 It is thought that this is due to the widespread presence of methylmalonyl-CoA mutase throughout the central nervous system.Due to the loss of functionality of the enzyme, substrate levels build up in the CNS.The substrate, L-methylmalonyl-CoA hydrolyzes to form methylmalonate (methylmalonic acid), a neurotoxic dicarboxylic acid that, due to the poor dicarboxylic acid transport capacities of the blood-brain barrier, is effectively trapped within the CNS, leading to neurological debilitation.