Cystathionine defect

Kluijtmans, L. A., Boers, G. H., Stevens, E. M. et al. Defective cystathionine beta-synthase regulation by S-adenosyl-methionine in a partially pyridoxine responsive homocys-tinuria patient./. Clin. Invest. 98 285-289,1996. 

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. 

Homocystinuria may result from one or several abnormalities in the mechanism whereby homocysteine is methylated to form methionine. About half of the patients respond to treatment with pyridoxine and it is thought that the vitamin overcomes a block at the homocysteine/cystathionine level by mass action (C23). However, Schuh et al. (S22) have recently described a patient who responded to vitamin B12. The infant presented with severe developmental delay, homocystinuria, and a megaloblastic anemia. Treatment with cyanocobalamin was without effect but treatment with hydroxocobalamin resulted in a rapid clinical improvement, and the homocystinuria disappeared. Methionine synthetase activity in cell extracts was normal, while cultured fibroblasts showed an absolute growth requirement for methionine. The defect appeared to be limited to methyleobalamin accumulation and an inability to transfer the methyl group from 5-methyltetrahydrofolate to homocysteine. 

There are two pyridoxal phosphate-requiring enzymes in the homocysteine degradation pathway, which are associated with genetic diseases. In homo-cystinuria, cystathionine synthase is defective, and large amounts of homocystine are excreted in the urine. Some homocystinurics respond to the administration of large doses of vitamin B6. In cystathioninuria, cystathionase is either defective or absent. These patients excrete cystathionine in the urine. Cystathionase is often underactive in the newborns with immature livers, and cysteine and cystine become essential amino acids. Human milk protein is especially rich in cysteine, presumably to prepare the newborn for such a contingency. 

The transsulfuration pathway involves conversion of homocysteine to cysteine by the sequential action of two pyridoxal phosphate (vitamin B6)-dependent enzymes, cystathionine- 5-synthase (CBS) and cystathionine y-lyase (Fig. 21-2). Transsulfuration of homocysteine occurs predominantly in the liver, kidney, and gastrointestinal tract. Deficiency of CBS, first described by Carson and Neill in 1962, is inherited in an autosomal recessive pattern. It causes homocystinuria accompanied by severe elevations in blood homocysteine (>100 (iM) and methionine (>60 (iM). Homocystinuria due to deficiency of CBS occurs at a frequency of about 1 in 300,000 worldwide but is more common in some populations such as Ireland, where the frequency is 1 in 65,000. Clinical features include blood clots, heart disease, skeletal deformities, mental retardation, abnormalities of the ocular lens, and fatty infiltration of the fiver. Several different genetic defects in the CBS gene have been found to account for loss of CBS activity. 

E-6) Homocysteinurla (defect in cystathionine synthase at this step). This is the most common form of ho-mocysteinuria). The enzyme defect leads to elevated levels of homocysteine, which can be detected in the urine. Serum methionine is also elevated. The clinical problems include dislocation of the lens, mental retardation, and various skeletal and neurologic problems. The mechanisms are unclear. Treatment may include administration of pyridoxine, decreasing dietary methionine and increasing cysteine. Elevated blood homocysteine is a risk factor for heart disease. 

Homocystinuria is a genetic disease lhal usually results from defects In the gene coding for cystathionine 3-synthase, and the consequent lack of activity of this enzyme. Typically, the resulting levels of enzyme activity are from 0 to 5 K> the normal value. The disease w as named because of the increased levels of homocysteine in the urine that arise because the plasma levels of this amino acid increase to over 100 jjlM. Actually, urinary homocysteine occurs as the disulfide-linked... 

Figure 7-10. Amino acids that can be converted to succinyl CoA. The amino acids methionine, threonine, isoleucine, and valine, which form succinyl CoA via methylmalonyl CoA, are all essential. The carbons of serine are converted to cysteine and do not form succinyl CoA by this pathway. A defect in cystathionine synthase (M) causes homocystinuria. SAM= S-adenosylmethionine PLP = pyridoxal phosphate.

In homocystinuria, cystathionine synthase is defective. Therefore, homocysteine does not react with serine to form cysteine (see Figure 7-10). The homocysteine that accumulates is oxidized to homocystine and excreted in the urine. Some cases respond to increased doses of vitamin B6, which forms pyridoxal phosphate, the cofactor for the synthase enzyme. 

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.)...

There are several vitamin Bg-responsive inborn errors of metabolism that include (1) cases of infantile convulsions in which the apoenzyme for glutamate decarboxylase has a poor affinity for the coenzyme (2) a type of chronic anemia in which the number but not morphological abnormality of erythrocytes is improved by pyridoxine supplementation (3) xanthurenic aciduria in which affinity of the mutant kynureninase for PLP is decreased (4) primary cystathion-inuria caused by similarly defective cystathionase and (5) homocystinuria in which there is less of the normal cystathionine synthetase. In these cases increased levels (200 to lOOOmg/day) of administered vitamin Bg are required for life. Low vitamin Bg status (together with low vitamin B12 and folate status) in humans has been linked to hyperho-mocysteinemia and as an independent risk factor for cardiovascular disease. ... 

A defect of cystathionine -synthase is an autosomal recessive disorder with an incidence of approximately 1 350,000 live births. In the United States, screening for homocystinuria is not offered in 23 states, corresponding to 53% of aU babies born each year (see Table 55-2). Clinical... 

Group 11 Defect in eataboHm high renal dearanee deledion preferable in urine Cystathionine... 

Vitamin B12 and folate are both required for conversion of homocysteine to methionine. Increased serum homocysteine may help define vitamin B12 or folate deficiency. Homocysteine levels can also be elevated in vitamin Bg deficiency, renal failure, hypothyroidism, and in persons with a genetic defect in cystathionine /S-synthase. Additionally, elevated levels have been caused by medications including nicotinic acid, theophylline, methotrexate, and L-dopa. 

In the genetic disease cystathioninuria, the enzyme cystathionase is defective or missing. In this case, cystathionine accumulates and is excreted in the urine. Thus, almost all of the equivalent of methionine intake is put out as cystathionine. If cystathionase is missing, an individual cannot make cysteine from methionine or homocysteine. 

If the blood levels of methionine and homocysteine are very elevated and cystine is low, cystathionine p-synthase could be defective, but a cystathionase deficiency is also a possibility. With a deficiency of either of these enzymes, cysteine could not be synthesized, and levels of homocysteine would rise. Homocysteine would be converted to methionine by reactions that require B12 and tetrahydrofolate (see Chapter 40). In addition, it would be oxidized to homocystine, which would appear in the urine. The levels of cysteine (measured as its oxidation product cystine) would be low. A measurement of serum cystathionine levels would help to distinguish between a cystathionase or cystathionine p-synthase deficiency. 

Homocystinuria is caused by deficiencies in the enzymes cystathionine p-synthase and cystathionase as well as by deficiencies of methyltetrahy-drofolate (CH3-FH4) or of methyl-B12. The deficiencies of CH3-FH4 or of methyl-B12 are due either to an inadequate dietary intake of folate or B12 or to defective enzymes involved in joining methyl groups to tetrahy-drofolate (FH4), transferring methyl groups from FH4 to B12, or passing them from B12 to homocysteine to form methionine (see Chapter 40). 

Treatment is directed toward early reduction of the elevated levels of homocysteine and methionine in the blood. In addition to a diet low in methionine, very high oral doses of pyridoxine (vitamin B6) have significantly decreased the plasma levels of homocysteine and methionine in some patients with cystathionine 3-synthase deficiency. (Genetically determined responders to pyridoxine treatment make up approximately 50% of type I homocystinurics.) PLP serves as a cofactor for cystathionine (3-synthase however, the molecular properties of the defective enzyme that confer the responsiveness to B6 therapy are not known. 

The terms hypermethioninemia, homocystinuria (or -emia), and cystathionin-uria (or -emia) designate biochemical abnormalities and are not specific clinical diseases. Each may be caused by more than one specific genetic defect. For example, at least seven distinct genetic alterations can cause increased excretion of homocystine in the urine. A deficiency of cystathionine (3-synthase is the most common cause of homocystinuria more than 600 such proven cases have been studied. 

A third way in which serum homocysteine levels can be elevated is by a mutated cystathinone-(3-synthase or a deficiency in vitamin B6, the required cofactor for that enzyme. These defects block the ability of homocysteine to be converted to cystathionine, and the homocysteine that does accumulate cannot all be accommodated by conversion to methionine. Thus, an accumulation of homocysteine results. 

Fig. 40.10. Reaction pathways involving homocysteine. Defects in numbered enzymes (1 = methionine synthase, 2 = N, methylene FH4 reductase, 3 = cystathionine-(3-synthase) lead to elevated homocysteine. Recall that as cysteine accumulates, there is feedback inhibition on cystathionine-P-synthase to stop further cysteine production.

Patients with this most common form of homocystinuria show evidence of involvement of the eye, the skeletal system, the vascular system, and the brain. It is important to note that individuals with cystathionine P-synthase deficiency do not manifest any abnormalities at birth and that the affected pregnancies are uneventful. Thus, this disorder, as opposed to the more rare remethylation defect variants of homocystinuria (described below), is not usually part of the differential diagnosis of the catastrophically ill newborn. Ectopia lentis does not usually appear before the age of 3 years, but most patients have some manifestations by the age of 10. The initial recognition of ocular abnormahties may be an observation by parent or physician that the iris shakes, when the head is moved rapidly. While a predilection for... 

Homocysteine lies at a metabolic crossroad it may condense with serine to form cystathionine, or it may undergo remethylation, thereby conserving methionine. There are two pathways for remethylation in humans. In one, betaine provides the methyl groups, while in the other 5-methyltetrahydrofolate is the methyl donor. This latter reaction is catalyzed by a Bj -containing enzyme, 5-methyltetrahydrofolate homocysteine methyltransferase. Two defects in this latter mechanism may account for the inability to carry out remethylation. In one of them, patients are unable to synthesize or accumulate methylcobalamin, while others cannot produce the second cofactor, 5 -methyltetrahydrofolate, because of adefect in 5,10-methylenetrahydrofolate reductase. 

As the name implies, renal clearance of abnormal levels of homocystine in the plasma causes excessive excretion of the amino acid in the urine. In cystathionine P-synthase deficiency, plasma methionine concentrations are elevated as well -this serves as a point of distinction from the remethylation defects. At present, it appears that the pyridoxal phosphate response may be explained by the fact that this vitamin increases the steady-state concentration of the active enzymes by decreasing the rate of apoenzyme degradation and possibly by increasing the rate of holoenzyme formation. The explanation is not entirely satisfactory, however, since in vitro studies have shown detectable levels of enzyme activity in mutant fibroblasts that have no response, while in other mutant lines without detectable enzyme activity, response has occurred. Once again, a distressing lack of correspondence between in vivo observations and in vitro experiments forces investigators to probe the secrets of these diseases more deeply. 

Fibroblasts from each patient had markedly reduced levels of the above enzyme. The defect results in an inability to synthesize 5-methyltetrahydrofolate in amounts sufficient for the remethylation of homocystine to methionine. Homocystine accumulates in plasma, and the plasma methionine is decreased. As in the other remethylation defect, there is accumulation of cystathionine. Treatment with high doses of folic acid has been beneficial in several patients (Mudd et al., 1989). 

Homocyslinuiia is an autosomal recessive condition caused by a deficiency of the enzyme, cystalhionine-p-synthase (CBS), which results in the accumulation of homocysteine and methionine and a deficiency of cystathionine and cysteine. There are other disorders to consider when an elevated homocysteine concentration is identified. These disorders include vitanun Bn uptake or activation defects, which may or may not have associated elevated methylmalonic acid, severe 5,10-methylenetetrahydrofolate reductase deficiency, and 5-methyl-THF-homocysteine meth-yltransferase deficiency. The latter two are typically associated with an elevated homocysteine, but low methionine concentrations, so it is relatively easy to disaiminate these conditions from homocystinuria. It is also important to consider that nongenetic causes of hyperhomocyste-inemia exist, such as dietary deficiencies, end-stage renal disease, and administration of several drugs [6]. 

Classical homocystinuria is caused by defective activity of cystathionine P-synthase. However, methionine synthase deficiency causes hyperhomocysteinaemia. 

Cystathioninuria, a deficiency of cystathionase, is a much rarer and less clearly defined disorder . While the disease has frequently been associated with mental retardation, this may only refiect the type of individual with which testing most frequently occurs. Patients with normal mental function are also known. Nonetheless, the high levels of cystathionine in brain and the mental defects associated with its faulty metabolism, have led to speculation that this thioether has some special role in nervous function. In tissues from at least one patient, there was evidence that the defect was in pyridoxal phosphate binding by cystathionase and that normal enzyme activity could be achieved at abnormally high levels of coenzyme. This is often quoted as the classical example of a binding or K mutant, but not all patients with the disorder give the same effect. 

The sulfur amino acids are methionine, homocyst(e)ine, cystathionine, cyst(e)ine, and taurine. Defects in several of the enzymatic steps of their metabolism are known some, but not all, result in human disease. The re-methylation of homocysteine to methionine is closely dependent on folate and cobalamin cofactors, and relevant defects of their metabolism are therefore included in this chapter. Cystinuria and cystinosis, defects of renal tubular and lysosomal transport of cystine, respectively, are described in Chap. 13. 

The deficiencies of cystathionine )5-synthase (CBS), sulfite oxidase, and methylenetetrahydrofolate reductase (MTHFR) may all result in central nervous system dysfunction, in particular mental retardation [1-3]. Defects of CBS and sulfite oxidase both cause dislocated lenses of the eyes, but the phenotypes are different otherwise. The manifestations of CBS deficiency, the most common of these disorders, and MTHFR deficiency range from severely affected to asymptomatic patients both may cause vascular occlusion. Deficiency of sulfite oxidase is clinically uniform, but genetically heterogeneous, and functional deficiency of the enzyme can result from several inherited defects of molybdenum cofactor biosynthesis [2, 4]. Hereditary folate malabsorption and defects of cobalamin transport (transcobala-min II deficiency) or cobalamin cofactor biosynthesis (cblC-G diseases) may cause megaloblastic anemia, in addition to CNS dysfunction [3, 5, 6]. 

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.

Corticosterone methyl oxidase II deficiency Costeff optic atrophy syndrome Coupling state defect Creatine deficiency Creatine transporter deficiency Cu-binding P-type ATPase deficiency y-Cystathionase deficiency Cystathionine gamma-lyase deficiency Cystathionine y -synthase deficiency Cystathioninuria... 

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