Methylmalonate semialdehyde, from

Fig. 6. Pathway leading from valine to methylmalonic acid. Metliylmalonic acid could arise through direct oxidation of methylmalonic semialdehyde, as shown in the scheme, or through deacylation of methylmalonyl-CoA, which would represent an intermediate (not shown on the scheme) between methylmalonic semialdehyde and methylmalonic acid.

The best-studied example of a CoA-dependent nonphosphorylating ALDH is the methylmalonate-semi-aldehyde dehydrogenase, which has been isolated from both mammalian and bacterial sources. This enzyme transforms malonate semialdehyde and methylmalonate semialdehyde into acetyl-CoA and propionyl-CoA, respectively, through an oxidation reaction as described above, followed by a decarboxylation reminiscent of other (3-keto acids. Mechanistic studies of the B. sukilis enzyme have shown that it is activated by NAD" " binding, that it exhibits half-of-sites reactivity (only two moles of NADH forms per tetrameric protein unit) and that the decarboxylation reaction occurs after formation of the acyl-enzyme intermediate. Acyl transfer from the enzyme to CoA completes the reaction. 

Methylmalonate semialdehyde dehydrogenase deficiency [8] has been described in a single patient. This patient, a boy, came to attention because of an elevated concentration of methionine on routine neonatal screening. The value exceeded 1000 imol/l. By 4 years of age he had developed normally. A valine load was followed by an increase in 3-hydroxyisobutyric acid excretion. Incubation of fibroblasts from the patient with 2-valine or p-[1- C]-alanine led to no production of C02 from valine and very little from )ff-alanine in contrast to control cells. 

The next product in the reaction sequence, methylmalonate semialdehyde, cannot be formed from the CoA thiol ester of hydroxyisobutyrate and requires free /3-hydroxy isobutyrate (87). Free /3-hydroxyisobutyrate and DPN react rapidly in the presence of the enzyme. Proof of the formation of the methylmalonate semialdehyde was obtained by cariying out the incubation in the presence of hydroxy lamine and converting the product to the methylmalonic semialdehyde phenylhydrazone. 

Methylmalonate semialdehyde has been shown to be transaminated to form /3-aminoisobutyrate, as well as being decarboxylated to propionyl aldehyde (89). Previously it had been determined that this compound was formed in the metabolism of dihydroth3mine (98, 99). Kupiecki and Coon (89) prepared a transaminase from pig kidney which catalyzes the transamination of methylmalonate semialdehyde with glutamic acid as the amino group donor to /3-aminoisobutyric acid according to Eq. (6). 

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