ABSTRACT
Patients with methylmalonic aciduria and homocystinuria have defective metabolism of cobalamin to both cofactors, methylcobalamin and deoxyadenoxylcobalamin. Accordingly, the activities of methionine synthase and methylmalonyl CoA mutase are defective (see Figure 4.1). Patients with impaired synthesis of methylcobalamin and deoxyadenosylcobalamin
fall into two distinct complementation groups designated Cbl C and Cbl D. Another group of patients designated Cbl F have defective transport of free cobalamin out of lysosomes. The differential diagnosis of methylmalonic acidemia and homocystinuria is given in Table 4.1.
