Succinylacetone (4,6-dioxoheptanoic acid) is an abnormal metabolite produced in patients with hereditary tyrosinemia as a consequence of an inherited deficiency of fumarylacetoacetate hydrolase. It is known that patients with this hereditary disease excrete excessive amounts of delta-aminolevulinic acid (ALA) in urine and that certain patients have an accompanying clinical syndrome resembling that of acute intermittent porphyria (AIP). In order to elucidate the relation of succinylacetone to the heme biosynthetic pathway, we have examined the effects of this metabolite on the cellular heme content of cultured avian hepatocytes and on the activity of purified ALA dehydratase from normal human erythrocytes and from mouse and bovine liver. Our data indicate that succinylacetone is an extremely potent competitive inhibitor of ALA dehydratase in human as well as in animal tissues. By using purified preparations of the enzyme from human erythrocytes and mouse and bovine liver, an inhibitor constant ranging from 2 x 10(-7) M to 3 x 10(-7) M was obtained. In cultured hepatocytes, succinylacetone also inhibited ALA dehydratase activity, decreased the cellular content of heme and cytochrome P-450, and greatly potentiated the induction response of ALA synthase to drugs such as phenobarbital, chemicals such as allylisopropylacetamide and 3,5-dicarbethoxy-1,4-dihydrocollidine, and natural steroids such as etiocholanolone. Four patients with hereditary tyrosinemia have been studied and all were found to have greatly depressed levels of erythrocyte ALA dehydratase activity and elevated concentrations of this inhibitor in urine. These findings indicate that tyrosinemia is a disorder of special pharmacogenetic interest because succinylacetone, an abnormal product of the tyrosine metabolic pathway, resulting from the primary gene defect of the disease, profoundly inhibits heme biosynthesis in normal cells through a blockade at the ALA dehydratase level, leading to clinical and metabolic consequences that mimic another genetic disease, AIP.