Remethylation pathway

Figure 29.6 Pathways for the metabolism of homocysteine. Normal transsulfuration requires cystathionine P-synthase with vitamin Bg as cofactor. Reme-thylation requires 5,10-methylenetetrahydrofolate reductase and methionine synthase. The latter requires folate as cosubstrate and vitamin Bi2 (cobalamin) as cofactor. An alternative remethylation pathway also exists using the cobalamin independent betaine-homocysteine methyltransferase (Robinson 2000).

Impairment of remethylation is strongly implicated as the cause of hyper-homocysteinemia in patients with CKD, which has been demonstrated in a radioisotope study in patients with CKD (Van Guldener et al. 1999), although reduced clearance of homocysteine has been suggested as the possible cause in some reports. We reported that supplementation with folic add and methyl-cobalamin normalized the remethylation pathway (Koyama et al. 2002). Compared with the decreases in homocysteine of 17.3 8.4% after supplementation with folic add alone and 18.7 7.5% after that with methylcoba-lamin alone, a combination of foUc acid and methyleobalamin decreased homocysteine by approximately 60% and normalized the findings of the methionine loading test Table 47.1. This result suggest that both coenzymes, folic acid and methyleobalamin, were insufficient due to reduced availability of these coenzymes in patients with CKD. There is also a report that increased MMA in dialysis patients was reduced by the administration of methylcoba-lamin (Nakamura et al. 2002). 

The best known biomarker associated with vitamin B12 is homocysteine. In patients with CKD, plasma total homocysteine level is elevated in an inverse relationship with the reduction in renal function (Bostom and Lathrop 1997) and homocysteine is a cardiovascular risk factor (Arnesen et al. 1995). Hyperhomocysteinemia in patients with CKD is thought to be associated with an impaired remethylation pathway (Van Guldener et al. 1999), but the cause of this disorder has not been fully clarified. 

Since homocysteine-lowering therapy, which activates the remethylation pathway, accelerates the transmethylation reaction (Koyama et at. 2010) (Figure 47.6, Table 47.1, the clinical assessment of homocysteine-lowering therapy may have to include examinations of wide-ranging factors (dementia, prevalence of cancer, etc.) as listed above (i-viii). 

Hyperhomocysteinemia in patients with CKD is thought to be associated with an impaired remethylation pathway. 

Remethylation pathway. In cells, homocysteine is either remethylated to methionine (via methionine synthase) or is transsulfurated to cysteine via cystathionine beta synthase). In remethylation, homocysteine receives a methyl group from 5-methyltetrahydrofolate or from betaine. Vitamin B12 is a necessary cofactor in the folate-dependent remethylation. 

A newer therapeutic approach is the administration of betaine (6-12 g daily), which lowers homocysteine levels by favoring remethylation [33], A theoretical hazard of betaine treatment is increasing the blood methionine, sometimes to an extravagant degree ( 1 mmol/1). Experience to date indicates that betaine administration is safe, with no major side effects except for a fishy odor to the urine. Other therapeutic approaches have included the administration of salicylate to ameliorate the thromboembolic diathesis. Patients also have been treated with dietary supplements of L-cystine, since the block of the transsulfura-tion pathway in theory could diminish the synthesis of this amino acid. 

Important pathways requiring SAM include synthesis of epinephrine and of the 7-methylgua-nine cap on eukaryotic mRNA, Synthesis of SAM from methionine is shown in Figure T17-3. After donating the methyl group, SAM is converted to homocysteine and remethylated in a reaction catalyzed by N-methyl THF-homocysteine methyltransferase requirii both vitamin Bj2 and N-meth d-THF. The methionine produced is once again used to make SAM. 

As well as remethylation, Hey can be degraded in the trans-sulphuration pathway, which first involves condensation of Hey with serine forming cystathionine, then breakdown of this compound to cysteine and a-oxo-butyrate. These reactions... 

Hydrolysis of SAM After donation of the methyl group, S-adenosylhomocysteine is hydrolyzed to homocysteine aid adenosine. Homocysteine has two fates. If there is a deficiency of methionine, homocysteine may be remethylated to methionine (see Figure 20.8). If methionine stores are adequate, homocysteine rmty enter the transsulfuration pathway, where it is converted to cysteine. 

As shown in Figure 10.9, the methyl donor is S-adenosyl methionine, which is demethylated to S-adenosyl homocysteine. After removal of the adenosyl group, homocysteine may undergo one of two metabolic fates remethylation to methionine or condensation with serine to form cystathionine, foUowed by cleavage to yield cysteine - the transulfuration pathway (Section 9.5.5). Cystathionine synthetase has a relatively low Tni compared with normal intra-ceUular concentrations of homocysteine. It functions at a relatively constant rate, and under normal conditions, most homocysteine wUl be remethylated to methionine. 

Methylation of homocysteine by 5-methyltetrahydrofolate-homocysteine methyl reductase depends on an adequate supply of 5-methyltetrahydrofoIate. The unmethylated folate is recycled in a cobalamin-dependent pathway, by remethylation to 5,10-methylene-tetrahydrofolate, and subsequent reduction to 5-methyltetrahydrofolate. The transferase enzyme, also named 5,10-methyltretrahydrofolate reductase catalyzes the whole cycle [3,91]. S-adenosylmethionine and 5-methyltetrahydrofolate are the most important methyl unit donors in biological system. S-adenosylmethionine is reported to regulate methylation and transsulfuration pathways in the homocysteine metabolism [3,91]. 

Once inside the cell, folates participate in a number of interconnected metabolic pathways involving (1) thymidine and purine biosynthesis necessary for DNA synthesis, (2) methionine synthesis via homocysteine remethylation, (3) methylation reactions involving S-adenosylmethionine (AdoMet), (4) serine and glycine interconversion, and (5) metabolism of histidine and formate (see Figure 8). Via these pathways. 

Methionine metabolism. The major pathway of methionine metabolism is presented. This includes the donation of methyl groups, cysteine formation, and potential remethylation of homocysteine. The purple lettering under an enzyme indicates the resulting disease when... 

Lipotrophic factors. This figure outlines various pathways for lecithin (phosphatidylcholine) formation. Lecithin is essential for transport of triglycerides from liver to adipose (or muscle). These include complete de novo synthesis, formation from choline, and remethylation of homocysteine from betaine (a catabolic product of choline). 

Homocysteine lies at a metabolic crossroad it may condense with serine to form cystathionine, or it may undergo remethylation, thereby conserving methionine. There are two pathways for remethylation in humans. In one, betaine provides the methyl groups, while in the other 5-methyltetrahydrofolate is the methyl donor. This latter reaction is catalyzed by a Bj -containing enzyme, 5-methyltetrahydrofolate homocysteine methyltransferase. Two defects in this latter mechanism may account for the inability to carry out remethylation. In one of them, patients are unable to synthesize or accumulate methylcobalamin, while others cannot produce the second cofactor, 5 -methyltetrahydrofolate, because of adefect in 5,10-methylenetrahydrofolate reductase. 

Since methionine has several pathways open to it, it is essential to know what factors control the direction that its metabolism takes. Studies in young adults have shown that the utilization of methyl groups is normally accounted for chiefly by creatinine formation. This reaction consumes more 5-adenosylmethionine than all other transmethylations together. However, examination of enzyme activities from these two pathways in fetal animals leads to the conclusion that remethylation preponderates over transsulfuration. Indeed, since y-cystathionase activity is immeasurable in human fetal liver and brain, not only is the remethylation sequence favored, but also cysteine then becomes an essential amino acid for the fetus and infant. 

The remethylation cycle allows the conversion of homocysteine back to methionine by two pathways. The first and major pathway is catalyzed by the enzyme, methionine synthase, and links the folate cycle with homocysteine metabolism. Methionine synthase requires the cofactor, meth-ylcobalamin. The second pathway utilizes the enzyme, betaine-homocysteine methyltransfer-ase [8]. This pathway remethylates homocysteine using a methyl group derived from betaine, formed via oxidation of choline, and is presumably responsible for up to 50 % of homocysteine remethylation [10]. Both methionine and homocysteine play important roles in protein synthesis, folding, and function. 

Homocysteine is an intermediate metabolite generated during the metabolism of methionine, an essential sulfur-containing amino acid. The biochemical pathways involved in homocystinuria perform two important processes transsulfuration and remethylation (Fig. 14.1). 

The S-adenosyl homocysteine produced in the transmethylation reactions is generally cleaved to adenosine and homocysteine. The latter can be degraded as previously discussed or be remethylated to methionine and eventually regenerate S-adenosyl methionine. Thus the operation of a methionine cycle provides a route whereby one-carbon metabolites reduced through the tetrahydrofolic acid sequence provide methyl groups for biosynthetic pathways. Certain other sulphonium compounds such as... 

Folate metabolism is not limited to the cytoplasmic compartment. Most of the folate in tissues is found in the mitochondrion and cytosol (Horne et al. 1997). Individual folate-dependent pathways are compartmentalized within organelles. The cytoplasmic and mitochondrial compartments each possess a parallel array of enzymes catalysing the interconversion of folate coenzymes that carry one-carbon units. The mitochondrial folate metabolism favours incorporation of one-carbon groups from serine and release of formate, while the cytoplasmic metabolism favours incorporation of one-carbon units from formate with purine and thymidine synthesis and homocysteine remethylation. 

Homocysteine is an intermediate metabolic product at the junction of two metabolic pathways, transsulfuration (requiring vitamin Bg as a coenzyme)and remethylation (requiring vitamin B12 as a coenzyme). When vitamin B12 and vitamin Bg are viewed in terms of homocysteine metabolism, both vitamins may be linked with each other at the degradation of homocysteine. The reduced remethylation in patients with CKD may stimulate sulfur transfer. We consider that the linkage between vitamin B12 and vitamin Bg may be mediated by SAM, since it has been demonstrated that accumulated SAM accelerates the transfer of sulfur (Purohit et al. 2007). Based on the fact as observed in patients with CKD that the transsulfuration pathway did not deteriorate and that the... 

Homocysteine is formed as an intermediate metabolic product of methionine at the junction of two metabolic pathway remethylation and transsulfuration. 

N -methylxanthine (II) which is subsequently demethylated at the N -position (III) and remethylated on C-2 to form the methoxy derivative (IV) and oxidized on C-8 (V) which can be converted to compound VI, which is one of the compounds recently isolated (see Fig. 6.27c, Wanner et aL, 1975) (2) the direct conversion to compound VI which can proceed by several alternate pathways. Compound VI can be oxidized to allantoin (VII) and allantoic acid (VIII) and then to CO2 (IX). Caffeine may be degraded by an unknown pathway into allantoin, allantoic acid, and finally, to CO2. 

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