Methionine metabolism cystathionine synthase

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

Homocystinuria Usually a failure of cystathionine synthase (Fig. 40-2 reaction 6). Rarely associated with aberrant vitamin B12 metabolism (Fig. 40-2) Thromboembolic diathesis, marfanoid habitus, ectopia lentis. Mental retardation is frequent. Diet low in methionine Vitamin B6 in pyridoxine-responsive syndromes Vitamin B12 in responsive syndromes Anticlotting agents... 

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]. 

Deficiencies of methionine adenosyltransferase, cystathionine 8-synthase, and cystathionine )/-lyase have been described. The first leads to hypermethioninemia but no other clinical abnormality. The second leads to hypermethioninemia, hyperhomocysteinemia, and homo-cystinuria. The disorder is transmitted as an autosomal recessive trait. Its clinical manifestations may include skeletal abnormalities, mental retardation, ectopia lentis (lens dislocation), malar flush, and susceptibility to arterial and venous thromboembolism. Some patients show reduction in plasma methionine and homocysteine concentrations and in urinary homocysteine excretion after large doses of pyridoxine. Homocystinuria can also result from a deficiency of cobalamin (vitamin B12) or folate metabolism. The third, an autosomal recessive trait, leads to cystathioninuria and no other characteristic clinical abnormality. 

Several inherited disorders of methionine metabolism (Chapter 17) give rise to exeessive production of homocysteine, HS-CH2-CH2CH(NH3 )COO , and its excretion in urine. The most common form of homocystinuria is due to a deficiency of cystathionine synthase (Chapter 17). A major clinical manifestation of homocystinuria is connective tissue abnormalities that are probably due to the accumulation of homocysteine, which either inactivates the reactive aldehyde groups or impedes the formation of polyfunctional cross-links. 

Cysteine synthesis is a primary component of sulfur metabolism. The carbon skeleton of cysteine is derived from serine (Figure 14.7). In animals the sulfhydryl group is transferred from methionine by way of the intermediate molecule homocysteine. (Plants and some bacteria obtain the sulfhydryl group by reduction of SOj to S2 as H2S. A few organisms use H2S directly from the environment.) Both enzymes involved in the conversion of serine to cysteine (cystathionine synthase and y-cystathionase) require pyridoxal phosphate. 

Fig. 20.3 Pathway of methionine metabolism. The numbers represent the following enzymes or sequences (1) methionine adenosyltransferase (2) S-adenosylmethionine-dependent transmethylation reactions (3) glycine methyltransferase (4) S-adenosylhomocysteine hydrolase (5) betaine-homocysteine methyltransferase (6) 5-methyltetrahydrofolate homocysteine methyltransferase (7) serine hydroxymethyltransferase (8) 5,10-methylenetetrahydrofolate reductase (9) S-adenosylmethionine decarboxylase (10) spermidine and spermine synthases (11) methylthio-adenosine phosphorylase (12) conversion of methylthioribose to methionine (13) cystathionine P-synthase (14) cystathionine y-lyase (15) cysteine dioxygenase (16) cysteine suplhinate decarboxylase (17) hypotaurine NAD oxidoreductase (18) cysteine sulphintite a-oxoglutarate aminotransferase (19) sulfine oxidase. MeCbl = methylcobalamin PLP = pyridoxal phosphate...

Eto et al. (2002) showed that the levels of H2S were severely decreased in the brains of Alzheimer s disease patients (76.4 2.3 years) compared with the brains of the age matched normal individuals (71.5 7.2 years). In addition to HjS production cystathionine P-synthase also catalyses another metabolic pathway in which cystathionine is produced from the substrate homocysteine. S-adenyl-L-methionine, a cystathionine P-synthase activator, is much reduced in Alzheimer s disease brains (Morrison et al. 1996, Eto et al. 2002) and homocysteine accumulates in the serum of Alzheimer s disease patients (Clarke etal. 1998, Eto etal. 2002). 

Fig. 10.1. Defects of transmethylation (methioninehomocysteine), transsulfuration (methionine sulfate), and remethylation (homocysteine - methionine) enzymes of sulfur amino acid metabolism 10.1, methionine adenosyltransferase 10.2, cystathionine ) -synthase 10.3, y-cystathionase 10.4, sulfite oxidase 10.5, molybdenum cofactor 10.6, methylenetetrahydrofolate reductase 10.7 and 10.8, methionine synthase.

The major developmental change which takes place In both brain and liver is the postnatal activation of the transsulfuration pathway of methionine metabolism. The net result of this pathway is the transfer of the sulfur atom from homocysteine to the carbon skeleton of serine to form cysteine. This conversion is mediated by two enzymes cystathionine synthase (L-serine hydro-lyase adding homocysteine, EC 4.2.1.22) which catalyzes the 3-activation of serine and the addition of homocysteine to form the thio-ether, cystathionine cystathionase (EC 4.4.1.1) which catalyzes the y-cleavage of cystathionine to form cysteine (Fig. 1). Both of these enzymes catalyze reactions other than those described above although their importance vivo is uncertain (Tallan et al., 1974). In mature mammals, activities both of cystathionine synthase and of cystathionase are present in brain and liver, although cystathionase activity in... 

Reaction 4 is catalysed by cystathionine synthase (EC 4.2.1.13), an enzyme widely distributed in the tissues. In homocystinuria, cystathionine synthase is virtually completely absent or inactive in all tissues examined liver, brain and fibroblasts grown in tissue culture [33]. In some cases 1 to 2% of the normal enzymic activity can be demonstrated, in others no enzymic activity has been found [34]. As a result of the metabolic block, homocysteine accumulates and is partly converted to homocystine, partly to homocysteine-cysteine mixed disulphide and partly S-methylated to methionine by reactions 6 and 7 with, respectively, N -methyltetrahydrofolic acid and betaine as methyl donors. In infancy methionine and homocysteine are present in high concentrations in the plasma while homocystine and homocysteine-cysteine mixed disulphide are excreted in the urine later the concentration of methionine in the plasma drops. Cystathionine is normally present in highest concentration in the cells of the brain, though traces are found elsewhere and in the urine in homocystinuria no cystathionine can usually be demonstrated in the brain or urine [35]. The body s cysteine and cystine are also largely biosynthesized from methionine, though some is obtained from cysteine and cystine in dietary proteins in homocystinuria, cysteine/cystine becomes an essential amino acid. 

A number of inborn errors of metabolism are due to an enzyme deficiency which leads to the accumulation of the precursor of that particular pathway. This type of clinical expression was among the first to be described, since simple clinical screening tests often uncovered the increased excretion in the urine, or accumulation in the blood, of early metabolites of the pathway which had been blocked. An important example of such a defect is homocystinuria, where both methionine and homocystine accumulate as the result of the deficiency of activity of cystathionine synthase (Mudd and Levy, 1978). A variant of the foregoing occurs when both the precursor of the main pathway and metab-... 

A few of the enzymes of methionine metabolism have been studied further since the review by Giovanelli et al. (1980). Cystathionine y-synthase has still not been purified to establish whether a single protein supports both the synthesis of cystathionine from phospbohomoserine and cysteine (cystathionine y-synthase) and the synthesis of homocysteine from phospbohomoserine and... 

A large elevation of Hey in body fluids and tissues is found in several genetic enzyme deficiencies, the homocystinurias. These include cystathionine /3-synlhase deficiency [9], the remethylation defects due to deficiency of MTHF reductase [10], methionine synthase and methionine synthase reductase deficiencies, as well as defects of intracellular cobalamin metabolism [11], namely the cblF, cblC and cblD defects. It is noteworthy that low levels of total Hey (tHcy) have been described in sulphite oxidase deficiency [12]. 

The homocystinurias are a group of disorders involving defects in the metabolism of homocysteine. The diseases are inherited as autosomal recessive illnesses, characterized by high plasma and urinary levels of homocysteine and methionine and low levels of cysteine. The most common cause of homocystinuria is a defect in the enzyme cystathionine /3-synthase, which converts homocysteine to cystathionine (Figure 20.21). Individuals who are homozygous for cystathionine [3-synthase deficiency exhibit ectopia lentis (displace ment of the lens of the eye), skeletal abnormalities, premature arte rial disease, osteoporosis, and mental retardation. Patients can be responsive or non-responsive to oral administration of pyridoxine (vitamin B6)—a cofactor of cystathionine [3-synthase. Bg-responsive patients usually have a milder and later onset of clinical symptoms compared with B6-non-responsive patients. Treatment includes restriction of methionine intake and supplementation with vitamins Bg, B, and folate. 

When present in excess methionine is toxic and must be removed. Transamination to the corresponding 2-oxoacid (Fig. 24-16, step c) occurs in both animals and plants. Oxidative decarboxylation of this oxoacid initiates a major catabolic pathway,305 which probably involves (3 oxidation of the resulting acyl-CoA. In bacteria another catabolic reaction of methionine is y-elimination of methanethiol and deamination to 2-oxobutyrate (reaction d, Fig. 24-16 Fig. 14-7).306 Conversion to homocysteine, via the transmethylation pathway, is also a major catabolic route which is especially important because of the toxicity of excess homocysteine. A hereditary deficiency of cystathionine (3-synthase is associated with greatly elevated homocysteine concentrations in blood and urine and often disastrous early cardiovascular disease.299,307 309b About 5-7% of the general population has an increased level of homocysteine and is also at increased risk of artery disease. An adequate intake of vitamin B6 and especially of folic acid, which is needed for recycling of homocysteine to methionine, is helpful. However, if methionine is in excess it must be removed via the previously discussed transsulfuration pathway (Fig. 24-16, steps h and z ).310 The products are cysteine and 2-oxobutyrate. The latter can be oxidatively decarboxylated to propionyl-CoA and further metabolized, or it can be converted into leucine (Fig. 24-17) and cysteine may be converted to glutathione.2993... 

Figure 21-1. Structural and metabolic relationships between methionine, homocysteine, and cysteine. CBS, cystathionine b-synthase CTH, cystathionine y-lyase MAT, methionine adenosyltransferase MS, methionine synthase 5-MTHF, 5-methyltetrahydrofoIate MTs, methyl transferases PLR pyridoxal phosphate SAH, S-adenosylhomocysteine SAHH, SAH hydrolase THF, tetrahydrofolate.

Figure 8 Extended folate metabolism, including compartmentation. MTHFR, methylenetetrahydrofolate reductase SHMT, serine hydroxymethyltransferase BHMT, betaine homocysteine methyltransferase, MAT, methionine adenosyltransferase SAH-hydrolase, S-adenosylhomocysteine hydrolase MT, methyltransferase CBS, cystathionine /i-synthase SAM, S-adenosylmethionine SAH, S-aden-osylhomocysteine THF, tetrahydrofolate and 5-MeTHF, 5-methyltetrahydrofolate. (Reproduced from Van der Put etal. (2001) Folate, homocysteine and neural tube defects An overview. Experimental Biology and Medicine 226 243-270.)...

Autotrophic organisms synthesize methionine from asparfafe as shovm in the lower right side of Fig. 24-13. This involves fransfer of a sulfur atom from cysfeine info homocysfeine, using the carbon skeleton of homoserine, the intermediate cystathionine, and two PLP-dependent enzymes, cystathionine y-synthase and cystathionine p-lyase. This transsulfuration sequence (Fig. 24-13, Eq. 14-33) is essentially irreversible because of the cleavage to pyruvate and NH4+ by the P-lyase. Nevertheless, this transsulfuration pathway operates in reverse in the animal body, which uses two different PLP enzymes, cystathionine P s3mthase (which also contains a bound heme) and cystathionine y-lyase (Figs. 24-13,24-16, steps h and i), in a pathway that metabolizes excess methionine. 

An increased plasma level of homocysteine is regarded as a risk factor for cardiovascular disease and the development of arteriosclerosis. Homocysteine concentrations in plasma are reduced by remethylation and transsulfuration (Komarnisky et al. 2003). The remethylation is catalyzed by methionine synthase, which in turn is influenced by vitamin B12 and folate. The transsulfura-tions depend on cystathionine 3-synthase. A dietary deficiency of vitamins B, B12 and folate, accompanied by a high protein intake, can cause hyperhomocystinemia in humans (Jacobsen 1998). Furthermore, a genetic disorder of enzymes involved in the metabolism of homocysteine leads to hypercystinuria (Mudd et al. 1989). 

H2S can be produced via the metabolism of sulfhydryl-bearing amino acids, specifically by several enzymes found in the methionine-homocysteine-cysteine pathway such as cystathionine 3 synthase (CBS) and cystathionine lyase (CGL) (Fig. 8.1) [6, 10, 11]. The sequence of CBS has been identified in genomes from bacteria to humans [12-14], and a gene similar to the sulfide quinone oxidoreductase gene has been identified in the genome of flies, worms, mice, rats, and humans [15], indicating that cellular H2S and its regulation may be widespread and essential. 

A second metabolic system similar to the first is shown in Figure 1, panel ii. In this schematic, the emphasis is a metabolite one or two steps proximal to the primary enzyme deficiency. For example, in normal individuals, metabolite C is converted by enzyme Y to metabolite A, which is subsequently converted by enzyme X to metabolite B. As described above, in individuals with an inherited deficiency of enzyme X, metabolite A accumulates. In this particular scenario, compounds that are converted to metabolite A, namely metabolite C, will increase as enzyme Y is inhibited by basic kinetics. An example of this enzyme system is homocystinuria. In this disorder, the metabolism of homocysteine to cystathionine by cystathionine S-synthase is blocked. An increase in homocysteine causes an accumulation of S-adenosyl homocysteine and S-adenosyl methionine. Due to an increase in these metabolites, the metabolism of methionine to S-adenosyl methionine by methionine adenosyl transferase is decreased. Hence, methionine increases in the blood of individuals with homocystinuria. Note that in this example there were two enzymatic steps before the metabolism of homocysteine. 

Figure 29.6 Pathways for the metabolism of homocysteine. Normal transsulfuration requires cystathionine P-synthase with vitamin Bg as cofactor. Reme-thylation requires 5,10-methylenetetrahydrofolate reductase and methionine synthase. The latter requires folate as cosubstrate and vitamin Bi2 (cobalamin) as cofactor. An alternative remethylation pathway also exists using the cobalamin independent betaine-homocysteine methyltransferase (Robinson 2000).

Figure 44.1 Folate-mediated one carbon metabolism network. Enzymes and transport proteins are enclosed in rectangular boxes. AHCY S-adenosyDiomocys-teine hydrolase AICART 5-aminoimidazole carboxamide ribonucleotide transferase BHMT betaine homocysteine methyltransferase CBS cystathionine beta-synthase DHFR dihydrofolate reductase FR folate receptor FTCD formimidoyltransferase cyclodeaminase GART glycinamide ribonucleotide transformylase MATs (MATI/MATIII) adenosylmethionine transferase enzyme I/III MS methionine synthase MSR methionine synthase reductase MT methyltransferase MTHFD methylenetetrahydrofolate dehydrogenase MTHFR 5,10-methylenete-trahydrofolate reductase MTHFS 5,10-methylenetetrahydrofolate synthase. RFC reduced folate AdoMet 5-adenosylmethionine AdoHcy S-adenosylhomocysteine Hey homocysteine SHMT serine hydroxymethyltransferase TS thymidylate synthase.

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