title: Haem control in experimental porphyria. The effect of haemin on the induction of delta-aminolaevulinate synthase in isolated chick-embryo liver cells.
abstract: 2-Allyl-2-isopropylacetamide-mediated induction of hepatic porphyria was studied in isolated chick-embryo liver cells. Increased delta-aminolaevulinate synthase activity occurred within 1h of induction and continued to increase for 8h. Protoporphyrins synthesized during this time accumulated to a concentration 10-fold greater than that in the control. Removal of 2-allyl-2-isopropylacetamide from the cells by washing at 3h immediately inhibited further increases in delta-aminolaevulinate synthase synthesis. However substitution of 2-allyl-2-isopropylacetamide at 3h by deferoxamine methane-sulphonate, an inhibitor of haem synthesis, allowed continued delta-aminolaevulinate synthase induction at an unaltered rate, even though this agent did not, by itself, induce enzyme synthesis. Exogenously added haemin was shown completely to inhibit 2-allyl-2-isopropylacetamide-mediated delta-aminolaevulinate synthase induction at concentrations as low as 20nm, a value that is less than the reported physiological one. The duration of inhibition was dependent on the concentration of added haemin and was followed by a period of delta-aminolaevulinate synthase synthesis at a rate similar to that of the control. These data are consistent with the hypothesis that delta-aminolaevulinate synthase synthesis is regulated by the concentration of intracellular haem and that induction is initiated by 2-allyl-2-isopropylacetamide-mediated destruction of haem. Induction of delta-aminolaevulinate synthase was shown to be dependent on both RNA and protein synthesis, and a study of the comparative effects of cordycepin, cycloheximide and haem has shown that, at haemin concentrations up to 50nm, the inhibition of delta-aminolaevulinate synthase synthesis followed kinetics similar to the effect of cordycepin, with no synergism between cordycepin and 50nm-haemin. However, at a haemin concentration of 2mum, the inhibition of delta-aminolaevulinate synthase synthesis followed similar kinetics to the effect of cycloheximide. These data demonstrate the control of delta-aminolaevulinate synthase synthesis by low concentrations of haemin and suggests that the primary effect of haemin is at the level of transcription.
mesh_terms: D000624:5-Aminolevulinate Synthetase / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D000502:Allylisopropylacetamide / Q000494:pharmacology; D000818:Animals; D002642:Chick Embryo; D004790:Enzyme Induction / Q000187:drug effects; D006418:Heme / Q000031:analogs & derivatives* / Q000378:metabolism; D006427:Hemin / Q000494:pharmacology*; D066298:In Vitro Techniques; D008099:Liver / Q000187:drug effects / Q000196:embryology / Q000201:enzymology*; D011164:Porphyrias / Q000378:metabolism
chemical_list: D000502:Allylisopropylacetamide; D006418:Heme; D006427:Hemin; D000624:5-Aminolevulinate Synthetase