CHAPTER 4 - δ-Aminolevulinic Acid Synthetase and the Control of Heme and Chlorophyll Synthesis

Abstract

This chapter provides an overview of δ-aminolevulinic acid synthetase and the control of heme and chlorophyll synthesis. Specialized cells that make large amounts of heme or chlorophyll have been examined to discover the controls involved in the synthesis of these pigments. In most of the cell types studied, the generalization can be made that all the enzymes of heme synthesis, except the first one, are normally present in excess. The activity of this enzyme not only controls the rate of heme synthesis, but in plants it also controls the rate of chlorophyll and bacteriochlorophyll synthesis. In Rhodopseudomonas spheroids, a photosynthetic bacterium, one and the same enzyme controls both heme and bacteriochlorophyll synthesis. In this organism, the enzymes of the Mg branch may be present in only limited sufficiency, but in higher plants the activities of the enzymes of the Mg branch do not limit chlorophyll synthesis. In animals, the physiological stimulus to increased activity of ALA-synthetase appears to be a group of steroids. In the mouse, the induction by these steroids has been shown to be independent of erythropoietin action. Recent experiments suggest that these steroids may act at the transcriptional level possibly to overcome a repressor. Heme administration to rats appears not only to suppress the induction of ALA-synthetase by chemicals, but also to suppress an increase in some other liver microsomal enzymes. Heme administration to rats may also inhibit the transport of soluble ALA-synthetase of the cytosol into the mitochondria.