5- Aminolevulinic acid synthesis

Werck-Reichhart, D., Jones, O.T.G., and Durst, F., Heme synthesis during cytochrome P-450 induction in higher plants. 5-Aminolevulinic acid synthesis through a five-carbon pathway in Helianthus tuberosus tuber tissues aged in the dark, Biochem. J., 249, 473 -80, 1988. 

Regulation of development, with the assembly of multimeric complexes, often requires the integration of diverse biosynthetic pathways. Thylakoid biogenesis is a notable example, since it requires cytosolic and chloroplastic protein synthesis, chlorophyll (Chi) production, iron-sulfur center biosynthesis, etc. Two biochemical probes, gabaculine (3-amino-2,3-dihydro-benzoic acid) and AHA (4-amino-hexynoic acid), permit the study of thylakoid assembly when one of these pathways, Chi biogenesis, is restricted by inhibition at the level of aminolevulinic acid synthesis (1,3). 

Moss CW, Dowell VR, Farshthi D, Raines LJ and Cherry WB (1967) Cultural characteristics and fatty acid composition of Corynebacterium acnes. J Bacteriol 94 1300-1305 Moss CW, Dowell VR, Lewis VJ and Schekter MA (1969) Cultural characteristics and fatty acid composition of propionibacteria. J Bacteriol 97 561-570 Murakami K, Hashimoto Y and Murooka Y (1993) Cloning and characterization of the gene encoding glutamate 1-semialdehyde 2,1-aminomutase, which is involved in 6-aminolevulinic acid synthesis in Propionibacterium freudenreichii. Appl Environ Microbiol 59 347-350... 

FIGURE 2.1.3 Synthesis of 5-aminolevulinic acid (ALA) by the C-5 pathway (from a-ketoglutarate or glutamate) and the C-4 pathway (condensation of succinyl CoA with glycine). 

The most common method used to monitor inorganic Pb is the determination of Pb in whole blood by GF-AAS. Exposure to organic lead (i.e. tetraethyl lead) can be monitored by the determination of Pb in mine by GF-AAS (Christensen and Kristiansen 1994). Early effects of exposure to Pb on the heme synthesis can be monitored by determination of the inhibition of the enayme 8-aminolevulinic acid dehydratase in whole blood or 8-aminolevulinic acid in urine by spectrophotometry. 

Alterations in blood heme metabolism have been proposed as a possible indicator of the biological effects of hydrogen sulfide (Jappinen and Tenhunen 1990), but this does not relate to the mechanism of toxicity in humans. The activities of the enzymes of heme synthesis, i.e., delta-aminolevulinic acid synthase (ALA-S) and heme synthase (Haem-S), were examined in 21 cases of acute hydrogen sulfide toxicity in Finnish pulp mill and oil refinery workers. Subjects were exposed to hydrogen sulfide for periods ranging from approximately 1 minute to up to 3.5 hours. Hydrogen sulfide concentrations were considered to be in the range of 20-200 ppm. Several subjects lost consciousness for up to 3 minutes. 

Lead (soft, Pb2+) Injuries to peripheral nervous system, disturbs heme synthesis and affects kidneys Pb2+ may replace Ca2+ with loss of functional and structural integrity. Reacts with sulfhydryl groups, replaces Zn2+ in 8-aminolevulinic acid dehydratase. 

An extremely important role of iron is the synthesis of haem for formation of erythrocytes and also for proliferating cells for synthesis of the mitochondrial enzymes that contain haem (e.g. cytochromes). The flux-generating enzyme in the synthesis of haem is aminolevulinic acid synthase (ALS) (Figure 15.20). If the cellular iron concentration is low, the concentration of this enzyme is increased in an attempt to maintain the rate of synthesis. As with the other two proteins, the concentration of ALS is controlled at the level of translation in a similar manner to that for transferrin, i.e. by increased stability of the mRNA, which is achieved by the binding of the IRP to the mRNA. 

Figure 15.20 Control of the rate of haem synthesis. The concentration of the enzyme aminolevulinic acid synthase, the first enzyme in the synthesis of haem, and the flux-generab ng enzyme, is increased by IRP. This ensures an adequate rate of synthesis of haem, even though the iron level in the cell may be low. This is achieved by stimulation of translation. Full details of the pathway are presented in Appendix 15.3.

Fig. 7.3.1 The heme synthesis pathway starts in the mitochondrion. The next four steps proceed in the cytosol. Coproporphyrinogen oxidase is in the intermembrane space of the mitochondrion, and the last two enzymes reside at the mitochondral matrix side of the inner membrane. The product heme represses the first and rate-limiting enzyme -aminolevulinic acid (5-ALA) synthase at transcription, during the translation step, and by its transport into the mitochondrion... 

Correct answer = B. The activity of 6-aminole-vulinic acid synthase controls the rate of por phyrin synthesis. The enzyme is increased in patients treated with certain drugs, and requires pyridoxal phosphate as a coenzyme. Another enzyme in the pathway ( -aminolevulinic acid dehydrase) is extremely sensitive to the pres ence of heavy metals. 

Enzyme Inhibition/Activation. A major site of toxic action for metals is interaction with enzymes, resulting in either enzyme inhibition or activation. Two mechanisms are of particular importance inhibition may occur as a result of interaction between the metal and sulfhydryl (SH) groups on the enzyme, or the metal may displace an essential metal cofactor of the enzyme. For example, lead may displace zinc in the zinc-dependent enzyme 5-aminolevulinic acid dehydratase (ALAD), thereby inhibiting the synthesis of heme, an important component of hemoglobin and heme-containing enzymes, such as cytochromes. 

Figure 10.3 Path of synthesis of delta-aminolevulinic acid (coenzyme A abbreviated as CoA). Cadmium tends to inhibit the enzyme responsible for this process.

Figure 10.4 Synthesis of porphobilinogen from delta-aminolevulinic acid, a major step in the overall scheme of heme synthesis that is inhibited by lead in the body.

More specifically, compounds like podolactone A (Fig. 10.1) inhibit proton efflux from plant cells induced by fusicoccin, without affecting ATP levels.42 The related compound, podolactone E is a strong inhibitor of 6-aminolevulinic acid and chlorophyll synthesis.34 The authors concluded that this was caused by suppression of synthesis of proteins needed in the porphyrin pathway because podolactones also inhibited gibberellic acid-induced a-amylase synthesis in barley embryos. The molecular target site(s) of this class of terpenoid phytotoxins remains to be determined. 

A second major lead-induced toxicity involves interruption of heme synthesis. Lead interacts at several steps in the heme biosynthetic pathway (Figure 21.13). As mentioned above, Pb inhibits the enzyme 8-aminolevulinic acid dehydratase (ALA-D), which catalyzes the second step of heme synthesis involving the condensation of two molecules of aminolevulinic acid (ALA) to form porphobilinogen. The result of this inhibition is the accumulation of aminolevulinic acid in the serum and increased excretion of ALA in the urine. A second major disruption of the heme biosynthetic pathway is Pb inhibition of ferrochelatase. This enzyme is responsible for the incorporation of the ferrous ion (Fe2+) into protoporphrin IX to produce heme (Figure 21.2). Accumulated protoporphrin is incorporated into red blood cells and chelates zinc as the cells circulate. This zinc-protoporphrin complex is fluorescent and used to diagnose Pb poisoning. 

Synthesis of S-aminolevulinic acid (ALA) regioselectively labelled with 13C... 

Biomarkers are related to different toxicants and organizational levels. Several examples are given in Table 9.12. Biomarkers range from those that are highly specific to those that are non-specific (a) those highly specific, as the enzyme aminolevulinic acid dehydratase (ALAD), which catalyzes a reaction involved in heme synthesis and is very sensitive to inhibition by inorganic lead (b) those non-specific, as effects in the immune system that can be caused by a wide variety of pollutants. 

---