Inhibition of aconitase by glyoxylate plus oxaloacetate

Oxalomalate, a competitive inhibitor of aconitase, modulates the RNA-binding activity of iron-regulatory proteins

We investigated the effect of oxalomalate (OMA, alpha-hydroxy-beta-oxalosuccinic acid), a competitive inhibitor of aconitase, on the RNA-binding activity of the iron-regulatory proteins (IRP1 and IRP2) that control the post-transcriptional expression of various proteins involved in iron metabolism. The RNA-binding activity of IRP was evaluated by electrophoretic mobility-shift assay of cell lysates from 3T3-L1 mouse fibroblasts, SH-SY5Y human cells and mouse livers incubated in vitro with OMA, with and without 2-mercaptoethanol (2-ME). Analogous experiments were performed in vivo by prolonged incubation (72 h) of 3T3-L1 cells with OMA, and by injecting young mice with equimolar concentrations of oxaloacetate and glyoxylate, which are the precursors of OMA synthesis. OMA remarkably decreased the binding activity of IRP1 and, when present, of IRP2, in all samples analysed. In addition, the recovery of IRP1 by 2-ME in the presence of OMA was constantly lower versus control values. These findings suggest that the severe decrease in IRP1 RNA-binding activity depends on: (i) linking of OMA to the active site of aconitase, which prevents the switch to IRP1 and explains resistance to the reducing agents, and (ii) possible interaction of OMA with some functional amino acid residues in IRP that are responsible for binding to the specific mRNA sequences involved in the regulation of iron metabolism.

Enzymological characterization of a feline analogue of primary hyperoxaluria type 2: a model for the human disease

This paper concerns an enzymological investigation into a putative feline analogue of the human autosomal recessive disease primary hyperoxaluria type 2. The hepatic activities of D-glycerate dehydrogenase, using both D-glycerate and hydroxypyruvate as substrates, and glyoxylate reductase, which are the deficient enzyme activities in human primary hyperoxaluria type 2, were markedly depleted in four affected cats (0-6% of controls). The activities of a number of other enzymes, lactate dehydrogenase, glutamate dehydrogenase, D-amino acid oxidase, aspartate:2-oxoglutarate amino-transferase, glutamate:glyoxylate aminotransferase and alanine:glyoxylate aminotransferase (the deficient enzyme in primary hyperoxaluria type 1) were unaltered. The intracellular distribution of D-glycerate dehydrogenase and glyoxylate reductase in cat liver was shown to be cytosolic, as they are in human liver. The activities of D-glycerate dehydrogenase and glyoxylate reductase were determined in unaffected related cats and putative heterozygotes were identified. The correlation between D-glycerate dehydrogenase and glyoxylate reductase activities in the related cats and their combined deficiency in the affected cats confirmed previous suggestions that they are identical gene products.

Peroxisomal localization and activation by bivalent metal ions of ureidoglycolate lyase, the enzyme involved in urate degradation in Candida tropicalis

Ureiodoglycolate lyase (EC 4.3.2.3) was found only in the peroxisomes in urate-induced Candida tropicalis. The enzyme was markedly activated by the bivalent metal ions Mn2+, Fe2+, and Ni2+. The activation by Mn2+ was suggested to be the result of its binding to the apoenzyme.

The effect of vitamin B6 deficiency on alanine: glyoxylate aminotransferase isoenzymes in rat liver

Endogenous synthesis of oxalate has been reported to increase in vitamin B6 deficiency probably due to defective transamination of glyoxylate, the direct source of oxalate, to glycine. Alanine:glyoxylate aminotransferase (AGT) in the liver catalyzes most of the glyoxylate transamination in mammalian tissues (E. V. Rowsell, K. Snell, J. A. Carnie, and K. V. Rowsell (1972) Biochem. J. 127, 155-165). The effects of vitamin B6 deficiency on hepatic AGT isoenzymes, designated AGT 1 and AGT 2, respectively, were examined with male rats; AGT 1 is located both in the peroxisomes and in the mitochondria, and AGT 2 only in the mitochondria. The holo activity of combined peroxisomal and mitochondrial AGT 1 with a low Km for L-alanine rapidly decreased after a lag time of about 2 days during feeding of the vitamin B6-deficient diet (by 50% in 5 days, by 86% in 14 days). Peroxisomal AGT 1 activity was more affected than the mitochondrial. The holo activity of AGT 2 with a high Km for L-alanine decreased more slowly than AGT 1 (by 33% in 14 days, by 60% in 28 days). Urinary excretion of oxalate began to increase in 8-9 days, when AGT 2 remained intact but most of AGT 1 is depleted. When the defect in the glyoxylate transamination in vivo in vitamin B6 deficiency is considered, these findings suggest that it is due to the deficiency of AGT 1. The importance of peroxisomal AGT 1 is discussed, since peroxisomes have been described to be probably the major site of glyoxylate formation.

Co-purification of alanine-glyoxylate aminotransferase with 2-aminobutyrate aminotransferase in rat kidney

Alanine-glyoxylate aminotransferase and 2-aminobutyrate aminotransferase were co-purified from rat kidney to a single protein (about 500-fold purified from the homogenate). The activity ratios of alanine-glyoxylate aminotransferase to 2-aminobutyrate aminotransferase were constant during co-purification steps suggesting the 2-aminobutyrate aminotransferase activity was catalysed by only alanine-glyoxylate aminotransferase. The molecular weight of the enzyme was estimated to be approx. 213 000, 220 000 and 236 000 by analytical ultracentrifugation, Sephadex G-150 gel filtration and sucrose density gradient centrifugation, respectively. From the polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate, the enzyme consisted of four apparently similar subunits having a molecular weight of approx. 56 000. The enzyme was almost specific to L-alanine and L-2-aminobutyrate as amino donor and to glyoxylate, pyruvate and 2-oxobutyrate as amino acceptor. The enzyme was identified with rat liver alanine-glyoxylate aminotransferase isoenzyme 2 but not with rat liver alanine-glyoxylate aminotransferase isoenzyme 1 from Ouchterlony double diffusion analysis. Absorption spectra and some kinetic properties of the enzyme were clarified.

Alternate pathways of metabolism of short-chain fatty acids

Images

The metabolism of phenylacetic acid by a Pseudomonas

A Pseudomonas species adapted to grow on phenylacetic acid is simultaneously adapted to the utilization of phenylacetic acid, p-hydroxyphenylacetic acid, and 3,4-dihydroxyphenylacetic acid. Extracts of the organism catalyze the oxidation of p-hydroxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid, but not phenylacetic acid. The addition of NAD or NADH to extracts is required for maximum utilization of p-hydroxyphenylacetic acid; ferrous ion is required for maximum utilization of 3,4-dihydroxyphenylacetic acid. The oxidation of 3,4-dihydroxyphenylacetic acid results in the production of a compound having an absorption maximum at 318 mμ in acid and at 380 mμ in alkali, which was identified as δ-carboxymethyl-α-hydroxymuconic semialdehyde. The degradation of phenylacetic acid by this organism follows a pathway that utilizes p-hydroxyphenylacetic acid and 3,4-dihydroxyphenylacetic acid as intermediates.