzyme for nitric oxide metabolism because ADMA is a competitive inhibitor for nitric oxide synthase. In this study, we examined the distribution of D-AIB AT in the rat brain further to understand its role. We measured D-AIB AT mRNA and protein expression using quantitative RT-PCR and Western blotting, and monitored its distribution using immunohistochemical staining.

Results

D-AIB AT was distributed throughout the brain, with high expression in the cortex and hippocampus. Immunohistochemical staining revealed that D-AIB AT was highly expressed in the retrosplenial cortex and in hippocampal neurons.

Conclusion

Our results suggest that D-AIB AT is distributed in the examined- just the regions and may play an important role there.

A gene duplication led to specialized γ-aminobutyrate and β-alanine aminotransferase in yeast

In humans, beta-alanine (BAL) and the neurotransmitter gamma-aminobutyrate (GABA) are transaminated by a single aminotransferase enzyme. Apparently, yeast originally also had a single enzyme, but the corresponding gene was duplicated in the Saccharomyces kluyveri lineage. SkUGA1 encodes a homologue of Saccharomyces cerevisiae GABA aminotransferase, and SkPYD4 encodes an enzyme involved in both BAL and GABA transamination. SkPYD4 and SkUGA1 as well as S. cerevisiae UGA1 and Schizosaccharomyces pombe UGA1 were subcloned, over-expressed and purified. One discontinuous and two continuous coupled assays were used to characterize the substrate specificity and kinetic parameters of the four enzymes. It was found that the cofactor pyridoxal 5'-phosphate is needed for enzymatic activity and alpha-ketoglutarate, and not pyruvate, as the amino group acceptor. SkPyd4p preferentially uses BAL as the amino group donor (V(max)/K(m)=0.78 U x mg(-1) x mm(-1)), but can also use GABA (V(max)/K(m)=0.42 U x mg(-1) x mm(-1)), while SkUga1p only uses GABA (V(max)/K(m)=4.01 U x mg(-1) x mm(-1)). SpUga1p and ScUga1p transaminate only GABA and not BAL. While mammals degrade BAL and GABA with only one enzyme, but in different tissues, S. kluyveri and related yeasts have two different genes/enzymes to apparently 'distinguish' between the two reactions in a single cell. It is likely that upon duplication approximately 200 million years ago, a specialized Uga1p evolved into a 'novel' transaminase enzyme with broader substrate specificity.

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.

Identity of D-3-aminoisobutyrate-pyruvate aminotransferase with alanine-glyoxylate aminotransferase 2

D-3-Aminoisobutyrate-pyruvate aminotransferase (EC 2.6.1.40) and alanine-glyoxylate aminotransferase 2 (EC 2.6.1.44) were co-purified from rat liver as a single protein. The ratio of the two activities remained constant after Sephacryl S-200 chromatography and chromatofocussing. The Km value for beta-alanine as a substrate with 1 mM glyloxylate as amino group acceptor was 1.4 mM. The activity was inhibited by (S)-alanine with Ki = 2.2 mM. The Km for (S)-alanine as substrate with 1 mM glyoxylate as amino group was 6 mM. This activity was inhibited competitively by beta-alanine with Ki = 0.7 mM. (R)-3-aminoisobutyric acid, 5-aminolevulinic acid, NG,NG'-dimethyl-(S)-arginine, and (S)-2-aminobutyric acid were active competitively with respect to beta-alanine with Km of 0.12 mM, 2.1 mM, 6.4 mM and 11.3 mM, respectively. Antiserum to rat liver D-3-aminoisobutyrate-pyruvate aminotransferase inhibited alanine-glyoxylate aminotransferase activity in rat liver in the same way as that of D-3-aminoisobutyrate-pyruvate aminotransferase. Alanine-glyoxylate aminotransferase activity and D-3-aminoisobutyrate-pyruvate aminotransferase activities were inactivated competitively with respect to beta-alanine by 5-fluorouracil and 6-azauracil, which are chemotherapeutic reagents used to cancer. These experiments indicate that D-3-aminoisobutyrate-pyruvate aminotransferase is identical with alanine-glyoxylate aminotransferase 2, aminolevulinate aminotransferase, 2-aminobutyrate aminotransferase and dimetylarginine-pyruvate aminotransferase.

Inhibition of D-3-aminoisobutyrate-pyruvate aminotransferase by 5-fluorouracil and alpha-fluoro-beta-alanine

Among pyrimidine derivatives, we found that 5-fluorouracil potently inhibited purified rat liver D-3-aminoisobutyrate-pyruvate aminotransferase, whereas 5-fluorouridine did so to a much lesser extent. 5-Fluorouracil acted as a competitive inhibitor against beta-alanine with a Ki of 56 microM, and was uncompetitive against pyruvic acid, with a Ki of 73 microM. alpha-Fluoro-beta-alanine, a metabolite of 5-fluorouracil, was also a competitive inhibitor with respect to beta-alanine with a Ki of 8.0 mM. 5-Fluorouracil acted also as a competitive inhibitor of 4-aminobutyrate aminotransferase with respect to beta-alanine with a Ki value of 1.9 mM, and was uncompetitive against 2-oxoglutaric acid, with a Ki of 1.8 mM.

Further studies on D-3-aminoisobutyrate-pyruvate aminotransferase

D-3-Aminoisobutyrate-pyruvate aminotransferase (EC 2.6.1.40, D-BAIB aminotransferase) participates in the metabolism of thymine. Recently we purified this enzyme from rat liver. We have studied D-BAIB aminotransferase further to clarify its physiological function. Among our findings were the following. (1) The enzyme activity was widely distributed in the organs of guinea pigs and rats. The kidney, liver, and lung showed high specific activities. (2) Using the livers of six vertebrates, differences between species were studied. Activity was detected in all species, the human liver showing the lowest activity among them. (3) Developmental study using rat liver showed that the activity was low at birth, increased sharply thereafter for 10 days, and then subsequently declined to the adult level. (4) Intraperitoneal injection of BAIB and beta-alanine in rats was performed to determine whether they induce activity of this aminotransferase. Only BAIB increased the activity of the aminotransferase in the liver significantly. (5) Subcellular distribution study of this aminotransferase in rat liver revealed that it is a mitochondrial enzyme.