ALA synthetase

Various minor hematological effects have been noted in animals. Rats exposed to 50-800 ppm of trichloroethylene continuously for 48 or 240 hours showed time- and dose-related depression of delta-aminolevulinate dehydratase activity in liver, bone marrow, and erythrocytes (Fujita et al. 1984 Koizumi et al. 1984). Related effects included increased delta-aminolevulinic acid (ALA) synthetase activity, reduced heme saturation of tryptophan pyrrolase and reduced cytochrome P-450 levels in the liver and increased urinary excretion of... 

Fawade MM, Pawar SS. 1983. Effect of NIC12 and cycloheximide on microsomal drug metabolism and ALA-synthetase during thiodemeton toxicity. Indian J Exp Biol 21 343-346. 

Although generalizations regarding the hematological effects of fuel oils on humans cannot be made, the effect of kerosene on the first two steps of the heme synthetic pathway has been studied in an animal model. Both hepatic -aminolevulinic acid ( -ALA) dehydratase and -ALA synthetase activities were decreased in female rats after intraperitoneal injection of kerosene, while heme oxygenase was unaffected (Rao and Pandya 1980). Since -ALA synthetase is the rate-limiting enzyme of the heme biosynthesis pathway, hepatic heme biosynthesis may be inhibited by kerosene. It is conceivable that this may be related to the extramedullary hematopoiesis reported in other studies (NTP/NIH 1986) however, there are no direct data to support this. 

Although not specific for kerosene, aminolevulinic acid (ALA) could potentially be used as an adjunct or supplemental biomarker for kerosene exposure. Kerosene may affect heme metabolism by decreasing the activities of enzymes in the heme biosynthetic pathway (hepatic -ALA dehydratase and -ALA synthetase) (Rao and Pandya 1980). Therefore, it may be possible that this effect would generate increased ALA in the urine of exposed individuals. Additional studies of acute, intermediate, and chronic exposure are needed to identify biomarkers of effects for specific target organs following exposure to fuel oils. 

Scheme 17 Proposed mechanism and stereochemistry for ALA-synthetase RCHO = pyridoxal phosphate...

This mitochondrial enzyme (ALA synthetase) catalyzes the formation of 6-aminolevulinic acid (ALA) from glycine and succinyl-CoA. This is the initial step in heme biosynthesis. 

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. 

All of the nitrogen in heme is derived from glycine and all of the carbons are derived from succinate and glycine. Thus, the process by which heme is synthesized is also called the succinate-glycine pathway. The first step in the process is catalyzed by a pyridoxal phosphate-containing enzyme, 5-aminolevulinic acid synthetase (ALA synthetase)... 

The major regulatory step in heme synthesis is the ALA synthetase reaction. Through feedback inhibition, heme regulates the enzyme. Heme also inhibits the translation of ALA synthetase. At even higher levels, heme blocks transport of ALA synthetase to the mitochondrion - its site of action. 

Note, there are other pyridoxal-P-dependent enzymes that have an inversion step (ALA-synthetase, racemases). 

Pyridoxal-P-dependent 5-aminolevulinate synthetase catalyses the formation of 5-aminolevulinic acid from glycine and succinyl-CoA, and this constitutes the first irreversible step on the main pathway leading to the biosynthesis of tetrapyrroles. ALA synthetase [61] has been purified from several bacterial, avian and mammalian sources, but the amount of enzyme obtained in these preparations is relatively small. Light-grown Rhodopseudomonas spheroides is at present the best source of the... 

Fig. 21. A mechanism for the formation of 3-dehydrosphinganine by a pyridoxal-P-dependent enzyme. Unlike the ALA synthetase reaction, in this case the C-C bond formation only occurs following decarboxylation.

Fig. 23. Two possible steric courses for the ALA synthetase reaction. In the upper sequence the replacement of a C-H bond by a C-C bond occurs with retention and the decarboxylation process with inversion. The converse is the case in the lower sequence.

With these key pieces of information in hand the overall steric course of the ALA synthetase reaction can be deduced. Since the transformation affects two different bonds at C it is necessary that the intermediate (Fig. 20, 3) should undergo a substantial conformational change at the active site if the cleavage of both are to be stereoelectronically assisted by the w-system of the Schiff base. Of course it is not known whether the cleavages of the C -C02H bond and the C -H bond occur on the same face or different faces of the w system. Nevertheless the overall steric course... 

V.P. Sweeney, M.A. Pathak, A.K. Asbury (1970). Acute intermittent porphyria. Increased ALA-synthetase activity during an acute attack. Brain, 3, 369-380. 

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