The nature of the inhibition of δ-aminolevulinic acid synthetase by hemin

Abstract

The feedback inhibition of δ-aminolevulinic acid (ALA) synthetase by hemin 2 2 Hemin is ferric heme and hemichrome is ferric haemochrome; heme and hemochrome unless otherwise specified are used as generic names denoting no particular iron valence state. in cell-free extracts of Rhodopseudomonas spheroides was greatly diminished by the prior conversion of the hemin to the bis-imidazole chelate, suggesting that hemin inhibits by forming, through its iron atom, a coordination complex with the enzyme. This suggestion was further confirmed by the finding that the ability of a number of ligands to diminish hemin inhibition was correlated with their affinities for hemin. Increase in imidazole concentration to approximately 0.1 m partly reversed hemin inhibition of ALA synthetase of R. spheroides but above this concentration imidazole inhibited ALA synthetase even in the absence of hemin. The protection afforded ALA synthetase by albumin against inhibition by hemin was found to be due to nonspecific adsorption of the metalloporphyrin to the albumin. The affinities of a number of ligands for ferrous and ferric protoheme were determined and the extinction coefficients for these hemochromes are reported. In addition, the effect of pH on the affinity of imidazole for ferrous and ferric protoheme is described.