Methionine activation and

Methylmercaptopropionaldehyde is also used to make the methionine hydroxy analog CH2SCH2CH2CH(OH)COOH [583-91 -5] which is used commercially as an effective source of methionine activity (71). AH commercial syntheses of methionine and methionine hydroxy analog are based on the use of acrolein as a raw material. More than 170,000 tons of this amino acid are produced yearly (30) (see Amino acids). One method for the preparation of methionine from acrolein via 3-methyhnercaptopropionaldehyde is as follows. 

Not only the smallest optically active amino acid (alanine), but also leucine, several (substituted) aromatic amino acids, heterosubstituted amino acids (methionine, homomethionine and thienylglydne) and even an iminoacid, proline, are obtainable in both the L- and D-form. 

Oxidation of methionine is particularly favoured under conditions of low pH, and in the presence of various metal ions. Methionine residues on the surface of a protein are obviously particularly susceptible to oxidation. Those buried internally in the protein are less accessible to oxidant. hGH contains three methionine residues (at positions 14, 125 and 170). Studies have found that oxidation of methionine 14 and 125 (the more readily accessible ones) does not greatly effect hGH activity. However, oxidation of all three methionine residues results in almost total inactivation of the molecule. 

Figure 22.7 Homocysteine formation from methionine and formation of thiolactone from homocysteine. The homocysteine concentration depends upon a balance between the activities of homocysteine methyltransferase (methionine synthase) and cystathionine p-synthase. Both these enzymes require vitamin B12, so a deficiency can lead to an increase in the plasma level of homocysteine. (For details of these reactions, see Chapter 15.) Homocysteine oxidises spontaneously to form thiolactone, which can damage cell membrane.

In a totally different field, studies were being carried out on children who had a deficiency of methionine synthase and an impaired ability to convert homocysteine to methionine, so that they had increased blood levels of homocysteine. It was noted that these children had an increased incidence of thrombosis in cerebral and coronary arteries. This led to a study which eventually showed that an increased level of homocysteine was a risk factor for coronary artery disease in adults. Since methionine synthase requires the vitamins, folic acid and B12, for its catalytic activity, it has been suggested that an increased intake of these vitamins could encourage the conversion of homocysteine to methionine and hence decrease the plasma level of homocysteine. This is particularly the case for the elderly who are undernourished (see Chapter 15 for a discussion of nutrition in the elderly). 

Drugs that may affect aspirin include activated charcoal, ammonium chloride, ascorbic acid or methionine, antacids and urinary alkalinizers, carbonic anhydrase inhibitors, corticosteroids, and nizatidine. Drugs that may be affected by aspirin include alcohol, ACE inhibitors, anticoagulants (oral), beta-adrenergic blockers, heparin, loop diuretics, methotrexate, nitroglycerin, NSAIDs, probenecid and sulfinpyrazone, spironolactone, sulfonylureas and exogenous insulin, and valproic acid. 

Distinct changes in several properties of lysozyme occur after reaction with ozone. The lytic activity of the ozonized lysozyme shows the same trend at various pHs as the native enz3mie (Fig. 2) this may suggest that the pK values of the ionizable groups involved in catalysis have not been altered by ozonplysis. The amino acid composition of ozonized lysozyme differs from that of the native enz3mie in three residues — methionine, tryptophan and t3H osine. None of the other amino acids is affected by ozone. The extensive loss of enz5miic activity must be ascribed to the oxidative modification of these three amino acid residues in the lysozyme. 

The oxidative stability of subtilisin has been extensively studied and improved stability has been engineered. In subtilisin BPN two methionines, MET " and MET are especially susceptible to oxidation. To prevent the negative influenee eaused by the formation of methioiune sulfoxide the MET can be substituted with ALA, SER or LEU, without loosing more than 12-53% of the activity. One such mutant MET222. ALA is currently in use as a commercial detergent enzyme Durazyme (Riisgard, 1990). 

Prolonged exposure to nitrous oxide decreases methionine synthase activity and theoretically can cause megaloblastic anemia, a potential occupational hazard for staff working in inadequately ventilated dental operating suites. 

The metabolic control of methionine metabolism is complex and involves, for example, changes of enzyme levels in particular tissues, mechanisms linked to the kinetic properties of the various enzymes and their interaction with metabolic effectors [6, 7]. A particularly important metabolic effector is AdoMet. This inhibits the low Km isoenzymes of MAT, and MTHF reductase, inactivates betaine methyltransferase, but activates MAT III (the high-Km isoenzyme) and cystathionine /1-synthase. Therefore, high methionine intake and thus higher AdoMet levels favour trans-sulphuration, and when levels are low methionine is conserved. AdoHcy potently inhibits AdoMet-dependent methyltransferases and both Hey remethylating enzymes. Another important control mechanism is the export of Hey from cells into the extracellular space and plasma, which occurs as soon as intracellular levels increase [8]. 

With the exception of indolylacetonitrile and dichloroacetonitrile, the compounds identified in the XAD-2/ethyl ether extracts of the chlorinated amino acids have also been identified by Glaze et al. (14), and Trehy and Bieber (15) have shown that dichloroacetonitrile can be produced by chlorination of certain amino acids. Of the compounds identified by GC-MS, only dichloroacetonitrile is a known mutagen (8). However, as in extracts of drinking water, dichloroacetonitrile is unlikely to account for a significant proportion of the activity observed in the extract of chlorinated methionine at the level detected. With the exception of chlorohydroxybenzyl cyanide, for which no authentic standard is available, all the compounds identified were tested for mutagenic activity, and no mutagenic response was obtained. 

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