ALA synthetase induction

Species Control on ALA-synthetase Induction of ALA-synthetase Effect of heme Other controls... 

It seems likely that under normal conditions there is a mechanism for repression of this enzyme induction, and it has indeed been shown that haem and other metal-containing porphyrins repress ALA-synthetase induction in foetal liver cells [82]. 

In a clinical study it has been emphasized not only how the decreased food intake in for example slimming, induced attacks in 11% of patients with acute intermittent porphyria, but they also demonstrated how the urinary output of porphyrin precursors varied inversely with the amount of carbohydrate and protein in the diet [94]. The inhibitory effect of carbohydrate feeding on ALA-synthetase induction has also been demonstrated experimentally [95, 96, 97]. 

Commercial PCB Mixtures. Urinary coproporphyrin levels were increased in rats that ingested 0.3 or 1.5 mg/kg/day Aroclor 1242 in the diet for 2-6 months (Bruckner et al. 1974). Rats treated with 5 mg/kg/day Aroclor 1254 in the diet had maximum increases in liver microsomal P-450 concentration and liver weight after 1 week, but onset of porphyria and induction of 5-aminolevulinic acid (ALA) synthetase was delayed until 2-7 months of treatment (Goldstein et al. 1974). A marked accumulation of uroporphyrins occurred in the liver, and urinary excretion of coproporphyrin and other porphyrins was increased the largest increase was in uroporphyrins. The uroporphyrins in the liver and urine of the treated rats consisted primarily of 8- and 7-carboxyporphyrins. 

Normally, the activity of ALA-synthetase in mitochondria is very low, and in the cytosol it is not detectable. When ALA-synthetase is induced in an animal the activity rises ten to forty times, and the enzyme is then found also outside the mitochondria [Granick and Urata, 35]. Studies by Hayashi et al. [33] have shown that as much as 35% of the enzyme may be obtained in the supernatant from an induced rat liver homogenate after centrifugation at 77,000 g for 2 hours. The soluble enzyme may represent newly synthesized enzyme on the way to being incorporated into the mitochondrion, as suggested by studies with marker enzymes of the mitochondria. When the activities were determined in vivo using inhibitors of protein synthesis, the soluble enzyme was estimated to have a half-life of 20 minutes, compared to 1 hour for the mitochondrial enzyme. ALA-synthetase is relatively more stable in the isolated mitochondrion [35] than in vivo. Whether its first-order decay in vivo occurs inside or outside the mitochondrion is unknown, nor is it known whether the enzyme, once transported into the mitochondrion, can be transported out again. More recently Beattie and Stuchell [35a] have shown that after induction with AIA, ALA synthetase activity in the rat liver first accumulates in the cytosol and then is transferred into the mitochondria. 

One of the most active inducing compounds is Lindane. It is about 100 times as active as AIA in porphyrin induction in chick embyro liver culture as well as in the increase in ALA-synthetase activity [21]. The relative activities of isomers of hexahydrohexachlorobenzene for porphyrin production, are Lindane, the y-isomer = 1, a =0.5, P = 0.3, E =0.25, 5 =0.15. These results may in part reflect the relative stability of the isomers whose structures are given by Hornstein [67b]. The hexahydrobromo derivative had no activity [67a]. 

The use of chick embryo liver culture in vitro as contrasted to induction in the whole animal permitted the inference that the inducing chemicals acted directly on the hepatic cells and not via other stimuli generated from other organs. The porphyrins formed could be seen by fluorescence microscopy to accumulate in the c)doplasm of the liver parenchyma cells and were identified mainly as coproporphyiin with some protoporphyrin [25]. However, Doss [70] found mainly protoporphyrin. This difference may depend on conditions of culture the older cultures had more coproporphyrin and uroporphyrin. The increase in porphyrins in tissue culture has been assumed to be a result of an increase in ALA-synthetase. With the aid of 13-cm-diameter petri dishes, it has been possible to grow sufficient numbers of cells and demonstrate with inducing chemicals an increase in vitro of ALA-synthetase activity of seven to ten times that of controls, in a 20-hour period [21]. 

The chemicals induce only in liver, not in other tissues. The steroids, as will be discussed later, induce ALA-synthetase in liver cells as well as in erythroid cells of the chick blastoderm. Although the kidneys of the chick embryo may rapidly convert ALA to porphyrins, neither steroids nor other inducing chemicals have an inducing effect on this tissue. Thus, response to inducing compounds depends on the tissue. In the liver, at least of the chick embryo, both steroids and other chemicals induce in erythropoietic blastoderm only steroids induce and in kidneys and other tissues none of these compounds induce. Steroid induction in liver and erythropoietic tissue appears to involve a transcriptional mechanism induction by chenucals appears to involve mainly the translational mechanism (see Section V). 

After it had been shown in chick embryo liver culture treated with inhibitors that RNA and protein synthesis were required for induction of ALA-synthetase [24,25], these results were confirmed in whole-animal studies [77-79]. Narisawa and Kikuchi [79] were able to detect an increase of ALA-synthetase in the liver 1 to 2 hours after a single subcutaneous dose of 400 mg of ALA per kilogram. It was also shown that not only porphyrins but heme synthesis was increased in chemical porphyria. DeMatteis and Rimington [80], using 2- " C-glycine or Fe, found an increased heme labeling in the presence of the inducers sedormid, AIA, and griseofulvin. It was not determined whether total heme was increased or whether heme turnover had increased. 

In this section, the activities of inducing chemicals are discussed in relation to their destruction by the liver microsomal oxygenase system the rate of heme synthesis and breakdown is considered in relation to the inhibitory properties of heme on the synthesis of ALA-synthetase and the contrasting effects of glucose and starvation are summarized in relation to their effects on induction. 

J. Effects of Fasting and Glucose Feeding on the Induction of ALA-Synthetase and the Microsomal Oxygenase System... 

The induction is specific, not general. The increase in ALA-synthetase induced by AlA or DDC is not due to an overall rate of synthesis of mRNA or protein, for neither radioactive orotic acid nor uridine, nor radioactive leucine was incorporated at a greater rate in the presence than in the absence of the inducers [25, 128]. 

Direct evidence supporting the idea that chemical induction results in an increased synthesis of mRNA for ALA-synthetase has been reported by Hickman et al. [129]. After induction in the rat with AIA, the RNA was isolated from the liver and added to a culture medium containing chick embryo liver fragments. After incubation for 18 hours, the induced RNA, as compared to noninduced RNA, had caused a... 

A summary of the compounds that directly or indirectly affect the chemical induction of ALA-synthetase in in vitro culture is provided in Table IV. 

We have discussed the effect of heme in the liver to decrease the induction of ALA-synthetase, and to decrease the induction of microsomal oxygenase enzymes. Heme also has a positive metabolic action in erythroid cells it stimulates globin synthesis. 

A role for heme in the repression of ALA synthetase has been proposed by Burnham and Lascelles (1963) and by Graniek and Kappas (1967). Heme has been shown to repress the synthesis of ALA synthetase in Khodopseudomonas spheroides (Bmnham and Lascelles, 1963). In liver cells and erythroid cells of the chick embryo, Gio and C21 j8-H steroids induce heme synthesis. This induction can be blocked by actinomycin D, by puromycin, and by added hemin (Graniek and Kappas, 1967). Graniek and his associates have proposed that heme... 

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