8-Aminolevulinate, biosynthesis

Aminolevulinate is the precursor from which the large class of alkaloids called tetrapyrroles are hiosynthesized. It arises by a PLP-dependent reaction of glycine and succinyl CoA. Review the mechanism of the formation of dopamine from L-dopa in Figure 25.7, and propose a mechanism for 5-aminolevulinate biosynthesis. 

Beyond pharmaceutical screening activity developed on aminothiazoles derivatives, some studies at the molecular level were performed. Thus 2-aminothiazole was shown to inhibit thiamine biosynthesis (941). Nrridazole (419) affects iron metabohsm (850). The dehydrase for 5-aminolevulinic acid of mouse liver is inhibited by 2-amino-4-(iS-hydroxy-ethyl)thiazole (420) (942) (Scheme 239). l-Phenyl-3-(2-thiazolyl)thiourea (421) is a dopamine fS-hydroxylase inhibitor (943). Compound 422 inhibits the enzyme activity of 3, 5 -nucleotide phosphodiesterase (944). The oxalate salt of 423, an analog of levamisole 424 (945) (Scheme 240),... 

Lead-induced anemia results from impairment of heme biosynthesis and acceleration of red blood cell destmction (10,13). Lead-induced inhibition of heme biosynthesis is caused by inhibition of S-aminolevulinic acid dehydratase and ferrochelatase which starts to occur at blood lead levels of 10 to 20 pu gjdL and 25 to 30 //g/dL, respectively (10,13). Anemia, however, is not manifested until higher levels are reached. 

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

The biosynthesis of the tetrapyrrole macrocycle and its branches leading to haem and chlorophylls has been covered in detail in several reviews - - and will be concisely described in this section. Tetrapyrrole biosynthesis occurs entirely in the plastids and is composed of several enzymatic steps starting from 5-aminolevulinic acid (ALA), which is the key precursor of porphyrins and the source of their carbon and nitrogen. 

Beale, S.I. and Castelfranco, P.A., The biosynthesis of 5-aminolevulinic acid in plants. II. Formation of C-5-Aminolevulinic acid from labeled precursors in greening plant tissues, Plant Physiol, 53, 297, 1974. 

Beale, S.I., 5-Aminolevulinic acid in plants its biosynthesis, regulation and role in plastid development, Annu. Rev. Plant Physiol, 29, 95, 1978. 

In summary, lead inhibits the activity of certain enzymes involved in heme biosynthesis, namely, 5-aminolevulinic acid dehydratase (ALAD), and ferrochelatase. As a consequence of these changes, heme biosynthesis is decreased and the activity of the rate limiting enzyme of the pathway,... 

Severe hypochromic microcytic anemia, responding only to vitamin B6 and not to iron, a typical symptom of B6 deficiency in many species of animals, is related to the dependence of porphyrin biosynthesis on vitamin Be, preceding the 8-aminolevulinic acid stage, at the condensation of glycine with succinate to yield a-amino- 3-ketoadipate, the immediate precursor of 8-aminolevulinic acid. 

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. 

Figure 9-7. Pathway for heme biosynthesis. Regulation by hemin at the key aminolevulinic acid (ALA) synthase-catalyzed step and by lead at two steps is indicated.

Wang, W.-Y. Gough, S.P. Kannangara, C.G. Biosynthesis of A-aminolevulinate in greening barley leaves IV. Isolation of three soluble enzymes required for the conversion of glutamate to A-aminolevulinate. Garlsberg Res. Gom-mun., 46, 243-257 (1981)... 

FIGURE 22-23 Biosynthesis of S-aminolevulinate. (a) In mammals and other higher eukaryotes, S-aminolevulinate is synthesized from glycine and succinyl-CoA. The atoms furnished by glycine are shown... 

Formation of S-aminolevulinic acid (ALA) All the carbon and nitrogen atoms of the porphyrin molecule are provided by two simple building blocks glycine (a nonessential amino acid) and succinyl CoA (an intermediate in the citric acid cycle). Glycine and succinyl CoA condense to form ALA in a reaction catalyzed by ALA synthase (Figure 21.3) This reaction requires pyridoxal phosphate as a coenzyme, and is the rate-controlling step in hepatic porphyrin biosynthesis. 

The biosynthesis of S-aminolevulinate is known to occur with the loss of one glycine carbon-bound hydrogen, producing the intermediate shown in figure 22.13. What coenzyme would you expect to participate in this process How does the same coenzyme stabilize the carbanion formed in the decarboxylation part of the reaction ... 

Aminolevulinic acid, an element in the biosynthesis of heme proteins, may be overexpressed under certain pathological conditions and its accumulation has been correlated with some hepatitic cancers. It is in equilibrium with its enol form and can complex transition-metal ions. In the presence of 02, this may lead to the formation of OH and hence in the presence of DNA to DNA damage which is enhanced in the presence of ferritin (Douki et al. 1998 di Mascio et al. 2000). Superoxide radicals have been assumed to be intermediates in these reactions. Mechanistically, the formation of 02 is certainly very complex, because even in the case of the Fe(II)-EDTA-complex the reduction potential is not low enough to reduce O2 by simple one-electron donation. 

Figure 7.13 The heme biosynthesis pathway. ALA, 5-aminolevulinic acid PBG, porphobilinogen uro gen, uroporphyrinogen copro gen, coproporphyrinogen prot gen, protoporphyrinogen Ac, acetate Pr, propionate vi, vinyl. (Reproduced by permission from Preface. Enzyme 28 91-93, 1982.)...

Primary amine catalysis (usually involving a lysine residue) has been recognised to play an important role in various enzyme-catalysed reactions. Examples are the conversion of acetoacetate to acetone catalysed by acetoacetate decarboxylase, the condensation of two molecules of S-aminolevulinic acid catalysed by -aminolevulinic deshydratase during the biosynthesis of porphyrins, and the reversible aldol condensation of dihydroxy-acetone phosphate with glyceraldehyde which in the presence of aldolase yields fructose-1-phosphate (64) (For reviews see, for example, Snell and Di Mari,... 

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. 

Several other iron metabolism proteins contain IREs, including ferroportin, an iron exporter, the erythrocyte form of aminolevulinic acid synthase, an enzyme important in heme biosynthesis, an alternatively spliced transcript of the iron transporter DMTl, and mammalian mitochondrial aconitase. The importance of these IREs in regulation of these transcripts is the subject of ongoing research. 

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