text: Although enzymes are generally specific towards their substrate, enzymatic side activities (enzyme promiscuity) can lead to toxic or useless products. These side reactions proceed at much lower rates than their normal physiological reactions, but build-up of damaged metabolites can still be significant over time. For example, the mitochondrial malate dehydrogenase reduces alpha-ketoglutarate to L-2-hydroxyglutarate 10 times less efficiently than its regular substrate oxaloacetate, but L-2-hydroxyglutarate can still accumulate to several grams per day in a human adult.