Molecular basis of 3-hydroxy-3-methylglutaric aciduria

The specific molecular architecture of plant 3-hydroxy-3-methylglutaryl-CoA lyase

3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase (HMGL) is involved in branched-chain amino acid catabolism leading to acetyl-CoA production. Here, using bioinformatics analyses and protein sequence alignments, we found that in Arabidopsis thaliana a single gene encodes two HMGL isoforms differing in size (51 kDa, HMGL51 and 46 kDa, HMGL46). Similar to animal HMGLs, both isoforms comprised a C-terminal type 1 peroxisomal retention motif, and HMGL51 contained a mitochondrial leader peptide. We observed that only a shortened HMGL (35 kDa, HMGL35) is conserved across all kingdoms of life. Most notably, all plant HMGLs also contained a specific N-terminal extension (P100) that is located between the N-terminal mitochondrial targeting sequence TP35 and HMGL35 and is absent in bacteria and other eukaryotes. Interestingly, using HMGL enzyme assays, we found that rather than HMGL46, homodimeric recombinant HMGL35 is the active enzyme catalyzing acetyl-CoA and acetoacetate synthesis when incubated with (S)-HMG-CoA. This suggested that the plant-specific P100 peptide may inactivate HMGL according to specific physiological requirements. Therefore, we investigated whether the P100 peptide in HMGL46 alters its activity, possibly by modifying the HMGL46 structure. We found that induced expression of a cytosolic HMGL35 version in A. thaliana delays germination and leads to rapid wilting and chlorosis in mature plants. Our results suggest that in plants, P100-mediated HMGL inactivation outside of peroxisomes or mitochondria is crucial, protecting against potentially cytotoxic effects of HMGL activity while it transits to these organelles.

Combined effects of a high-fat diet and chronic valproic acid treatment on hepatic steatosis and hepatotoxicity in rats

AIM

To investigate the potential interactive effects of a high-fat diet (HFD) and valproic acid (VPA) on hepatic steatosis and hepatotoxicity in rats.

METHODS

Male SD rats were orally administered VPA (100 or 500 mg·kg⁻¹·d⁻¹) combined with HFD or a standard diet for 8 weeks. Blood and liver samples were analyzed to determine lipid levels and hepatic function biomarkers using commercial kit assays. Low-molecular-weight compounds in serum, urine and bile samples were analyzed using a metabonomic approach based on GC/TOF-MS.

RESULTS

HFD alone induced extensive hepatocyte steatosis and edema in rats, while VPA alone did not cause significant liver lesions. VPA significantly aggravated HFD-induced accumulation of liver lipids, and caused additional spotty or piecemeal necrosis, accompanied by moderate infiltration of inflammatory cells in the liver. Metabonomic analysis of serum, urine and bile samples revealed that HFD significantly increased the levels of amino acids, free fatty acids (FFAs) and 3-hydroxy-butanoic acid, whereas VPA markedly decreased the levels of amino acids, FFAs and the intermediate products of the tricarboxylic acid cycle (TCA) compared with the control group. HFD aggravated VPA-induced inhibition on lipid and amino acid metabolism.

CONCLUSION

HFD magnifies VPA-induced impairment of mitochondrial β-oxidation of FFAs and TCA, thereby increases hepatic steatosis and hepatotoxicity. The results suggest the patients receiving VPA treatment should be advised to avoid eating HFD.

In vivo experimental evidence that the major metabolites accumulating in 3-hydroxy-3-methylglutaryl-CoA lyase deficiency induce oxidative stress in striatum of developing rats: a potential pathophysiological mechanism of striatal damage in this disorder

3-Hydroxy-3-methylglutaryl-CoA lyase (HL) deficiency is a genetic disorder biochemically characterized by predominant accumulation of 3-hydroxy-3-methylglutaric (HMG) and 3-methylglutaric (MGA) acids in tissues and biological fluids of affected individuals. Clinically, the patients present neurological symptoms and basal ganglia injury, whose pathomechanisms are partially understood. In the present study, we investigated the ex vivo effects of intrastriatal administration of HMG and MGA on important parameters of oxidative stress in striatum of developing rats. Our results demonstrate that HMG and MGA induce lipid and protein oxidative damage. HMG and MGA also increased 2',7'-dichlorofluorescein oxidation, whereas only HMG elicited nitric oxide production, indicating a role for reactive oxygen (HMG and MGA) and nitrogen (HMG) species in these effects. Regarding the enzymatic antioxidant defenses, both organic acids decreased reduced glutathione concentrations and the activities of superoxide dismutase and glutathione reductase and increased glutathione peroxidase activity. HMG also provoked an increase of catalase activity and a diminution of glucose-6-phosphate dehydrogenase activity. We finally observed that antioxidants fully prevented or attenuated HMG-induced alterations of the oxidative stress parameters, further indicating the participation of reactive species in these effects. We also observed that MK-801, a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, prevented some of these effects, indicating the involvement of the NMDA receptor in HMG effects. The present data provide solid evidence that oxidative stress is induced in vivo by HMG and MGA in rat striatum and it is presumed that this pathomechanism may explain, at least in part, the cerebral alterations observed in HL deficiency.

Characterization of a novel HMG-CoA lyase enzyme with a dual location in endoplasmic reticulum and cytosol

A novel lyase activity enzyme is characterized for the first time: HMG-CoA lyase-like1 (er-cHL), which is a close homolog of mitochondrial HMG-CoA lyase (mHL). Initial data show that there are nine mature transcripts for the novel gene HMGCLL1, although none of them has all its exons. The most abundant transcript is called “variant b,” and it lacks exons 2 and 3. Moreover, a three-dimensional model of the novel enzyme is proposed. Colocalization studies show a dual location of the er-cHL in the endoplasmic reticulum (ER) and cytosol, but not in mitochondria or peroxisomes. Furthermore, the dissociation experiment suggests that it is a nonendoplasmic reticulum integral membrane protein. The kinetic parameters of er-cHL indicate that it has a lower V_max and a higher substrate affinity than mHL. Protein expression and lyase activity were found in several tissues, and were particularly strong in lung and kidney. The occurrence of er-cHL in brain is surprising, as mHL has not been found there. Although mHL activity is clearly associated with energy metabolism, the results suggest that er-cHL is more closely related to another metabolic function, mostly at the pulmonary and brain level.

A single-residue mutation, G203E, causes 3-hydroxy-3-methylglutaric aciduria by occluding the substrate channel in the 3D structural model of HMG-CoA lyase

3-Hydroxy-3-methylglutaric aciduria is a rare autosomal recessive genetic disorder that affects ketogenesis and leucine metabolism. The disease is caused by mutations in the gene coding for 3-hydroxy-3-methylglutaryl-coenzyme A lyase (HL). To date 26 different mutations have been described. A (betaalpha)(8) TIM barrel structure has been proposed for the protein, and almost all missense mutations identified so far localize in the beta sheets that define the inside cavity. We report an Italian patient who bears homozygously a novel HL mutation, c.608G > A (p. G203E) in beta sheet six. A structural model of the mutated protein suggests that glutamic acid 203 impedes catalysis by blocking the entrance to the inner cavity of the enzyme. Loss of functionality has been confirmed in expression studies in E. coli, which demonstrate that the G203E mutation completely abolishes enzyme activity. Beta sheet six and beta sheet two are the two protein regions that accumulate most missense mutations, indicating their importance in enzyme functionality. A model for the mechanism of enzyme function is proposed.

Crystal structures of two bacterial 3-hydroxy-3-methylglutaryl-CoA lyases suggest a common catalytic mechanism among a family of TIM barrel metalloenzymes cleaving carbon-carbon bonds

The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase catalyzes the terminal steps in ketone body generation and leucine degradation. Mutations in this enzyme cause a human autosomal recessive disorder called primary metabolic aciduria, which typically kills victims because of an inability to tolerate hypoglycemia. Here we present crystal structures of the HMG-CoA lyases from Bacillus subtilis and Brucella melitensis at 2.7 and 2.3 A resolution, respectively. These enzymes share greater than 45% sequence identity with the human orthologue. Although the enzyme has the anticipated triose-phosphate isomerase (TIM) barrel fold, the catalytic center contains a divalent cation-binding site formed by a cluster of invariant residues that cap the core of the barrel, contrary to the predictions of homology models. Surprisingly, the residues forming this cation-binding site and most of their interaction partners are shared with three other TIM barrel enzymes that catalyze diverse carbon-carbon bond cleavage reactions believed to proceed through enolate intermediates (4-hydroxy-2-ketovalerate aldolase, 2-isopropylmalate synthase, and transcarboxylase 5S). We propose the name "DRE-TIM metallolyases" for this newly identified enzyme family likely to employ a common catalytic reaction mechanism involving an invariant Asp-Arg-Glu (DRE) triplet. The Asp ligates the divalent cation, while the Arg probably stabilizes charge accumulation in the enolate intermediate, and the Glu maintains the precise structural alignment of the Asp and Arg. We propose a detailed model for the catalytic reaction mechanism of HMG-CoA lyase based on the examination of previously reported product complexes of other DRE-TIM metallolyases and induced fit substrate docking studies conducted using the crystal structure of human HMG-CoA lyase (reported in the accompanying paper by Fu, et al. (2006) J. Biol. Chem. 281, 7526-7532). Our model is consistent with extensive mutagenesis results and can guide subsequent studies directed at definitive experimental elucidation of this enzyme's reaction mechanism.

Skipping of exon 2 and exons 2 plus 3 of HMG-CoA lyase (HL) gene produces the loss of beta sheets 1 and 2 in the recently proposed (beta-alpha)8 TIM barrel model of HL

HMG-CoA lyase (HL) deficiency is a rare autosomal recessive genetic disorder that affects ketogenesis and leucine catabolism. We report a new Spanish patient who bears the frequent nonsense mutation G109T (Mediterranean mutation). This mutation can produce aberrant splicing with three mRNA variants: one of the expected size, the second with deletion of exon 2, and the third with deletion of exons 2 and 3. Recently our group proposed a 3D model for human HL containing a (beta-alpha)(8) (TIM) barrel structure. We have studied the effect of the deletions of exon 2 and exons 2 plus 3 on the proposed HL model. Exon 2 skipping led to the loss of beta-sheet 1, and the skipping of exons 2 and 3 caused the disappearance of alpha helix 1 and beta-sheets 1 and 2.-