Regulatory regions of the mitochondrial and cytosolic isoenzyme genes participating in the malate-aspartate shuttle

Genetics of MDH in humans

Malate dehydrogenase (MDH) performs key roles in metabolism, but little is known about its function specifically in human health and disease. In this minireview, we describe the incomplete state of our knowledge of human MDH genetics. Humans have three MDH genes with a total of four validated isoforms. MDH1 and MDH2 are widely expressed, while MDH1B is only expressed in a small subset of tissues. Many mutations in MDH1 and MDH2 have been identified in patients, but only a few have been studied to determine what symptoms they cause. MDH1 has been associated with cancer and a neurodevelopmental disorder. MDH2 has been associated with diabetes, neurodevelopmental disorders, and cancer.

Malate dehydrogenase: a story of diverse evolutionary radiation

Malate dehydrogenase (MDH) is a ubiquitous enzyme involved in cellular respiration across all domains of life. MDH's ubiquity allows it to act as an excellent model for considering the history of life and how the rise of aerobic respiration and eukaryogenesis influenced this evolutionary process. Here, we present the diversity of the MDH family of enzymes across bacteria, archaea, and eukarya, the relationship between MDH and lactate dehydrogenase (LDH) in the formation of a protein superfamily, and the connections between MDH and endosymbiosis in the formation of mitochondria and chloroplasts. The development of novel and powerful DNA sequencing techniques has challenged some of the conventional wisdom underlying MDH evolution and suggests a history dominated by gene duplication, horizontal gene transfer, and cryptic endosymbiosis events and adaptation to a diverse range of environments across all domains of life over evolutionary time. The data also suggest a superfamily of proteins that do not share high levels of sequential similarity but yet retain strong conservation of core function via key amino acid residues and secondary structural components. As DNA sequencing and 'big data' analysis techniques continue to improve in the life sciences, it is likely that the story of MDH will continue to refine as more examples of superfamily diversity are recovered from nature and analyzed.

Progesterone causes metabolic changes involving aminotransferases and creatine kinase in cryopreserved bovine spermatozoa

Progesterone (P4) is capable of inducing acrosome reaction in many species. The objective of this study was to determine the activity of enzymes involved in metabolism that contribute to the redox state and supply energy for acrosome reaction in cryopreserved bull spermatozoa. To accomplish this aim, acrosome reaction was induced by P4 in capacitated and non-capacitated samples. Alanine and aspartate aminotransferases (ALT, AST) and creatine kinase (CK) activities were measured spectrophotometrically at 340 nm after acrosome reaction with P4. Oxygen consumption was measured polarographically. ALT and AST activities increased by the addition of P4 capacitated and non-capacitated samples. P4 addition provoked an increase in CK activity in non-capacitated spermatozoa compared to heparin capacitated spermatozoa with or without P4 addition. P4 increased oxygen consumption, the percentage of acrosome reacted spermatozoa as well as the absence of acrosome integrity in both capacitated and non-capacitated bovine spermatozoa, but oxygen consumption in P4 samples was significantly lower than in heparin capacitated spermatozoa (P<0.05). Acrosome reaction induction by P4 required different creatine kinase activity with the same oxygen consumption and transaminases level to maintain oxidative metabolism and redox state through reducing equivalents transfer between cytosolic and mitochondrial compartment. In conclusion, P4 induces a lower oxidative metabolism during acrosome reaction in bovine cryopreserved spermatozoa, compared to heparin induced capacitation process.

Over-expression of aspartate aminotransferase genes in rice resulted in altered nitrogen metabolism and increased amino acid content in seeds

Aspartate aminotransferase (AAT) is a key enzyme in the synthesis of amino acids. It plays an important role in regulating carbon and nitrogen metabolism in almost all organisms. In this study, we over-expressed in rice separately all three AAT genes from rice (OsAAT1~3) and one AAT gene from Escherichia coli (EcAAT). Over-expression was driven by the CaMV 35S promoter and constructs were introduced into rice by Agrobacterium tumefaciens-mediated transformation. Compared with control plants, the transformants showed significantly increased leaf AAT activity and greater seed amino acid and protein contents. No other phenotypic changes were observed. The total leaf AAT activities in plants over-expressing OsAAT1, OsAAT2, and EcAAT were 26.6, 23.6, and 19.6 A min(-1) mg(-1) FW (A: units of activity, defined as increase of absorbency per min per mg; FW: fresh weight), which were significantly higher than that in the wild-type control (17.7 A min(-1) mg(-1) FW). The amino acid content in seeds of transgenic plants over-expressing OsAAT1, OsAAT2, and EcAAT was 119.36, 115.36, and 113.72 mg g(-1), respectively, which were 16.1, 12.0, and 5.4% higher, respectively, than that in the control plants. The transgenic plants over-expressing OsAAT1, OsAAT2, and EcAAT had significantly higher protein contents (increased 22.2, 21.1, and 11.1%, respectively) than wild-type plants. No significant changes were found in leaf AAT activity, seed amino acid content or protein content in OsAAT3 over-expressed plants. The expression patterns of the three OsAAT genes and their different functions are also discussed.

Exercise training increases electron and substrate shuttling proteins in muscle of overweight men and women with the metabolic syndrome

Aerobic conditioned muscle shows increased oxidative metabolism or glucose relative to untrained muscle at a given absolute exercise intensity. The studies of a targeted risk reduction intervention through defined exercise (STRRIDE) study is an aerobic exercise intervention in men and women with features of metabolic syndrome (Kraus WE, Torgan CE, Duscha BD, Norris J, Brown SA, Cobb FR, Bales CW, Annex BH, Samsa GP, Houmard JA, and Slentz CA, Med Sci Sports Exerc 33: 1774–1784, 2001), with four muscle biopsies taken during training and detraining time points. Here, we expanded a previous study (Hittel DS, Kraus WE, and Hoffman EP, J Physiol 548: 401–410, 2003) and used mRNA profiling to investigate gene transcripts associated with energy and substrate metabolism in STRRIDE participants. We found coordinate regulation of key metabolic enzymes with aerobic training in metabolic syndrome (aspartate aminotransferase 1, lactate dehydrogenase B, and pyruvate dehydrogenase-α1). All were also quickly downregulated by detraining, although the induction was not an acute response to activity. Protein and enzymatic assays were used to validate mRNA induction with aerobic training and loss with detraining (96 h to 2 wk) in 10 male and 10 female STRRIDE subjects. We propose that training coordinately increases the levels of aspartate aminotransferase 1, lactate dehydrogenase B, and pyruvate dehydrogenase-α1subunit, increasing glucose metabolism in muscle by liberating pyruvate for oxidative metabolism and, therefore, limiting lactate efflux. Serial measurement of fasting plasma lactate from 62 subjects from the same exercise group demonstrated a significant decrease of circulating lactate with training. We also found evidence for sex-specific molecular remodeling of muscle with ubiquinol-cytochrome c reductase core protein II, a component of mitochondrial respiratory complex III, which showed an increase after training that was specific to women. These biochemical adaptations complement existing molecular models for improved glucose tolerance with exercise intervention in prediabetic individuals.

Fenofibrate modifies transaminase gene expression via a peroxisome proliferator activated receptor alpha-dependent pathway

Fibrates modify the expression of genes implicated in lipoprotein and fatty acid metabolism via the peroxisome proliferator-activated receptor alpha(PPARalpha), leading to reductions in serum triglycerides and cholesterol. The expression of certain genes regulated by PPARalpha have been shown to be modified in a species dependent manner. Aspartate aminotransferase (AspAT or GOT) and alanine aminotransferase (AlaAT or GPT) are enzymes involved in intermediate metabolism in all cells and in hepatic gluconeogenesis. These enzymes are also widely used as serum markers of possible tissue damage. This study investigated whether fenofibrate could modify the expression of liver AspAT and/or AlaAT and thus possibly alter transaminase levels independently of a cytotoxic effect. In human Hep G2 cells, fenofibrate increased cytosolic AspAT (cAspAT) activity by 40% and AlaAT activity by 100%, as well as both mRNAs. Nuclear run on assays showed that this effect was, at least in part, transcriptional. Increases in mRNA were also observed in human hepatocyte cultures at concentrations of the drug attained in patients. In C57BL/6 mice, fenofibrate decreased cAspAT and cAlaAT mRNA, while these effects were abolished in PPARalpha knock-out mice. In conclusion, fenofibrate has been shown to modify cAspAT and AlaAT gene expression in a species and PPARalpha dependent manner. This is the first demonstration that cAspAT and AlaAT activities may be pharmacologically altered, independently of a toxic phenomenon.

Molecular cloning, characterization, and regulation of the human mitochondrial serine hydroxymethyltransferase gene

The human mitochondrial serine hydroxymethyltransferase (mSHMT) gene was isolated, sequenced, and characterized. The 4.5-kilobase gene contains 10 introns and 11 exons, with all splice junctions conforming to the GT/AG rule. The 5' promoter region contains consensus motifs for several regulatory proteins including PEA-3, Sp-1, AP-2, and a CCCTCCC motif common to many genes expressed in liver. Consensus TATA or CAAT sequence motifs are not present, and primer extension and 5'-rapid amplification of cDNA ends studies suggest that transcription initiation occurs at multiple sites. The mitochondrial leader sequence region of the deduced mRNA contains two potential ATG start sites, which are encoded by separate exons. The intervening 891-base pair intron contains consensus promoter elements suggesting that mSHMT may be transcribed from alternate promoters. 5'-Rapid amplification of cDNA ends analysis demonstrated that the first ATG is transcribed in human MCF-7 cells. However, transfection of Chinese hamster ovary cells deficient in mSHMT activity with the human mSHMT gene lacking exon 1 overcame the cell's glycine auxotrophy and restored intracellular glycine concentrations to that observed in wild-type cells, showing that exon 1 is not essential for mSHMT localization or activity and that translation initiation from the second ATG is sufficient for mSHMT import into the mitochondria. Mitochondrial SHMT mRNA levels in MCF-7 cells did not vary during the cell cycle and were not affected by the absence of glycine, serine, folate, thymidylate, or purines from the media.

Lack of correlation between mRNA expression and enzymatic activity of the aspartate aminotransferase isoenzymes in various tissues of the rat

Little is known about control of expression of basal levels of the aspartate aminotransferases which are ubiquitous 'house keeping' enzymes in vertebrates. We have measured both mRNA and activity levels for both isoenzymes in various rat tissues as a function of age. Patterns of mRNA expression for the two isoenzymes were similar in a particular tissue about differed widely between tissues. Surprisingly, there was no simple correlation between mRNA levels and specific activities of the enzyme products. We conclude that translation for mRNA for these two isoenzymes is subject to tissue-specific, and in some cases age-related, regulation.

Androgen modulation of multiple transcription start sites of the mitochondrial aspartate aminotransferase gene in rat prostate

Mitochondrial aspartate aminotransferase (mAAT) is one of two key enzymes in the pathway of citrate production in prostate. Expression of mAAT is modulated by testosterone and prolactin in prostate. We cloned the promoter and 5'-flanking region of the rat mAAT gene and sequenced 2.0 kilobases of the DNA. This fragment contains the 5'-regulatory promoter region that lacks a TATA and a CCAAT box but is G+C rich. The 5'-upstream flanking region contains sequences that have high homology with the consensus glucocorticoid response element/androgen response element (ARE) and a reported ARE sequence that is different from the consensus sequence. Functional transcription studies showed that a 481-base region containing the two ARE sequences was sufficient for androgen-regulated gene expression. There are multiple transcription start sites that are regulated by testosterone in prostate. In liver, on the other hand, castration did not affect transcription from any of the start sites. Therefore, these data provide evidence that transcriptional regulation of the rat pmAAT gene occurs through an ARE located in the 5'-region. In addition, not only is gene expression modulated by testosterone, but the effect of testosterone on transcription is cell specific.

Structure of genes that encode isozymes of aspartate aminotransferase in Panicum miliaceum L., a C4 plant

The cytosolic and mitochondrial isozymes of aspartate aminotransferase (AspAT) function in the C4 photosynthetic cycle in NAD-malic enzyme-type C4 plants and are expressed at high levels in mesophyll cells and bundle sheath cells, respectively. We constructed a genomic library from Panicum miliaceum, a NAD-malic enzyme-type C4 plant, and cloned the genes for these isozymes. The sequence of the cloned gene for cytosolic AspAT spans 7800 bp and consists of 12 exons. The sequence of the cloned gene for mitochondrial AspAT spans 9000 bp and consists of 10 exons. The results of primer-extension analysis suggest that transcription may be initiated from multiple adjacent sites. Both genes have significant GC-rich regions around the site of initiation of transcription, and these regions showed no CpG suppression. The 5'- flanking regions of both genes include several short sequences similar to the regulatory elements found in other genes for components of the photosynthetic machinery. In particular, the cytosolic AspAT gene contains sequences similar to nuclear protein-binding sites in other mesophyll-expressed C4 photosynthetic genes and the mitochondrial AspAT gene contains elements for light-sensitive and constitutive expression of a bundle sheath-expressed gene. The results of Southern analysis indicated that there are at least two genes that encode each isozyme in the genome of P. miliaceum. A comparison of intron-insertion positions between AspAT genes of plants and animals revealed that several introns are located at identical positions. On the basis of a phylogenetic tree among AspATs and tyrosine aminotransferase, we have shown that the introns of aminotransferase genes antedate the divergence of eubacteria, archaebacteria, and eukaryotes.

Regulation of malate dehydrogenases from neonatal, adolescent, and mature rat brain

Since the malate-aspartate shuttle in brain has been shown to be closely linked to brain energy metabolism and neurotransmitter synthesis, the activity of MDH, one of the enzymes of the malate-aspartate shuttle, was studied in cortical non-synaptic mitochondria (mMDH) and cytosol (cMDH) in 1-4 day, 18-20 day and 7-8 week old rats. The mean mMDH activity (nmol/min/mg protein) was 10,517 +/- 734 (mean +/- SEM), 8,882 +/- 241 and 10,323 +/- 561 and cMDH activity was 2,453 +/- 99, 4,673 +/- 152 and 6,821 +/- 205 in 1-4 day, 18-20 day and 7-8 week old rats, respectively. While cMDH activity increased with age (p < 0.0001), mMDH activity showed no change. This study also determined if endogenous compounds, previously shown to alter malate metabolism, affected MDH activities. Lactate inhibited only cMDH activity, by a competitive mechanism. Oxaloacetate inhibited mMDH by partial non-competitive inhibition and cMDH by competitive inhibition. Alpha-ketoglutarate competitively inhibited both enzymes; however, the inhibition of mMDH activity was more pronounced than that of cMDH activity. Citrate inhibited mMDH via an uncompetitive mechanism and cMDH via a noncompetitive mechanism. The mechanisms of inhibition of mMDH and cMDH by each of the effectors were the same over the three ages. The results suggest mMDH and cMDH activities show a dissimilar developmental pattern and may be regulated differently by endogenous effectors. The greater sensitivity of mMDH, compared to cMDH, to certain effectors may be related to the dual role of mMDH in the tricarboxylic acid cycle and the malate-aspartate shuttle.

Malate dehydrogenase isoenzymes: cellular locations and role in the flow of metabolites between the cytoplasm and cell organelles

Malate dehydrogenases belong to the most active enzymes in glyoxysomes, mitochondria, peroxisomes, chloroplasts and the cytosol. In this review, the properties and the role of the isoenzymes in different compartments of the cell are compared, with emphasis on molecular biological aspects. Structure and function of malate dehydrogenase isoenzymes from plants, mammalian cells and ascomycetes (yeast, Neurospora) are considered. Significant information on evolutionary aspects and characterisation of functional domains of the enzymes emanates from bacterial malate and lactate dehydrogenases modified by protein engineering. The review endeavours to give up-to-date information on the biogenesis and intracellular targeting of malate dehydrogenase isoenzymes as well as enzymes cooperating with them in the flow of metabolites of a given pathway and organelle.

Structural organization and regulatory regions of the human medium-chain acyl-CoA dehydrogenase gene

Medium-chain acyl-CoA dehydrogenase (MCAD) is a highly regulated mitochondrial flavo-enzyme that catalyzes the initial reaction in fatty acid beta-oxidation. Deficiency of MCAD is a common inherited defect in energy metabolism. We have previously shown that the mRNA encoding MCAD in an MCAD-deficient child is homozygous for the point mutation A985 to G [Kelly et al. (1990) Proc. Natl. Acad. Sci. U.S.A. 87, 9236-9420]. To define the molecular basis of MCAD deficiency and as an initial step in the study of the regulation of MCAD gene expression, we determined the structure and organization of the human MCAD gene. The gene is comprised of 12 exons which span 44 kb of DNA. Comparison of the MCAD gene to MCAD mRNAs from the MCAD-deficient child revealed that missplicing was common, resulting in a variety of exon deletions and intron insertions. The MCAD gene promoter region is extremely GC-rich and lacks prototypical TATA and CAAT boxes. Several regions upstream of the promoter are homologous with mitochondrial enhancers purportedly involved in coordinate expression of nuclear genes encoding mitochondrial proteins. Transfection of chimeric plasmid constructs with 299 bp of upstream sequence into HepG2 cells revealed high-level transcriptional activity. We conclude that the precursor MCAD mRNA is misspliced to a high degree and complexity in association with the G985 mutation and the MCAD gene contains a strong promoter which shares some structural features with other "housekeeping" genes encoding mitochondrial proteins.

Concepts of citrate production and secretion by prostate: 2. Hormonal relationships in normal and neoplastic prostate

A unique and major function of prostate secretory epithelial cells is to synthesize, accumulate, and secrete extraordinarily high levels of citrate. This function is regulated by testosterone and by prolactin. Concepts of the mechanisms of hormonal regulation are presented. The relationship of testosterone and prolactin to the origin and homologies of different prostate cell lines is described. The metabolic differentiation of citrate and non-citrate producing prostate secretory epithelial cells is discussed. Concepts of the pathogenesis of prostatic neoplasms are presented based on hormonal, metabolic, and homologous relationships associated with citrate production. Characterization of normal and neoplastic secretory epithelial cells by their citrate function is emphasized. The urgency and necessity for research relating to all aspects of prostate citrate production in normal and pathological prostate are emphasized.