N-methyltransferases

Sherman et al. have analyzed the function of the MitM methyltransferase in more detail [113]. They knocked out the MitM gene and found that mitomytin C production was abolished, and were able to isolate a small quantity of 9a-demethyl mitomycin A (62) from this mutant (Figure 11.9). MitM was expected to act as an O-methyltransferase, but surprisingly recombinant MitM converted 62 into 9-epi-mitomydn B (63), hence acting as an aziridine N-methyltransferase. Mitomy-... 

Figure 42-10. Biosynthesis of catecholamines. (PNMT, phenylethanolamine-N-methyltransferase.)...

Because LCEC had its initial impact in neurochemical analysis, it is not, surprising that many of the early enzyme-linked electrochemical methods are of neurologically important enzymes. Many of the enzymes involved in catecholamine metabolism have been determined by electrochemical means. Phenylalanine hydroxylase activity has been determined by el trochemicaUy monitoring the conversion of tetrahydro-biopterin to dihydrobiopterin Another monooxygenase, tyrosine hydroxylase, has been determined by detecting the DOPA produced by the enzymatic reaction Formation of DOPA has also been monitored electrochemically to determine the activity of L-aromatic amino acid decarboxylase Other enzymes involved in catecholamine metabolism which have been determined electrochemically include dopamine-p-hydroxylase phenylethanolamine-N-methyltransferase and catechol-O-methyltransferase . Electrochemical detection of DOPA has also been used to determine the activity of y-glutamyltranspeptidase The cytochrome P-450 enzyme system has been studied by observing the conversion of benzene to phenol and subsequently to hydroquinone and catechol... 

Roberts, M.F. and Waller, G.R., N-methyltransferases and 7-methyl-N9-nucleo-side hydrolase activity in Coffea arabica and the biosynthesis of caffeine, Phytochemistry, 18,451,1979. 

Mazzafera, P., Wingsle, G., Olsson, O., Sandberg, G., S-Adenosyl-L-methionine theobromine 1-N-methyltransferase, an enzyme catalyzing the synthesis of caffeine in coffee, Phytochemistry, 37(6), 1577, 1994. (CA122 259411n)... 

Figure 1. Biosynthetic pathways for biogenic amines. In Drosophila and vertebrates decarboxylation of DOPA and 5-hydroxy-tryptophan is catalyzed by the same enzyme, DDC. In vertebrates this enzyme is called amino acid decarboxylase (AADC). Only vertebrates further metabolize dopamine to norepinephrine and epinephrine. TH, tryosine hydroxylase DDC, DOPA decarboxylase DBH, dopamine b-hydroxylase PNMT, phenylethanolamine N-methyltransferase. Tryp-OH tryptophan hydroxylase.

HA turnover is rapid in the brain, with a half-life of about 30 min. This can change very quickly depending on neuronal activity. There is no high-affinity uptake system for HA once released, HA is inactivated by catabolism. In the brain, released HA is methylated almost exclusively by the enzyme histamine-N-methyltransferase (E.C. 2.1.1.8). The tele-methyl-HA is subsequently degraded by monoamine oxidase-B (MAO-B) and aldehyde dehydrogenase to produce tele-methylimidazoleacetic acid (Brown et ah, 2001). 

Orr, E. Quay, W. B. (1975). Hypothalamic 24-hour rhythms in histamine, histidine, decarboxylase and histamine-N-methyltransferase. Endocrinology 96, 941-5. 

In cells that synthesize epinephrine, the final step in the pathway is catalyzed by the enzyme phenylethanolamine /V-methyltransferase. This enzyme is found in a small group of neurons in the brainstem that use epinephrine as their neurotransmitter and in the adrenal medullary cells, for which epinephrine is the primary hormone secreted. Phenylethanolamine N-methyltransferase (PNMT) transfers a methyl group from S-adenosylmethionine to the nitrogen of norepinephrine, forming a secondary amine [5]. The coding sequence of bovine PNMT is contained in a... 

Baetge, E. E., Suh, Y. H. and Joh, T. H. Complete nucleotide and deduced amino acid sequence of bovine phenylethanolamine N-methyltransferase partial amino acid homology with rat tyrosine hydroxylase. Proc. Natl Acad. Sci. U.S.A. 83 5454-5458,1986. 

Biosynthesis is performed in one step by the enzyme L-histidine decarboxylase (HDC, E.C. 4.1.1.22). Histamine metabolism occurs mainly by two pathways. Oxidation is carried out by diamine oxidase (DAO, E.C. 1.4.3.6), leading to imidazole acetic acid (IAA), whereas methyla-tion is effected by histamine N-methyltransferase (HMT, E.C. 2.1.1.8), producing fe/e-methylhistamine (t-MH). IAA can exist as a riboside or ribotide conjugate. t-MH is further metabolized by monoamine oxidase (MAO)-B, producing fe/e-methylimidazole acetic acid (t-MIAA). Note that histamine is a substrate for DAO but not for MAO. Aldehyde intermediates, formed by the oxidation of both histamine and t-MH, are thought to be quickly oxidized to acids under normal circumstances. In the vertebrate CNS, histamine is almost exclusively methylated... 

Barnes, W. G. and Hough, L. B. Membrane-bound histamine N-methyltransferase in mouse brain possible role in the synaptic inactivation of neuronal histamine. /. Neurochem. 82 1262-1271, 2002. 

Nishibori, M., Tahara, A, Sawada, K., Sakiyama, J., Nakaya, N. and Saeki, K. Neuronal and vascular localization of histamine N-methyltransferase in the bovine central nervous system. Eur. J. Neurosci. 12 415-424,2000. 

A high intracerebral level of S-adenosylhomocysteine may inhibit methylation reactions involving S-adenosyl-methionine. The metabolic repercussions would be extensive, including deficient methylation of proteins and of phos-phatidylethanolamine as well as an inhibition of catechol-O-methyltransferase and histamine-N-methyltransferase. 

Weinshilboum, R.M., et al., "Methylation Pharmacogenetic Catechol O-Methyltrans-ferase, Thiopurine Methyltransferase, and Histamine N-Methyltransferase," Annu. Rev. Pharmacol. Toxicol., 39,19-52 (1999). 

Weinshilboum RM, Otterness DM, Szumlanski CL. Methylation pharmacogenetics catechol O-methyltransferase, thiopurine methyltransferase, and histamine N-methyltransferase. Annu Rev Pharmacol Toxicol 1999 39 19-52. 

Crooks PA, Godin CS, Damani LA, et al. Formation of quaternary amines by N-methylation of azaheterocycles with homogeneous amine N-methyltransferases. Biochem Pharmacol 1988 37(9) 1673-1677. 

Figure 1. Pathways for the synthesis of phosphatidylcholine, phosphatidylethanolamine and sphingomyelin. Abbreviations CK, choline kinase CPT, cholinephosphotransferase CT, CTP phosphooholine cytidylyltransferase DAG, diacylglycerol PC, phosphatidylcholine PE, phosphatidylethanolamine PEMT, phosphatidylethanolamine-N-methyltransferase SM, sphingomyelin SMase, sphingomyelinase SMsyn, sphingomyelin synthase.

Houweling, M., Cui, Z., and Vance, D.E., 1995, Expression of phosphatidylethanolamine N-methyltransferase-2 cannot compensate for an impaired CDP-choUne pathway in mutant Chinese hamster ovary cells. J. Biol. Chem. 270 16277-16282 Ishidate, K.. 1997, Chohne/ethanolamine kinase from mammalian tissues. Biochim. Biophys. Acta 1348 70-78... 

Uckun, P.M., Schieven, G.L., Tuel-Ahlgren, L.M., Etihirdik, L, Myers, D.E., Ledhetter, J.A., and Song, C.W., 1993, Tyrosine phosphorylation is a mandatory proximal step in radiation induced activation of the protein kinase C signaling pathway in human B-lymphocyte precursors. Proc. Natl. Acad Sci. U.S.A. 90 252-256 Vance, D.E., Walkey, C.J., and Cui Z, 1997, Phosphatidylethanolamine N-methyltransferase from tiver. Biochim. Biophys. Acta 1318 142-150... 

Lysine is the most common site for N-methylation, but methylation can also occur on arginine, histidine, glutamine, and asparagine. The enzymes responsible for N-methylation are known as N-methyltransferases aided with SAM as a cosubstrate. All forms of methylation share the same mechanism the nucleophilic amino acid side chain attacks the electrophilic methyl group of SAM and releases S-adenosylhomocysteine (SAH) (Scheme 7). 

Figure 2.16. Pathways for the synthesis and metabolism of the catecholamines. A=phenylalanine hydroxylase+pteridine cofactor+Oj B tyrosine hydroxylase+ tetrahydropteridme+Fe+ +Oj C=dopa decarboxylase+pyridoxal phosphate D= dopamine beta-oxidase+ascorbate phosphate+Cu+ +Oj E=phenylethanolamine N-methyltransferase+S-adenosylmethionine l=monoamine oxidase and aldehyde dehydrogenase 2=catechol-0-methyltransferase+S-adenosylmethionine.

Lin, W.J., Gary J.D., Yang, M.C., Clarke, S. and Herschman, H.R. (1996) The mammalian immediate-early TIS21 protein and the leukemia-associated BTGl protein interact with a protein-arginine N-methyltransferase. The Journal of Biological Chemistry, 271, 15034—15044. 

Pawlak, M.R., Scherer, C.A., Chen, J., Roshon, M.J. and Ruley, H.E. (2000) Arginine N-methyltransferase 1 is required for early postimplantation mouse development, but cells deficient in the enzyme are viable. Molecular and Cellular Biolcgy, 20, 4859-4869. 

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