27-Methylation pathway

As with adults, the primary organ responsible for drug metabolism in children is the liver. Although the cytochrome P450 system is fully developed at birth, it functions more slowly than in adults. Phase I oxidation reactions and demethylation enzyme systems are significantly reduced at birth. However, the reductive enzyme systems approach adult levels and the methylation pathways are enhanced at birth. This often contributes to the production of different metabolites in newborns from those in adults. For example, newborns metabolize approximately 30% of theophylline to caffeine rather than to uric acid derivatives, as occurs in adults. While most phase I enzymes have reached adult levels by 6 months of age, alcohol dehydrogenase activity appears around 2 months of age and approaches adult levels only by age 5 years. 

Both compounds (Figure 4.7) are non-competitive inhibitors of chitin synthase and also interfere with the Bremner-Greenberg methylation pathway.6 Their activity is dependent upon the oxidation of the phosphorous - sulfur linkage, a feature that is the basis of resistance development, which is common sensitive strains of P. oryzae metabolise the compounds more rapidly than resistant strains. 

The source of the electrons in the reduction of arsenic outlined in Fig. 7 is not known, but a sound model involving reduction by thiols has been proposed (133). A reexamination of Challenger s proposed methylation pathway looked at the effects of adding low levels of As(III), As(V), MMA, and DMA to cultures of two microorganisms including... 

Kodaki, T., Hosaka, K., Nikawa, J.-I., and Yamashita, S., 1991a, Identification of the upstream activation sequences responsible for the expression and regulation of the PEM1 and PEM2 genes encoding the enzymes of the phosphatidylethanolamine methylation pathway in Saccharomyces cerevisiae. J. Biochem. 109 276-287. 

Kodaki, T., and Yamashita, S., 1987, Yeast phosphatidylethanolamine methylation pathway. J. Biol. Chem. 262 15428-15435. 

Li, L., Popko, J.L., Zhang, X.H., Osakabe, K., Tsai, C.J., Joshi, C.P and Chiang, V.L. (1997) A novel multifunctional O-methyltransferase implicated in a dual methylation pathway associated with Ugnin biosynthesis in loblolly pine. Proc. Natl. Acad. Sci. USA., 94,5461-6. 

Homocysteine is metabolized in the liver, kidney, small intestine and pancreas also by the transsulfuration pathway [1,3,89]. It is condensed with serine to form cystathione in an irreversible reaction catalyzed by a vitamin B6-dependent enzyme, cystathionine-synthase. Cystathione is hydrolyzed to cysteine that can be incorporated into glutathione or further metabolized to sulfate and taurine [1,3,89]. The transsulfuration pathway enzymes are pyridoxal-5-phosphate dependent [3,91]. This co-enzyme is the active form of pyridoxine. So, either folates, cobalamin, and pyridoxine are essential to keep normal homocysteine metabolism. The former two are coenzymes for the methylation pathway, the last one is coenzyme for the transsulfuration pathway [ 1,3,89,91 ]. 

The CDP-choline pathway to PC (Figure 6.1) occurs in a number of tissues, but the PE methylation pathway (Figure 6.2) occurs mainly in the liver and accounts... 

In neonates, the gastric pH is biphasic, being high in the first few days after birth and decreasing by day 30, but it takes 5-12 years for the adult pattern and value to emerge (Signer and Fridrich, 1975). On the contrary, the methylation pathway, unimportant in adults, is well developed in children. Furthermore, acetaminophen is less toxic to children than to adults, probably because it utilizes the sulfate metabolic pathway (Rane, 1992). 

Phosphatidylcholine can also be synthesized by the methylation pathway that converts phosphatidylethanolamine to phosphatidylcholine, principally in the liver. The methyl donor is S-adenosylmethionine (Chapter 17). Phosphatidylethanolamine-N-methyltransferase transfers three methyl groups in sequence to produce phosphatidylcholine. The fatty acid components of phosphatidylcholine can then be altered by deacylation-reacylation reactions. 

The position had now been reached where the biosynthetic route to hydrogenobyrinic acid 60, a precursor of vitamin B12, had been mapped out from ALA through several steps to uro gen III 10, before going forward along the methylation pathway to precorrin-2 12 and precorrin-3A 55. Then there was a gap to be filled before precorrin-6A 61 and subsequent intermediates. We will now... 

Fig. 1 GSH synthesis and methylation pathways in neuronal cells. Cysteine for GSH synthesis is provided by either uptake via EAAT3 or via transsulfuration of homocysteine (HCY), although transsulfuration is limited in neuronal cells, increasing the importance of uptake. Methionine synthase activity in neurons requires methylcobalamin (MeCbl), whose synthesis is GSH dependent. Dopamine-stimulated PLM is dependent upon methionine synthase activity. Methionine synthase activity determines levels of the methyl donor SAM and the methylation inhibitor SAH, affecting the efficiency of a large number of cellular methylation reactions.

As illustrated in Fig. 1, methionine synthase is positioned at the intersection between transsulfuration and methylation pathways. As a consequence, its level of activity exerts control over cellular redox status, since it determines the proportion of HCY that will be diverted toward cysteine and GSH synthesis. Methionine synthase activity is exceptionally sensitive to inhibition during oxidative stress, primarily because its cobalamin cofactor is easily oxidized (Liptak and Brunold, 2006). This allows methionine synthase to serve as a redox sensor, lowering its activity whenever the level of oxidation increases, until increased GSH synthesis brings the system back into balance. Electrophilic compounds, such as oxygen-containing xenobiotic metabolites, also react with cobalamin, inactivating the enzyme and increasing diversion of HCY toward GSH synthesis (Watson et al., 2004). Thus, methionine synthase is a sensor of both redox and xenobiotic status. 

During the catechol methylation under the conditions used, three parallel reactions take place (see Figure 2). The rates of these pathways (R,) can be easily calculated from the data of Table 1, since Rj = Si (-Ri), where Si is the selectivity towards the monomethylated product Ai. The rise of acidity affects differently the rates of the reaction pathways for the phosphated alumina the rate of guaiacol formation (R2, O-methylation ) doubled and the rate of C-methylation pathways simultaneously decreased 1.5 times, if compared to pure Y-AI2O3. 

The sulfur atom of methionine becomes the sulfur atom of cysteine. The sulfate generated in cysteine catabolism is excreted or used in several biosynthetic or catabolic pathways. The transulfuration and methylation pathways are intimately related. 

All nucleated cells contain PC and utilize the CDP-choline pathway. Thus, it was not obvious why the alternative pathway for PC synthesis, the PE methylation pathway (Fig. 4), survived during evolution. Nor was it obvious why PE methyltransferase (PEMT) activity is essentially a liver-specific enzyme whereas 2% or less of the hepatic PEMT activity is found in other tissues. In addition to producing PC, PEMT generates three... 

Phosphatidylcholine is the major phospholipid on the surface monolayer of all lipoproteins, including VLDLs. In the liver, phosphatidylcholine is synthesized by two biosynthetic pathways the CDP-choline pathway and the phosphatidylethanolamine A -methyltransferase pathway (Chapter 8). Choline is an essential biosynthetic precursor of phosphatidylcholine via the CDP-choline pathway. When cells or animals are deprived of choline, plasma levels of TG and apo B are markedly reduced and TG accumulates in the liver, resulting in fatty liver. These observations led to the widely held view that the fatty liver caused by choline deficiency is due to inhibition of PC synthesis, which in turn would decrease VLDL secretion. This hypothesis was tested in primary rat hepatocytes cultured in medium lacking choline. Upon removal of choline/methionine from culture medium, the TG content of hepatocytes was increased 6-fold, and the secretion of TG and apo B in VLDL was markedly reduced. The interpretation of these experiments was that hepatic VLDL secretion requires the synthesis of phosphatidylcholine from either the CDP-choline or methylation pathways which require choline or methionine, respectively, as precursors (D.E. Vance, 1988). However, since choline deprivation was induced in a background of methionine insufficiency, it was not clear whether the lack of choline per se, and inhibition of the choline pathway for phosphatidylcholine synthesis, decreased VLDL secretion. More recent experiments have shown, surprisingly, that deficiency of choline in primary mouse hepatocytes does not reduce, but increases, phosphatidylcholine synthesis via the CDP-choline pathway, and does not decrease VLDL secretion (J.E. Vance, 2004). Thus, a deficiency of dietary choline reduces plasma TG and apo B levels by a mechanism that does not involve reduction of phosphatidylcholine synthesis. 

To determine the role of the CDP-choline pathway of phosphatidylcholine synthesis for VLDL secretion, knockout mice were generated in which the gene encoding CTP phos-phocholine cytidylyltransferase-a (Pcytla), a key enzyme in phosphatidylcholine synthesis via the CDP-choline pathway (Chapter 8), was disrupted only in the liver. TG accumulated in livers of these mice and the secretion of TG and apo B was decreased (D.E. Vance, 2004). Thus, elimination of the CDP-choline pathway in the liver inhibits VLDL secretion. In addition, disruption of the gene encoding the liver-specific enzyme, phosphatidylethanolamine V-methyltransferase, in mice fed a high-fat/high-cholesterol diet, also markedly reduces the secretion of TG and apo B in VLDLs (D.E. Vance, 2003). Thus, the hepatic synthesis of phosphatidylcholine via both the CDP-choline and methylation pathways appears to be required for normal VLDL secretion. 

Despite major advances in understanding the mechanisms of assembly of VLDLs, important questions remain. Although MTP has been extensively studied, and although we know that an absence of MTP severely impairs the secretion of apo B-containing lipoproteins, the precise role of MTP in VLDL assembly remains unclear. It is widely assumed that MTP is involved in transferring lipids to apo B, but a lipid transfer function for MTP has not been directly demonstrated in vivo. One possible function of MTP is the transport of TG to a pool (perhaps lumenal TG apo B-free droplets) that is subsequently used for assembly with apo B. The composition and physiological imp)ortance of these lipid droplets are unknown. The phospholipid transfer function of MTP for VLDL assembly also requires further study. Moreover, it is not clear why the hepatic synthesis of phosphatidylcholine via both the CDP-choIine pathway and the methylation pathway is required for VLDL secretion. The intracellular site of addition of TG to apo B has not been unambiguously established and almost no information is available on the intracellular site of addition of phospholipids to VLDLs. 

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