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

Abstract

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.