Cloning, sequencing, and heterologous expression of rat methionine synthase cDNA

Effects of dietary supplements of folic acid and vitamin B12 on metabolism of dairy cows in early lactation

The present experiment was undertaken to determine the effects of dietary supplements of folic acid and vitamin B12 given from 3 wk before to 8 wk after calving on lactational performance and metabolism of 24 multiparous Holstein cows assigned to 6 blocks of 4 cows each according to their previous milk production. Supplementary folic acid at 0 or 2.6 g/d and vitamin B12 at 0 or 0.5 g/d were used in a 2 x 2 factorial arrangement. Supplementary folic acid increased milk production from 38.0 +/- 0.9 to 41.4 +/- 1.0 kg/d and milk crude protein yield from 1.17 +/- 0.02 to 1.25 +/- 0.03 kg/d. It also increased plasma Gly, Ser, Thr, and total sulfur AA, decreased Asp, and tended to increase plasma Met. Supplementary B12 decreased milk urea N, plasma Ile, and Leu and tended to decrease Val but increased homocysteine, Cys, and total sulfur AA. Liver concentration of phospholipids was higher in cows fed supplementary B12. Plasma and liver concentrations of folates and B12 were increased by their respective supplements, but the increase in plasma folates and plasma and liver B12 was smaller for cows fed the 2 vitamins together. In cows fed folic acid supplements, supplementary B12 increased plasma glucose and alanine, tended to decrease plasma biotin, and decreased Km of the methylmalonyl-coenzyme A mutase in hepatic tissues following addition of deoxyadenosylcobalamin, whereas it had no effect when cows were not fed folic acid supplements. There was no treatment effect on plasma nonesterified fatty acids as well as specific activity and gene expression of Met synthase and methylmalonyl-coenzyme A mutase in the liver. Ingestion of folic acid supplements by cows fed no supplementary B12 increased total lipid and triacylglycerols in liver, whereas these supplements had no effect in cows supplemented with B12. The increases in milk and milk protein yields due to folic acid supplements did not seem to be dependent on the vitamin B12 supply. However, when vitamin B12 was given in combination with folic acid, utilization of the 2 vitamins seems to be increased, probably more so in extrahepatic tissues. Metabolic efficiency seems also to be improved as suggested by similar lactational performance and dry matter intake for cows fed supplementary folic acid but increased plasma glucose and decreased hepatic lipids in cows fed folic acid and vitamin B12 together.

Testicular Injury to Rats Fed on Soybean Protein-Based Vitamin B12-Deficient Diet Can Be Reduced by Methionine Supplementation

We have previously reported that rats fed on a vitamin B12 (B12)-deficient diet containing 180 g soybean protein per kg diet showed marked histologic damage in their testes. In this paper, we report the effect of B12-deficiency on B12-dependent methionine synthase in the rats' testes and the effect of methionine supplementation of the diet on testicular damage. Rats were fed the soybean protein-based B12-deficient diet for 120 d. We confirmed that those rats were in serious B12-deficiency by measuring urinary methylmalonic acid excretion and B12 content in tissues. Methionine synthase activity in the testis of the B12-deficient rats was less than 2% of that in B12-supplemented (control) rats. To complement disrupted methionine biosynthesis, methionine was supplied in the diet. A supplement of 5 g D,L-methionine per kg diet to the B12-deficient diet did not affect urinary methylmalonic acid excretion of B12-deficient rats. The testicular histology of rats fed the methionine-supplemented B12-deficient diet was almost indistinguishable from that of control rats. Thus, we conclude that the lowered testicular methionine synthase activity is the primary cause of the histologic damage due to B12-deficiency and that methionine supplementation to the diet can reduce the damage. These findings would indicate the importance of the methionine synthase activity, especially for testicular function.

Pyruvate:NADP+ oxidoreductase is stabilized by its cofactor, thiamin pyrophosphate, in mitochondria of Euglena gracilis

Pyruvate:NADP(+) oxidoreductase (PNO) is a thiamin pyrophosphate (TPP)-dependent enzyme that plays a central role in the respiratory metabolism of Euglena gracilis, which requires thiamin for growth. When thiamin was depleted in Euglena cells, PNO protein level was greatly reduced, but its mRNA level was barely changed. In addition, a large part of PNO occurred as an apoenzyme lacking TPP in the deficient cells. The PNO protein level increased rapidly, without changes in the mRNA level, after supplementation of thiamin into its deficient cells. In the deficient cells, in contrast to the sufficient ones, a steep decrease in the PNO protein level was induced when the cells were incubated with cycloheximide. Immunofluorescence microscopy indicated that most of the PNO localized in the mitochondria in either the sufficient or the deficient cells. These findings suggest that PNO is readily degraded when TPP is not provided in mitochondria, and consequently the PNO protein level is greatly reduced by thiamin deficiency in E. gracilis.

Hyperhomocysteinemia due to methionine synthase deficiency, cblG: structure of the MTR gene, genotype diversity, and recognition of a common mutation, P1173L

Mutations in the MTR gene, which encodes methionine synthase on human chromosome 1p43, result in the methylcobalamin deficiency G (cblG) disorder, which is characterized by homocystinuria, hyperhomocysteinemia, and hypomethioninemia. To investigate the molecular basis of the disorder, we have characterized the structure of the MTR gene, thereby identifying exon-intron boundaries. This enabled amplification of each of the 33 exons of the gene, from genomic DNA from a panel of 21 patients with cblG. Thirteen novel mutations were identified. These included five deletions (c.12-13delGC, c.381delA, c.2101delT, c.2669-2670delTG, and c.2796-2800delAAGTC) and two nonsense mutations (R585X and E1204X) that would result in synthesis of truncated proteins that lack portions critical for enzyme function. One mutation was identified that resulted in conversion of A to C of the invariant A of the 3' splice site of intron 9. Five missense mutations (A410P, S437Y, S450H, H595P, and I804T) were identified. The latter mutations, as well as the splice-site mutation, were not detected in a panel of 50 anonymous DNA samples, suggesting that these sequence changes are not polymorphisms present in the general population. In addition, a previously described missense mutation, P1173L, was detected in 16 patients in an expanded panel of 24 patients with cblG. Analysis of haplotypes constructed using sequence polymorphisms identified within the MTR gene demonstrated that this mutation, a C-->T transition in a CpG island, has occurred on at least two separate genetic backgrounds.

Identification of the cobalamin-dependent methionine synthase gene, metH, in Vibrio fischeri ATCC 7744 by sequencing using genomic DNA as a template

To confirm the presence of cobalamin-dependent methionine synthase (CDMS) in luminous bacteria, which is a prerequisite for the substantiation of our proposals on the physiological function of the lux operon, we identified the CDMS gene (metH) in Vibrio fischeri ATCC 7744. Two partial metH sequences, one located near the 5'-terminus of the gene and the other near the 3'-terminus, were sequenced by a PCR based method. To design a new set of PCR primers located on the two flanking regions of the gene, the genomic DNA was sequenced by SUGDAT method (sequencing using genomic DNA as a template) upstream or downstream from the respective partial gene sequences. Subsequently a 4.2 kb DNA fragment containing the whole metH was amplified by PCR and sequenced. The number of amino acid residues comprising the protein (1226 amino acids) was comparable to those of known CDMSs. The deduced amino acid sequence showed 85, 74, 55, 31, 30, 52, or 52% identity with that of Vibrio cholerae, Escherichia coli, Deinococcus radiodurans, Synechocystis PCC6803, Mycobacterium tuberculosis, Caenorhabditis elegans or Homo sapiens, respectively. All the predicted amino acid residues for the binding of cobalamin and S-adenosylmethionine were conserved. In the regulatory region of the V. fischeri metH, the binding site of the met repressor, MetJ, was present, although the site is atypically not present in E. coli metH or Salmonella typhimurium metH. It was shown that nucleotide sequences, even long ones, can be determined without a cloning step, if only parts of the DNA fragment to be sequenced are amplified by PCR.

Methionine synthase, a gene required for methionine synthesis, is expressed in planta by Cladosporium fulvum

Abstract The nutritional requirements of phytopathogenic fungi growing in planta has to date been largely ignored. We have begun to address this problem by investigating the methionine requirement for the biotrophic pathogen of tomato Cladosporium fulvum during infection. The Met6 gene from Cladosporium fulvum encoding a cobalamin-independent 5-methyltetrahydropteroyltriglutamate-homocysteinemethyltransferase, was cloned by functional yeast complementation. The open reading frame was found to be 2304 bp, containing no introns and encoding a protein of 87 kDa. In vitro Northern analysis demonstrated high levels of Met6 expression in the absence of externally supplied methionine. However in the presence of methionine or in the absence of carbon, expression of Met6 decreased significantly. Analysis of Met6 expression in planta revealed a strong increase during infection suggesting the requirement for methionine synthesis in planta by Cladosporium fulvum. This study demonstrates that Cladosporium fulvum is starving for methionine during infection and thus implies the essentiality of primary biosynthetic pathways to the infecting fungus.

Extremely low activity of methionine synthase in vitamin B-12-deficient rats may be related to effects on coenzyme stabilization rather than to changes in coenzyme induction

Severely vitamin B-12 (B-12)-deficient rats were produced by feeding a B-12-deficient diet. The status of B-12 deficiency was confirmed by an increase in urinary methylmalonate excretion and decreases in liver B-12 concentrations and cobalamin-dependent methionine synthase activity. Rat liver methionine synthase existed almost exclusively as the holoenzyme. In B-12-deficient rats, the level of methionine synthase protein was lower, although the mRNA level was not significantly different from that of control rats. When methylcobalamin, the coenzyme for methionine synthase, was administered to the B-12-deficient rats, growth, liver B-12 concentrations and urinary excretion of methylmalonate were reversed although not always to control (B-12-sufficient) levels in a short period. During this recovery process, methionine synthase activity and its protein level increased, whereas the mRNA level was unaffected. We reported previously that rat apomethionine synthase is very unstable and is stabilized by forming a complex with methylcobalamin. Thus, the extremely low activity of methionine synthase in B-12-deficient rats may be related to effects on "coenzyme stabilization" (stabilization of the enzyme by cobalamin binding) rather than to changes in "coenzyme induction."

Heterologous high level expression, purification, and enzymological properties of recombinant rat cobalamin-dependent methionine synthase

Rat methionine synthase was expressed chiefly as apoenzyme in recombinant baculovirus-infected insect cells (Yamada, K., Tobimatsu, T., and Toraya, T. (1998) Biosci. Biotech. Biochem. 62, 2155-2160). The apoenzyme produced was very unstable, and therefore, after complexation with methylcobalamin, the functional holoenzyme was purified to homogeneity. The specific activity and apparent K(m) values for substrates were in good agreement with those obtained with purified rat liver enzyme. The electronic spectrum of the purified recombinant enzyme resembled that of cob(II)alamin and changed to a methylcobalamin-like one upon incubation of the enzyme with titanium(III) and S-adenosylmethionine. The rate of oxidative inactivation of the enzyme in the absence of S-adenosylmethionine was slower with a stronger reducing agent like titanium(III). The nucleotide moiety, especially the phosphodiester group, was shown to play an important role in the binding of the coenzyme to apoprotein and thus for catalysis. Upon incubation with the apoenzyme in the absence of a reducing agent, cyano- and aquacobalamin were not effective or were effective only slightly in reconstituting holoenzyme. Ethyl- and propylcobalamin formed inactive complexes with apoenzyme, which were converted to holoenzyme by photolytic activation. Adenosylcobalamin was not able to form a complex with apoenzyme, which was convertible to holoenzyme by photoirradiation.

The energy conserving methyltetrahydromethanopterin:coenzyme M methyltransferase complex from methanogenic archaea: function of the subunit MtrH

In methanogenic archaea the transfer of the methyl group of N5-methyltetrahydromethanopterin to coenzyme M is coupled with energy conservation. The reaction is catalyzed by a membrane associated multienzyme complex composed of eight different subunits MtrA-H. The 23 kDa subunit MtrA harbors a corrinoid prosthetic group which is methylated and demethylated in the catalytic cycle. We report here that the 34 kDa subunit MtrH catalyzes the methylation reaction. MtrH was purified and shown to exhibit methyltetrahydromethanopterin:cob(I)alamin methyltransferase activity. Sequence comparison revealed similarity of MtrH with MetH from Escherichia coli and AcsE from Clostridium thermoaceticum: both enzymes exhibit methyltetrahydrofolate:cob(I)alamin methyltransferase activity.