Methylmalonic acid urinary excretion

Biochemical findings are variable. The blood cobala-min and folate levels often are normal. Patients often have homocysteinemia with hypomethioninemia, the latter finding discriminating this group from homocystinuria secondary to cystathionine- P-synthase deficiency. Urinary excretion of methylmalonic acid may be high, reflecting the fact that vitamin B12 serves as a cofactor for the methyl-malonyl-CoA (coenzyme A) mutase reaction. 

Methylmalonyl-CoA mutase is a cobalamin-linked enzyme of mitochondria that catalyzes the isomerization of methylmalonyl-CoA to succinyl-CoA. A reduction of this enzyme due to vitamin B12 deficiency will result in a metabolic block with the urinary excretion of methylmalonic acid, and the measurement of this metabolite has been used to confirm a deficiency of vitamin B12. The test has also been useful in investigating rare abnormalities of this enzyme that result in the excretion of methylmalonic acid in the presence of adequate vitamin B12. Given an oral loading dose of valine or isoleucine will increase the urinary excretion of methylmalonic acid in patients with a vitamin B12 deficiency (G4). However, Chanarin and his colleagues (CIO) found that one-quarter of their patients with pernicious anemia excreted a normal concentration of methylmalonic acid even after a loading dose of valine. Normal subjects excrete up to 15 mg of methylmalonic acid in their urine over a 24-hour period (Cll). 

S-Methylmalonyl-CoA mutase (EC 5.4.99.2) is a deoxyadenoxyladen-osylcobalamin-dependent enzyme of mitochondria required to catalyze the conversion of methylmalonyl-CoA to succinyl-CoA. A decrease in the activity of methylmalonyl-CoA mutase leads to the urinary excretion of large amounts of methylmalonic acid (C22). The biochemical lesion may be at the mutase level due to an abnormality of apoenzyme protein or an inability to elaborate the required coenzyme form of vitamin B12> i.e., adenosyl-cobalamin. In rare cases the abnormality may be due to an inability to convert the d form of methylmalonyl-CoA mutase to the l form as a result of a defective racemase (EC 5.1.99.1) (Kll). In patients, the nature of the abnormality can be determined by tissue culture studies (D13) and by clinical trial, since patients with a defect in adenosylcobalamin production will show clinical improvement when treated with very large doses of vitamin B12 (Mil). 

As a result of the reduced activity of the mutase in vitamin B12 deficiency, there is an accumulation of methyhnalonyl CoA, some of which is hydrolyzed to yield methylmalonic acid, which is excreted in the urine. As discussed in Section 10.10.3, this can be exploited as a means of assessing vitamin B12 nutritional status. There may also be some general metabolic acidosis, which has been attributed to depletion of CoA because of the accumulation of methyl-malonyl CoA. However, vitamin B12 deficiency seems to result in increased synthesis of CoA to maintain normal pools of metabolically useable coenzyme. Unlike coenzyme A and acetyl CoA, neither methylmalonyl CoA nor propionyl CoA (which also accumulates in vitamin B12 deficiency) inhibits pantothenate kinase (Section 12.2.1). Thus, as CoA is sequestered in these metabolic intermediates, there is relief of feedback inhibition of its de novo synthesis. At the same time, CoA may be spared by the formation of short-chain fatty acyl carnitine derivatives (Section 14.1.1), which are excreted in increased amounts in vitamin B12 deficiency. In vitamin Bi2-deficient rats, the urinary excretion of acyl carnitine increases from 10 to 11 nmol per day to 120nmolper day (Brass etal., 1990). 

Breakdown of isoleucine, valine, threonine, and methionine results in the production of propionyl-CoA. Propionyl-CoA, in turn, is catabolized to succinyl-CoA via the intermediate methylmalonyl-CoA. Methylmalonyl-CoA is a compoimd of imusual interest to nutritional scientists. This compound accumulates in the cell during a vitamin B12 deficiency. Vitamin B12 deficiency is not a rare disease, as it appears in a common autoimmune disease called pernicious anemia. Vitamin B12 deficiency also occurs in strict vegetarians who avoid meat, fish, poultry, and dairy products. Methylmalonyl-CoA can also build up with rare genetic diseases that involve the production of defective, mutant forms of methylmalonyl-CoAmutase. Most of the methylmalonyl-CoAthat accumulates to abnormally high levels in the cell is hydrolyzed to methylmalonic acid (MMA), which leaves the cell for the bloodstream and eventual excretion in the urine. Some of the MMA is converted back to propionyl-CoA, resulting in the production and accumulation of propionic acid in the cell. The measurement of plasma and urinary MMA has proven to be a method of choice for the diagnosis of vitamin B12 deficiency, whether induced by pernicious anemia or by dietary deficiency. 

Chiandetti L, ZaccheHo F. Acute metabolic decompensation in methylmalonic acidosis time sequence in the urinary excretion pattern of precursor organic acids and their major metabolites. Perspect Inherit Metab Dis 1987 7 57-70. 

A vitamin B12 coenzyme is needed to convert methyknalonyl coenzyme A to succinyl co-enzyme A. Patients with vitamin B12 deficiency almost always have increased urinary excretion of methylmalonic acid (MMA). MMA is amore specific marker for vitamin B12 deficiency compared to homocysteine. MMA levels are not elevated in folate deficiency, as folate does not participate in MMA metabolism. Levels of both MMA and homocysteine are usually elevated prior to the development of hematologic abnormalities and reductions in serum vitamin B12 levels. MMA levels need to be interpreted cau-... 

B3. Bamess, L. A., Moeksi, H., and Gyorgy, P., Urinary excretion of methylmalonic, tt-a-dimethylsuccinic and other ether-soluble acids in rats. J. Biol. Chem. 221, 93 (1956). 

Methylmalonyl CoA is formed as an intermediate in the catabolism of valine and by the carboxylation of propionyl CoA arising in the catabolism of isoleucine, cholesterol and (rare) fatty acids with an odd number of carbon atoms. As shown in Figure 11.18, it normally undergoes vitamin B dependent rearrangement to succinyl CoA, catalysed by methylmalonyl CoA mutase. The activity of this enzyme is greatly reduced in vitamin B deficiency, leading to an accumulation of methylmalonyl CoA, some of which is hydrolysed to yield methylmalonic acid, which is excreted in the urine urinary... 

Chalmers, R.A., Lawson, A.M. and Watts, R.W.E. (1974b), Studies on the urinary acidic metabolites excreted by patients with )S-methylcrotonylglycinuria, propionic acidaemia and methylmalonic aciduria, using gas-liquid chromatography and mass spectrometry. Clin. Chim. Acta, 52,43. 

Vitamin B12 deficiency must be excluded, even in the newborn (see Section 11.2.6 below), particularly when the mother is a vegetarian, and maternal serum vitamin B12 concentrations may be helpful. Patients without vitamin Bi2 deficiency have also been reported with methylmalonic aciduria at low levels (12-115 mg day) which is unresponsive to vitamin B12 therapy (Giorgio et aly 1976) and apparently occurs as a result of reduced activity of methylmalonyl-CoA mutase. Such patients with urinary methylmalonic acid concentrations about 10-50 times normal may also be observed during well-baby screening (Shih et al, 1976), but no confusion with the true congenital methylmalonic acidurias where concentrations are 1000 times or more, and metabolites of propionyl-CoA are also excreted, should occur if the proper analytical procedures are employed. 

Fig. 11.10 Chromatogram of organic acids extracted using DEAE-Sephadex from the urine of the same patient described in the legend to Fig. 11.9, after 1 year on vitamin B12 therapy. Separated as described for Fig. 11.9. Peak identifications are given in the legend to Fig. 11.9, additional identifications are 10, 2-deoxytetronate 17,4-hydroxyphenyl-acetate 18, 2-deoxypentonate. The reduced excretion of methylmalonate and the abolition of methylcitrate and 3-hydroxypropionate excretion together with the production of an otherwise normal urinary organic acid profile is apparent when compared to Fig. 11.9. (The horizontal axis represents the time elapsed in minutes from sample injection.) (Redrawn from the original, after Chalmers, 1980)...

---