Soluble -aminolevulinic acid synthetase of rat liver. II. Studies related to the mechanism of enzyme action and hemin inhibition.

The present study describes some characteristics of soluble hepatic d-aminolevulinic acid synthetase, including end product inhibition by hemin. The enzyme is shown to contain a -SH group at its active center which is associated with the initial binding of the cofactor pyridoxal-S/-PO4 to the enzyme. In addition, the -SH group may influence the activation of d-aminolevulinic acid synthetase by divalent cations. A mechanism is suggested for the catalytic formation of krninolevulinic acid from glycine and succinyl-CoA. From data derived from isotopic studies with [l-14C]glycine, and studies utilizing specific inhibitors, the following sequence of reactions resulting in the formation of d-aminolevulinic acid is proposed. The initial formation of a thiohemiacetal between the enzyme and pyridoxal-5’P04; the subsequent formation of a Schiff base with glycine which is followed by its condensation with succinyl-CoA; the enzymatic reaction is terminated with the decarboxylation of the glycyl carboxyl group in association with the release of krninolevulinic acid from the enzyme. End product inhibition of the mammalian enzyme was confirmed using hemin, which was demonstrated to have a KI of 2 x 1O-5 M. In addition, various metalloporphyrins, porphyrins, and bilirubin also inhibit d-aminolevulinic acid synthetase activity. The relative potencies of the inhibitors appear to be dependent on the net charge present on the tetrapyrrole.