The Regulation of Heme Synthesis: Studies on Erythrocyte Aminolevulinic Acid Synthetase

Abstract

The synthesis of aminolevulinic acid (ALA) is the first in a series of enzymatic reactions leading to the formation of tetrapyrrols and heme. The reactions shown on this slide as well as the final two steps in heme synthesis are localized within mitochondria. This fact has implicated the synthesis of ALA as the site of a feedback control mechanism whereby heme synthesis may be regulated i and integrated with the formation of other parts of the hemoglobin molecule. The following studies were performed: (1) to characterize the kinetics of the synthesis iff"% O$ ALA-for enzyme obtained from chicken erythrocytes and (2) to identify and ascertain the importance of potential physiological inhibitors in controlling the rate of heme synthesis. A crude preparation of mitochondria capable of carrying out the reactions shown on this slide was obtained from reticulocytes of anemic chickens. The preparation was free of hemoglobin and enzymes utilizing ALA so that its formation could be followed. Incubations for one hour were performed at pH optimal .for ALA synthetase varying the concentration of substrate to permit construction of standard doublereciprocal Lineweaver-Burke plots. In the present studies Succinyl CoA was generated either from c(ketoglutarate or from sudcinate in the presence of optimal concentrations of cofactors for the reactions, as npted on the slide, and relying upon dKG oxidase and succinyl CoA synthetase known to be present in mitochondria of the preparation.

The Relationship between Haem and Globin Synthesis by Erythroid Precursors in Refractory Normoblastic Anaemia

“…They found the most striking effects were observed in precursor cells derived from animals rendered iron deficient by diet or phlebotomy. The increased formation of globin secondary to haem synthesis is in keeping with the hypothesis of the control and co-ordination of the synthesis between haem and globin (Vavra 1967).…”

“…Recently it has been shown that the characteristic disorder in this disease is a defect in the rate of synthesis of either the alpha or beta chain with diminished formation of globin (Bank & Marks 1966). Since there is apparently a regulatory mechanism for haem formation by globin (Vavra 1967) diminished globin synthesis will reduce the formation of haem. The disproportionately low specific activity of glycine-2-I4C in haem as compared to that in globin can be explained by the striking reduction in ALA synthesis and haem synthesis enzymes in bone marrow aspirates from four patients with thalassaemia major (Steiner et al 1964).…”

Depression of Bone Marrow Delta‐Aminolaevulic Acid Synthetase Activity in Erythroleukaemia

“…Aoki et al (1971) reported an inhibitory effect of ferrous iron on partially purified ALA synthetase from rabbit reticulocytcs. However, in chicken reticulocyte mitochondria iron had an enhancing effect on ALA syiithetasc activity (Vavra, 1967) and in rabbit mitochondria, ferrous iron had no effect on ALA synthetase activity in concentratioiis ranging from 1.7x I O -~ to 1.7x I O -~ M (Bottomley & Smithee, 1968).…”