Congenital defect in intracellular cobalamin metabolism resulting in homocystinuria and methylmalonic aciduria. II. Biochemical investigations.

Biochemical investigations are reported in an infant with methylmalonic aciduria and homocystinuria who died at 4 months of age. Postmortem analysis of liver obtained 2 weeks after the child was treated with vitamin B12 revealed deficient activity of both cobalamin dependent enzymes: N5-methyltetrahydrofolate: homocysteine methyltransferase (requiring Me-Cbl), and methylmalonyl CoA mutase (requiring Ado-Cbl). MMA-CoA mutase activity could be restored to normal in vitro by added Ado-Cbl, but MeTHF-HCy transferase activity was not significantly enhanced by addition of Me-Cbl. Though the serum total cobalamin was normal, total cobalamin in liver and kidney was abnormally low. In the kidney Me-Cbl and Ado-Cbl were disproportionally decreased whereas in the liver only Ado-Cbl was significantly reduced. This suggests that at least some of the CN-Cbl administered was converted to the coenzymes in liver which would explain the reduction of MMA- and HCy-excretion during therapy. The results show 1. that this infant suffered from a congenital defect in one of the steps of intracellular cobalamin metabolism or transport common to the synthesis of both coenzymes, 2. that life-long treatment with vitamin B12 (OH-Cbl) may be of value in similar cases, particularly if given early in the course of the disease.