Homocysteine regulators

Certain amino acids and their derivatives, although not found in proteins, nonetheless are biochemically important. A few of the more notable examples are shown in Figure 4.5. y-Aminobutyric acid, or GABA, is produced by the decarboxylation of glutamic acid and is a potent neurotransmitter. Histamine, which is synthesized by decarboxylation of histidine, and serotonin, which is derived from tryptophan, similarly function as neurotransmitters and regulators. /3-Alanine is found in nature in the peptides carnosine and anserine and is a component of pantothenic acid (a vitamin), which is a part of coenzyme A. Epinephrine (also known as adrenaline), derived from tyrosine, is an important hormone. Penicillamine is a constituent of the penicillin antibiotics. Ornithine, betaine, homocysteine, and homoserine are important metabolic intermediates. Citrulline is the immediate precursor of arginine. 

In mammals and in the majority of bacteria, cobalamin regulates DNA synthesis indirectly through its effect on a step in folate metabolism, catalyzing the synthesis of methionine from homocysteine and 5-methyltetrahydrofolate via two methyl transfer reactions. This cytoplasmic reaction is catalyzed by methionine synthase (5-methyltetrahydrofolate-homocysteine methyl-transferase), which requires methyl cobalamin (MeCbl) (253), one of the two known coenzyme forms of the complex, as its cofactor. 5 -Deoxyadenosyl cobalamin (AdoCbl) (254), the other coenzyme form of cobalamin, occurs within mitochondria. This compound is a cofactor for the enzyme methylmalonyl-CoA mutase, which is responsible for the conversion of T-methylmalonyl CoA to succinyl CoA. This reaction is involved in the metabolism of odd chain fatty acids via propionic acid, as well as amino acids isoleucine, methionine, threonine, and valine. 

Kinetics of O-Methylaiion. The steady state kinetic analysis of these enzymes (41,42) was consistent with a sequential ordered reaction mechanism, in which 5-adenosyl-L-methionine and 5-adenosyl-L-homocysteine were leading reaction partners and included an abortive EQB complex. Furthermore, all the methyltransferases studied exhibited competitive patterns between 5-adenosyl-L-methionine and its product, whereas the other patterns were either noncompetitive or uncompetitive. Whereas the 6-methylating enzyme was severely inhibited by its respective flavonoid substrate at concentrations close to Km, the other enzymes were less affected. The low inhibition constants of 5-adenosyl-L-homocysteine (Table I) suggests that earlier enzymes of the pathway may regulate the rate of synthesis of the final products. 

Finkelstein JD (2000) Pathways and regulation of homocysteine metabolism in mammals. Semin Thromb Hemost 26 219-225... 

Since preliminary studies showed that 6-hydroxymellein-O-methyl-transferase activity was appreciably inhibited in the presence of the reaction products, the mode of product inhibition of the enzyme was studied in detail in order to understand the regulatory mechanism of in vivo methyltransfer. It is well known that S-adenosyl-Z.-homocysteine (SAH), which is a common product of many O-methyltransferases that use SAM as methyl donor, is usually a potent inhibitor of such enzymes. In the 6-hydroxymellein-Omethyltransferase catalyzing reaction another product of this enzyme, 6-methoxymellein, has pronounced inhibitory activity, in addition to SAH. Since the specific product of the transferase reaction, 6-methoxymellein, is capable of inhibiting transferase activity [88], this observation suggests that activity of the transferase is specifically regulated in response to increases in cellular concentrations of its reaction products in carrot cells. It has been also found that 6-methoxymellein inhibits transferase activity with respect not only to 6-hydroxymellein but also to SAM, competitively. This competitive inhibition was also found in SAH as a function of the co-substrates of the enzyme [89]. It follows that the reaction catalyzed by 6-hydroxymellein-O-methyltransferase proceeds by a sequential bireactant mechanism in which the entry of the co-substrates to form the enzyme-substrate complexes and the release of the co-products to generate free enzyme take place in random order [Fig. (7)]. This result also implies that 6-methoxymellein and SAH have to associate with the free transferase protein to exhibit their inhibitory activities, and cannot work as the inhibitors after the enzyme forms complexes with the the substrate. If, therefore, 6-hydroxymellein-O-methyltransferase activity is controlled in vivo by its specific product 6-methoxymellein, this compound should... 

Individuals on long-term diuretic therapy may also experience elevated levels of homocysteine, an amino acid regulated by folate. High homocysteine levels increase the risk of heart disease. Thiamin, or vitamin Bj, depletion is another possible side effect of loop diuretics. Individuals with thiamin deficiencies are at risk for fatigue, heart enlargement, muscle cramps, heart rate irregularities, and impaired mental function. 

Moshal K, Sen U, Tyagi N, et al. 2006a. Regulation of homocysteine-induced MMP-9 by ERK 1/2 pathway. Am J Physiol Cell Physiol 290 C883-C891. 

Moshal K, Singh M, Sen U, et al. 2006b. Homocysteine-mediated activation and mitochondrial translocation of calpain regulates MMP-9 in MVEC. Am J Physiol Heart Circ Physiol 291 H2825-H2835. 

Regulation of homocysteine metabolism appears to be especially important in the central nervous system, presumably because of the critical role of methyl transfer reactions in the production of neurotransmitters and other methylated products. It has been known for decades that mental retardation is a feature of the genetic diseases, such as CBS deficiency, that cause severe hyperhomocysteinemia and ho-mocystinuria. Impaired cognitive function is also seen in pernicious anemia, which causes hyperhomocysteinemia due to deficiency of cobalamin (see Chapter 28). Hyperhomocysteinemia also may be linked to depression, schizophrenia, multiple sclerosis, and Alzheimer s disease. The molecular mechanisms underlying these clinical associations have not yet been delineated. 

Methylation of homocysteine by 5-methyltetrahydrofolate-homocysteine methyl reductase depends on an adequate supply of 5-methyltetrahydrofoIate. The unmethylated folate is recycled in a cobalamin-dependent pathway, by remethylation to 5,10-methylene-tetrahydrofolate, and subsequent reduction to 5-methyltetrahydrofolate. The transferase enzyme, also named 5,10-methyltretrahydrofolate reductase catalyzes the whole cycle [3,91]. S-adenosylmethionine and 5-methyltetrahydrofolate are the most important methyl unit donors in biological system. S-adenosylmethionine is reported to regulate methylation and transsulfuration pathways in the homocysteine metabolism [3,91]. 

J. D. Finkelstein, The Metabolism of Homocysteine Pathways and Regulation, European Journal of Pediatry 157 (1998) S40-S44. 

Methylation is the addition of a carbon atom to a molecule, usually causing a change in the function of the methylated molecule. For example, methylation of the neurotransmitter dopamine by catechol-O-methyltransferase renders it inactive. With only two exceptions, 5-adenosylmethionine (SAM), an activated form of the essential amino acid methionine, is the methyl donor for each of the more than 150 methylation reactions, which regulate a large number of cellular functions. One exception is methylation of homocysteine (HCY) to methionine by the cobalamin (vitamin Bi2)-dependent enzyme methionine synthase, which utilizes 5-methyltetrahydrofolate (methylfolate) as the methyl donor, serving to complete the methionine cycle of methylation, as illustrated in Fig. 1 (lower right). Notably, HCY formation from S-adenosylhomocysteine (SAH) is reversible and, as a result, any decrease in methionine synthase activity will be reflected as an increase in both HCY and SAH. This is significant because SAH interferes with SAM-dependent methylation reactions, and a decrease in methionine synthase activity will decrease all of these reactions. Clearly methionine synthase exerts a powerful influence over cell function via its control over methylation. 

Vitvitsky V, Prudova A, Stabler S, Dayal S, Lentz SR, Baneijee R. (2007) Testosterone regulation of renal cystathionine beta-synthase implications for sex-dependent differences in plasma homocysteine levels. Am J Physiol Renal Physiol 293 F594-600. 

Regulation of liver D-fructose 1,6-diphosphatase could involve modification of the sulfhydryl group of the enzyme.397 Cystamine (2,2 -dithiobisethylamine) or homocysteine undergoes a disulfide exchange-reaction with two reactive cysteine residues. This exchange leads to a four-fold increase in catalytic activity.405 Other sulfhydryl reagents, such as l-fluoro-2,4-dinitrobenzene (FDNB), p-mercuri-benzoate (PMB), and 2-iodoacetamide, also activate liver D-fructose... 

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