High-performance liquid chromatography determination of methionine adenosyltransferase activity using catechol-O-methyltransferase-coupled fluorometric detection

Functional and structural characterization of a novel catechol-O-methyltransferase from Schizosaccharomyces pombe

Catechol-O-methyltransferase (COMT¹ ) catalyzes the transfer of a methyl group from S-adenosylmethionine (SAM) to various catechol substrates. COMTs play vital roles in physiological processes in animals, plants, and fungi, as well as bacteria, and have essential application values in industry. spCOMT is a probable COMT from Schizosaccharomyces pombe. It has an extraordinary intracellular distribution different from other homologs and would thus be predicted to perform a distinct physiological function. In this report, recombinant spCOMT was purified and kinetically characterized for the first time. The enzymology assays indicate that spCOMT is a metal-dependent enzyme and belongs to class I OMTs. In addition, the crystal structures of apo-spCOMT and SAM-complexed spCOMT were also presented, revealing that spCOMT possesses a conserved SAM-binding site and Mg²⁺ pocket, but a distinct substrate pocket was not present in homologs. The mutagenesis ITC analysis revealed the SAM recognition characteristics of spCOMT. Based on all of the above findings, we speculated about the putative substrates' characteristics and the substrate recognition mechanisms of spCOMT. This work will help in elucidating the physiological functions of spCOMT in S. pombe. © 2018 IUBMB Life, 71(3):330-339, 2019.

Human liver methionine cycle: MAT1A and GNMT gene resequencing, functional genomics, and hepatic genotype-phenotype correlation

The "methionine cycle" plays a critical role in the regulation of concentrations of (S)-adenosylmethionine (AdoMet), the major biological methyl donor. We set out to study sequence variation in genes encoding the enzyme that synthesizes AdoMet in liver, methionine adenosyltransferase 1A (MAT1A) and the major hepatic AdoMet using enzyme, glycine N-methyltransferase (GNMT), as well as functional implications of that variation. We resequenced MAT1A and GNMT using DNA from 288 subjects of three ethnicities, followed by functional genomic and genotype-phenotype correlation studies performed with 268 hepatic biopsy samples. We identified 44 and 42 polymorphisms in MAT1A and GNMT, respectively. Quantitative Western blot analyses for the human liver samples showed large individual variation in MAT1A and GNMT protein expression. Genotype-phenotype correlation identified two genotyped single-nucleotide polymorphisms (SNPs), reference SNP (rs) 9471976 (corrected p = 3.9 × 10(-10)) and rs11752813 (corrected p = 1.8 × 10(-5)), and 42 imputed SNPs surrounding GNMT that were significantly associated with hepatic GNMT protein levels (corrected p values < 0.01). Reporter gene studies showed that variant alleles for both genotyped SNPs resulted in decreased transcriptional activity. Correlation analyses among hepatic protein levels for methionine cycle enzymes showed significant correlations between GNMT and MAT1A (p = 1.5 × 10(-3)) and between GNMT and betaine homocysteine methyltransferase (p = 1.6 × 10(-7)). Our discovery of SNPs that are highly associated with hepatic GNMT protein expression as well as the "coordinate regulation" of methionine cycle enzyme protein levels provide novel insight into the regulation of this important human liver biochemical pathway.

Methionine adenosyltransferase 2A/2B and methylation: gene sequence variation and functional genomics

Methionine adenosyltransferase (MAT) catalyzes the synthesis of S-adenosylmethionine, the major biological methyl donor. MAT1A and MAT2A encode two distinct MAT isoforms in mammals. MAT2A is expressed in nonhepatic tissues, whereas MAT1A is expressed in the liver. A third gene, MAT2B, encodes a MAT2A regulatory protein. We resequenced MAT2A and MAT2B exons, splice junctions, and flanking regions using 288 DNA samples from three ethnic groups and also imputed additional single nucleotide polymorphisms (SNPs) across both genes using data from the 1000 Genomes Project. For MAT2A, resequencing identified 74 polymorphisms, including two nonsynonymous (ns) SNPs. Functional genomic studies of wild type and the two MAT2A variant allozymes (Val11 and Val205) showed that the Val11 allozyme had approximately 40% decreases in levels of enzyme activity and immunoreactive protein after COS-1 cell transfection. For MAT2B, 44 polymorphisms, 2 nonsynonymous, were identified during resequencing. Neither of the two MAT2B nsSNPs displayed alterations in levels of protein. Imputation using 1000 Genomes Project data resulted in 1730 additional MAT2A and 1997 MAT2B polymorphisms within ± 200 kilobases of each gene, respectively. Coexpression of MAT2A and MAT2B in COS-1 cells resulted in significantly increased MAT enzyme activity that correlated with increased MAT2A and MAT2B immunoreactive protein, apparently as a result of decreased degradation. Finally, studies of mRNA expression in lymphoblastoid cells showed that 7 SNPs in MAT2A and 16 SNPs in MAT2B were significantly associated with mRNA expression with p < 0.01. These observations provide a foundation for future mechanistic and clinical translational pharmacogenomic studies of MAT2A/2B.

Temporal study of acetaminophen (APAP) and S-adenosyl-L-methionine (SAMe) effects on subcellular hepatic SAMe levels and methionine adenosyltransferase (MAT) expression and activity

Acetaminophen (APAP) is the leading cause of drug induced liver failure in the United States. Previous studies in our laboratory have shown that S-adenosyl methionine (SAMe) is protective for APAP hepatic toxicity. SAMe is critical for glutathione synthesis and transmethylation of nucleic acids, proteins and phospholipids which would facilitate recovery from APAP toxicity. SAMe is synthesized in cells through the action of methionine adenosyltransferase (MAT). This study tested the hypothesis that total hepatic and subcellular SAMe levels are decreased by APAP toxicity. Studies further examined MAT expression and activity in response to APAP toxicity. Male C57BL/6 mice (16-22 g) were treated with vehicle (Veh; water 15 ml/kg ip injections), 250 mg/kg APAP (15 ml/kg, ip), SAMe (1.25 mmol/kg) or SAMe administered 1h after APAP injection (SAMe and SAMe+APAP). Hepatic tissue was collected 2, 4, and 6h after APAP administration. Levels of SAMe and its metabolite S-adenosylhomocysteine (SAH) were determined by HPLC analysis. MAT expression was examined by Western blot. MAT activity was determined by fluorescence assay. Total liver SAMe levels were depressed at 4h by APAP overdose, but not at 2 or 6h. APAP depressed mitochondrial SAMe levels at 4 and 6h relative to the Veh group. In the nucleus, levels of SAMe were depressed below detectable limits 4h following APAP administration. SAMe administration following APAP (SAMe+APAP) prevented APAP associated decline in mitochondrial and nuclear SAMe levels. In conclusion, the maintenance of SAMe may provide benefit in preventing damage associated with APAP toxicity.