... Oxidation of methionine (S-methyl-l-cysteine and sarcosine) methyl group in vivo proceeds primarily by way of free formate, and that conversion to formate is probably not catalysed by tetrahydrofolic acid.
... Methionine ... is catabolized to a large extent independently of initial activation to S-adenosyl-l-methionine. The system for catabolism ... appears analogous to one that catalyses oxidation of S-methyl-l-cysteine methyl group ... The methyl group of methionine ... /has been/ shown ... to yield formate in vitro and in vivo.
The mechanism of the possible anti-hepatotoxic activity of L-methionine is not entirely clear. It is thought that metabolism of high doses of acetaminophen in the liver lead to decreased levels of hepatic glutathione and increased oxidative stress. L-methionine is a precursor to L-cysteine. L-cysteine itself may have antioxidant activity. L-cysteine is also a precursor to the antioxidant glutathione. Antioxidant activity of L-methionine and metabolites of L-methionine appear to account for its possible anti-hepatotoxic activity. Recent research suggests that methionine itself has free-radical scavenging activity by virtue of its sulfur, as well as its chelating ability.
参考文献:M Chen, V Vijay, Q Shi, Z Liu, H Fang, W Tong. FDA-批准的药物标签用于研究药物诱导的肝损伤,《药物发现今天》,16(15-16):697-703, 2011. PMID:21624500 DOI:10.1016/j.drudis.2011.05.007
M Chen, A Suzuki, S Thakkar, K Yu, C Hu, W Tong. DILIrank: 按照在人类中发展药物诱导肝损伤风险排名的最大参考药物清单。《药物发现今天》2016, 21(4): 648-653. PMID:26948801 DOI:10.1016/j.drudis.2016.02.015
References:M Chen, V Vijay, Q Shi, Z Liu, H Fang, W Tong. FDA-Approved Drug Labeling for the Study of Drug-Induced Liver Injury, Drug Discovery Today, 16(15-16):697-703, 2011. PMID:21624500 DOI:10.1016/j.drudis.2011.05.007
M Chen, A Suzuki, S Thakkar, K Yu, C Hu, W Tong. DILIrank: the largest reference drug list ranked by the risk for developing drug-induced liver injury in humans. Drug Discov Today 2016, 21(4): 648-653. PMID:26948801 DOI:10.1016/j.drudis.2016.02.015
... Rats were fed diets containing [(14)C-methyl]l-methionine ... with 6% of sodium formate, and conversion of (14)C into [(14)C]formate was measured in urine and exhaled air (as (14)CO2) ... Total oxidation of [(14)C-methyl] into CO2, amounted to 60-87% for methionine ...
Although the free amino acids dissolved in the body fluids are only a very small proportion of the body's total mass of amino acids, they are very important for the nutritional and metabolic control of the body's proteins. ... Although the plasma compartment is most easily sampled, the concentration of most amino acids is higher in tissue intracellular pools. Typically, large neutral amino acids, such as leucine and phenylalanine, are essentially in equilibrium with the plasma. Others, notably glutamine, glutamic acid, and glycine, are 10- to 50-fold more concentrated in the intracellular pool. Dietary variations or pathological conditions can result in substantial changes in the concentrations of the individual free amino acids in both the plasma and tissue pools. /Amino acids/
After ingestion, proteins are denatured by the acid in the stomach, where they are also cleaved into smaller peptides by the enzyme pepsin, which is activated by the increase in stomach acidity that occurs on feeding. The proteins and peptides then pass into the small intestine, where the peptide bonds are hydrolyzed by a variety of enzymes. These bond-specific enzymes originate in the pancreas and include trypsin, chymotrypsins, elastase, and carboxypeptidases. The resultant mixture of free amino acids and small peptides is then transported into the mucosal cells by a number of carrier systems for specific amino acids and for di- and tri-peptides, each specific for a limited range of peptide substrates. After intracellular hydrolysis of the absorbed peptides, the free amino acids are then secreted into the portal blood by other specific carrier systems in the mucosal cell or are further metabolized within the cell itself. Absorbed amino acids pass into the liver, where a portion of the amino acids are taken up and used; the remainder pass through into the systemic circulation and are utilized by the peripheral tissues. /Amino acids/
Protein secretion into the intestine continues even under conditions of protein-free feeding, and fecal nitrogen losses (ie, nitrogen lost as bacteria in the feces) may account for 25% of the obligatory loss of nitrogen. Under this dietary circumstance, the amino acids secreted into the intestine as components of proteolytic enzymes and from sloughed mucosal cells are the only sources of amino acids for the maintenance of the intestinal bacterial biomass. ... Other routes of loss of intact amino acids are via the urine and through skin and hair loss. These losses are small by comparison with those described above, but nonetheless may have a significant impact on estimates of requirements, especially in disease states. /Amino acids/