Choline demethylation

DMN oxidative demethylation has been shown to be a liver mi-crosome cytochrome P-450 monooxygenase (10) Lotlikar et al. (11) found that a reconstituted enzyme system, consisting of cytochrome P-450, NADPH-cytochrome P-450 reductase and phosphatidyl choline was effective in catalyzing the demethylation of DMN. The most commonly accepted mechanism for the oxidative demethylation of DMN and, by extension, of other dialkyInltrosamlnes is shown in Scheme 1. 

Karlen et al. analyzed acetylcholine and choline by an ion pair extraction and gas phase method [134]. The gas chromatographic estimation of the isolated acetylcholine and choline was carried out after demethylation either with benzenethiolate or by controlled pyrolysis. Acetylcholine was quantitated by flame ionization detection at the nmol level. Mass fragment analysis was employed for the determination of acetylcholine in pmol amounts. 

Polak and Molenaar described a method for the determination of acetylcholine from brain tissue by pyrolysis-gas chromatography-mass spectrometry [200]. The deuterium-labeled acetyl-choline is pyrolytically demethylated with sodium benzenethiolate, followed by quantitative GC-MS analysis. In this method, care must be taken so that the samples do not contain appreciable amounts of choline since exchange of deuterium-labeled groups between acetylcholine and choline during pyrolysis may yield erroneous results. The same authors have also reported a method for the determination of acetylcholine by slow pyrolysis combined with mass fragment analysis on a packed capillary column [201]. 

Jenden and co-workers [109—111] determined acetylcholine and other choline derivatives by GC after demethylating them with sodium thiophenolate. The reaction is described schematically by Scheme 5.10 and is carried out by using the following proce-... 

The bowel, one of the largest and most metabolically active organs, contains bacteria that may change the chemical composition of the human body. In renal failure the altered bacterial flora cause the accumulation of aliphatic amines in the gut (09, S25). Bacteria transform part of the choline in the gut to trimethylamine, which is reabsorbed and then either oxidized or demethylated to dimethylamine in the liver (S24). Dimethylamine enters the circulation and is excreted in the bile and urine. The trimethylamine and dimethylamine in the exhaled air of uremic patients may contribute to the classic fishy breath, which can be improved by hemodialysis or by gut sterilization with nonabsorbable antibiotics (S23, S25). The overall role of these compounds as uremic toxins, however, remains to be defined. 

Formation of ethanolamine from betaine and of choline from ethanolamine was also shown by the experiments of Stetten. The data are reproduced in Table I. The observations support the main features of the cycle represented in Fig. 1, namely that ethanolamine is formed by the decarboxylation of serine, this in turn is methylated to choline, which is then oxidized to betaine, and the latter is demethylated to glycine. Further support for this scheme is supplied by the observations of Arn-stein that L-serine-S-C is converted to choline with about the same degree of efficiency as N -glycine and that glycine-l-C is not a precursor of choline. 

The demethylation reactions so far outlined take us to dimethylglycine. Nothing concrete is known on the further demethylation of this compound. Since N -betaine has been shown to be converted to labeled glycine the demethylation must go to completion. Similarly the role of sarcosine in the proposed cycle is obscure. In feeding experiments sarcosine was found not to be an effective methyl donor for choline formation. " This also was observed for the synthesis of methionine from homocysteine with liver slices and homogenates. The oxidation of the sarcosine methyl to formaldehyde, on the other hand, is a very active process and sarcosine oxidase is very widely distributed. This reaction could serve as a pathway for the catabolism of surplus methyl groups in the body. 

Dimethylglycine is formed by endogenous demethylation of betaine, a choline metabolite, with concomitant methylation of homocysteine to methionine. DMG is then converted to sarcosine by oxidative demethylation catalyzed by DMG-dehydrogenase, a flavin-containing enzyme requiring folate as a cofactor. Increased excretion of DMG has been observed in individuals with folate deficiency or receiving large doses of betaine as a... 

Hordenine occurs mainly in young barley plants, but has been found in millet or proso (Panicum miliaceum) too. It is found for a short time during plant development this is supposedly due to catabolism (Brady and Tyler, 1958 Neumann and Tschoepe, 1966). The course of the catabolic reactions is not known. In 1956, Frank and Marion studied the N-demethylation of hordenine in barley. Radioactivity from d-[ C]hordenine fed to barley seedlings was found in A/ -methyltyramine. Since no tyramine (Figure 6.17) was found at aU, it appears likely that the demethylation affects only one of the two methyl groups of hordenine. The mechanism of the reaction was not discussed in the cited paper, but the isolated choline was found nonradioactive, so transamination is not likely. 

Although this is not an anabolic process, discussion of the reactions involved is included here to complete the presentation of the steps of the cycle shown in Fig. 2. The process is initiated by the oxidation of choline to betaine. Evidence that methyl transfer involves betaine, not choline was indicated by the work of Dubnofif (78), who observed that methyl transfer can occur from betaine anaerobically, but from choline only aerobically. More definite proof was provided by Muntz (79), who demonstrated that incubation of choline and homocysteine with liver homogenates yielded dimethylglycine and not dimethylaminoethanol. The former would be expected if betaine was demethylated, the latter if it was choline. 

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