The methylmalonic acidurias

Carboxylation of propionyl-CoA leads to the formation of D(5)-methyl-malonyl-CoA. D-Methylmalonyl-CoA is converted by methylmalonyl-CoA racemase (EC 5.1.99.1) into the L(/ )-isomer which is in turn converted into succinyl-CoA by the vitamin B 12-dependent methylmalonyl-CoA mutase (EC 5.4.99.2) (formerly isomerase), the succinyl-CoA subsequently entering the tricarboxylic acid cycle. 

D(S)-Methylmalonyl-CoA L(/ )-Methylmalonyl-CoA SuccInyl-CoA The racemase enzyme acts by moving the a-hydrogen on D-methylmalonyl-CoA and the mutase by movement of the CoA carboxyl group. The mutase enzyme requires a cofactor form of vitamin B12, 5 -deoxyadenosylcobalamin 

Once in the blood, hydroxocobalamin, the precursor vitamin, is transported by two different serum globulins, transcobalamins (TC) I and II. Trans-cobalamin II is a /3-globulin and is concerned with the transport of newly absorbed vitamin while transcobalamin I, an a-globulin, carries the majority of the vitamin found in plasma and is also concerned with storage (Rosenberg, 1978). Hydroxocobalamin is transported into the cell bound to transcobalamin II via a specific cell-surface calcium-dependent receptor-mediated endocytic 

Isolated deficiencies of methylcobalamin or of 5 -deoxyadenosylcobalamin result in homocystinuria or methylmalonic aciduria respectively, whereas deficiencies in the earlier reductase steps result in combined disorders. Intrinsic factor deficiency also leads to combined methylmalonic aciduria and homocystinuria because of the low serum vitamin B12 levels, but the urinary concentrations recorded are only moderately increased in comparison to those encountered in methylmalonic aciduria due to primary enzyme deficiencies. Defects in vitamin B12 transport, TCI and TCII deficiency do not appear to produce abnormal organic or amino acidurias. Deficiencies of intrinsic factor or of ileal absorption of vitamin B12 respond clinically and biochemically to physiological doses (1-5 fig day ) of vitamin B12 given intravenously or intramuscularly, whereas deficiencies in transport proteins require pharmacological doses ( 500 pg day ). The involvement of 5 -deoxyadenosylcobalamin in methylmalonate metabolism also suggests the possibility of vitamin Bi2-responsive primary methylmalonic aciduria, and this is discussed further below. 

Cobalamin metabolism in intact cells AdoCbl synthesis - 

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Table 11.3. Biochemical features in vitro of cultured fibroblasts from patients in the various complementation groups of the methylmalonic acidurias (data from Willard and Rosenberg, 1978).

The catabolism of leucine, valine, and isoleucine presents many analogies to fatty acid catabolism. Metabolic disorders of branched-chain amino acid catabolism include hypervalinemia, maple syrup urine disease, intermittent branched-chain ketonuria, isovaleric acidemia, and methylmalonic aciduria. 

On rare occasions an organic aciduria occurs not because of an enzyme deficiency but from a failure to transport or activate a water-soluble vitamin that serves as a cofactor for the reaction in question. Thus, congenital deficiencies in the metabolism of vitamin B12 commonly give rise to methylmalonic aciduria (Fig. 40-1, Table 40-2). Similarly, deficiencies of biotin metabolism can cause a severe organic aciduria (Table 40-2). It is very important to be aware of the defects of vitamin metabolism because the administration of large doses of these cofactors may completely prevent brain damage. 

Patients typically present by 6-12 months with severe developmental retardation, convulsions, microcephaly and homocysteinemia (=50pmol/l) with hypomethioninemia (<20 pmol/1). A few individuals have had psychiatric disturbances. The blood concentration of vitamin B12 is normal, and, unlike individuals with defects of cobalamin metabolism, these patients manifest neither anemia nor methylmalonic aciduria. The blood folic acid level is usually low. 

Methionine synthase deficiency (cobalamin-E disease) produces homocystinuria without methylmalonic aciduria. This enzyme mediates the transfer of a methyl group from methyltetrahydrofolate to homocysteine to yield methionine (Fig. 40-4 reaction 4). A cobalamin group bound to the enzyme is converted to methylcobalamin prior to formation of methionine. 

In cobalamin-E (cblE) disease there is a failure of methyl-B12 to bind to methionine synthase. It is not known if this reflects a primary defect of methionine synthase or the absence of a separate enzyme activity. Patients manifest megaloblastic changes with a pancytopenia, homocystinuria and hypomethioninemia. There is no methylmalonic aciduria. Patients usually become clinically manifest during infancy with vomiting, developmental retardation and lethargy. They respond well to injections of hydroxocobalamin. 

The fibroblasts do not convert cyanocobalamin or hydroxocobalamin to methylcobalamin or adenosyl-cobalamin, resulting in diminished activity of both N5-methyltetrahydrofolate homocysteine methyltransferase and methylmalonyl-CoA mutase. Supplementation with hydroxocobalamin rectifies the aberrant biochemistry. The precise nature of the underlying defect remains obscure. Diagnosis should be suspected in a child with homocystinuria, methylmalonic aciduria, megaloblastic anemia, hypomethioninemia and normal blood levels of folate and vitamin B12. A definitive diagnosis requires demonstration of these abnormalities in fibroblasts. Prenatal diagnosis is possible. 

In this hereditary disease up to 1 - 2 g of methylmalonic acid per day (compared to a normal of <5 mg/day) is excreted in the urine, and a high level of the compound is present in blood. Two causes of the rare disease are known/ One is the lack of functional vitamin B12-containing coenzyme. This can be a result of a mutation in any one of several different genes involved in the synthesis and transport of the cobalamin coenzyme.6 Cultured fibroblasts from patients with this form of the disease contain a very low level of the vitamin B12 coenzyme (Chapter 16), and addition of excess vitamin B12 to the diet may restore coenzyme synthesis to normal. Among elderly patients a smaller increase in methylmalonic acid excretion is a good indicator of vitamin B12 deficiency. A second form of the disease, which does not respond to vitamin B12, arises from a defect in the methylmalonyl mutase protein. Methylmalonic aciduria is often a very severe disease, frequently resulting in death in infancy. Surprisingly, some children with the condition are healthy and develop normally.3 1... 

Both methylmalonic aciduria and propionyl-CoA decarboxylase deficiency are usually accompanied by severe ketosis, hypoglycemia, and hyperglycinemia. The cause of these conditions is not entirely clear. However, methylmalonyl-CoA, which accumulates in methylmalonic aciduria, is a known inhibitor of pyruvate carboxylase. Therefore, ketosis may develop because of impaired conversion of pyruvate to oxalo-acetate. 

Methylmalonic aciduria is rare and can be diagnosed incorrectly. In 1989 a woman in St. Louis, Missouri, was convicted and sentenced to life in prison for murdering her 5-month-old son by poisoning with ethylene glycol. While in prison she gave birth to another son who soon fell ill of methylmalonyl aciduria and was successfully treated. Reexamination of the evidence revealed that the first boy had died of the same disease and the mother was released.1... 

Whlean et al. (W7) described a follow-up, extending over several years, of two infants with methylmalonic aciduria unresponsive to treatment with vitamin B12. The first patient, a boy, was the child of two first cousins delivery followed an uneventful pregnancy. The child had convulsions 4 days after birth and was found to have a profound metabolic acidosis, and was excreting a large amount of methylmalonic acid in his urine. His serum vitamin B12 concentration was normal. Further studies confirmed a diagnosis of methylmalonic aciduria. 

As discussed in Section 10.8.2, moderate vitamin B12 deficiency results in increased accumulation of methylmalonyl CoA, and methylmalonic aciduria and methylmalonic acidemia. This can be exploited as both a means of detecting subclinical deficiency and monitoring vitamin B12 status in patients with pernicious anemia who have been treated with parenteral vitamin. As they become depleted, the excretion of methylmalonic acid, especially after a loading dose of valine, will provide a sensitive index of depletion of vitamin Bi2 reserves. 

Methylmalonyl CoA mutase is especially sensitive to vitamin B12 depletion, so methylmalonic aciduria is the most sensitive index of vitamin B12 status. Folate deficiency does not cause methylmalonic aciduria. However, up to 25% of patients with confirmed pernicious anemia excrete normal amounts of methylmalonic acid, even after a loading dose of valine (Chanarin et al., 1973). 

GC-MS has been responsible for the identification of a variety of unusual lipids associated with diseased conditions. Hydroxyocta-decadienoic esters of cholesterol for example, have been isolated from aortal atheroma placques [277] and branched chain and odd numbered fatty acids identified in the glycerolipids of brain, spinal cord and sciatic nerve [278] from a patient with methylmalonic aciduria. The latter compounds are thought to arise by the replacement of malonyl CoA with methyl malonyl CoA, and acetyl CoA with propionyl CoA at certain stages of fatty acid synthesis. In these and other examples, the lipids need to be hydrolysed to permit the identification of the constituent fatty acids. As the class of lipids is usually known from the separation procedure used, the nature of the fatty acids may allow the characterisation of the complete molecule. However, volatilisation of the intact lipid into the mass spectrometer when possible would be preferable, particularly when it is present in a mixture and separation of the components is first made by GC. 

An important aspect of metabolite profiles is the study of known inborn errors where the qualitative and quantitative interrelationship of the affected metabolites can be studied. A recent example of this is the comparative study of three diseases, )8-methylcrotonylglycinuria, propionic acidemia and methylmalonic aciduria [362]. The advantage of considering a complete class of compounds in a single experiment is that biochemical markers for a disorder can be detected in the context of any variations in other components. This is particularly important in monitor-... 

Inborn errors in the synthesis of adenosylcobalamin or of both adenosyl- and methylcobalamins have been described. They cause, respectively, methylmalonic aciduria alone or combined with homocystinuria (Table 38-1). These disorders respond to treatment with pharmacological doses of vitamin B12. Methylmalonic aciduria that does not respond to vitamin B12 is probably due to an abnormal methylmalonyl-CoA mutase. 

Not all patients who present the same clinical picture respond to vitamin therapy. Thus, if the structural gene for an apoenzyme or transport molecule is completely absent because of a gene deletion, no amount of vitamin or cofactor will correct the defect. If the mutation affects substrate rather than cofactor binding, the pathway is blocked just as effectively and cannot be relieved by increased concentration of cofactor. Thus, six mutations have been identified that cause methylmalonic aciduria. 

The answer is d. (Murray, pp 238-249. Scriver, pp 2165-2194. Sack, pp 121-144. Wilson, pp 287-324.) Propionic acidemia (232000) results from a block in propionyl CoA carboxylase (PCC), which converts propionic to methylmalonic acid. Excess propionic acid in the blood produces metabolic acidosis with a decreased bicarbonate and increased anion gap (the serum cations sodium plus potassium minus the serum anions chloride plus bicarbonate). The usual values of sodium (-HO meq/L) plus potassium ( 4 meq/T) minus those for chloride (-105 meq/L) plus bicarbonate (—20 meq/L) thus yield a normal anion gap of -20 meq/L. A low bicarbonate of 6 to 8 meq/L yields an elevated gap of 32 to 34 meq/L, a gap of negative charge that is supplied by the hidden anion (propionate in propionic acidemia). Biotin is a cofactor for PCC and its deficiency causes some types of propionic acidemia. Vitamin B deficiency can cause methylmalonic aciduria because vitamin Bn is a cofactor for methylmalonyl coenzyme A mutase. Glycine is secondarily elevated in propionic acidemia, but no defect of glycine catabolism is present. 

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