Aminolevulinate mechanism

Chlorophyll, heme, vitamin B,2, and a host of other substances are bio-synthesized from porphobilinogen (PEG), which is itself formed from condensation of two molecules of 5-aminolevulinate. The two 5-aminolevulinates are bound to lysine (Lys) amino acids in the enzyme, one in the enamine form and one in the imine form, and their condensation is thought to occur by the following steps. Using curved arrows, show the mechanism of each step. 

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. 

Goering PL, BA. 1985. Mechanisms of renal lead-binding protein protection against lead-inhibition of delta-aminolevulinic acid dehydratase. J Pharmacol Exp Ther 234 365-371. 

Novotny A, Xiang J, Stummer W, Teuscher NS, Smith DE, Keep RF. Mechanisms of 5-aminolevulinic acid uptake at the choroid plexus. J Neurochem 2000 75(1) 321—328. 

Hockin, L.J. and Paine, A.J. (1983). The role of 5-aminolevulinate synthetase, haem oxygenase and ligand formation in the mechanism of maintenance of cytochrome P-450 concentration in hepatocyte culture. Biochem. Pharmacol. 210 855-857. 

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. 

Iron regulatory proteins (IRPs) regulate the cellular iron level in mammalian cells. IRPs are known as cytosol mRNA binding proteins which control the stability or the translation rate of mRNAs of iron metabolism-related proteins such as TfR, ferritin, and 5-aminolevulinic acid synthetase in response to the availability of cellular iron [19-21] after uptake [5]. The regulatory mechanism involves the interaction between the iron-responsive element (IRE) in the 3 or 5 untranslated regions of the transcripts and cytosolic IRPs (IRP-1 and -2). IRP-1 is an iron-sulfur (Fe-S) protein with aconitase activity containing a cubane 4Fe-4S cluster. When Fe is replete, IRP-1 prevails in a 4Fe-4S form as a holo-form and is an active cytoplasmic aconitase. As shown in Fig. 3, when Fe is deplete, it readily loses one Fe from the fourth labile Fe in the Fe-S cluster to become a 3Fe-4S cluster and in this state has little enzymatic activity [22, 23]. 

Heme synthesis is controlled primarily by 8-aminolevulinate synthase (ALA synthase). There are two mechanisms of control, and each involves a process that affects the concentration of the enzyme. First, the half-life of ALA synthase, as shown by experiments in rat liver, is very short (60-70 min). Like many mitochondrial proteins, ALA synthase is encoded by nuclear genes, synthesized on cytoplasmic ribosomes, and the enzyme is translocated into the mitochondria. The second and main regulating factor is the inhibition of ALA synthase by hemin. Hemin differs from heme in that the Fe atom is in the Fe3+ oxidation state. Heme spontaneously oxidizes to hemin when there is no globin to form hemoglobin. Hemin serves a second function in the regulation of hemoglobin synthesis in reticulocytes. It controls the synthesis of globin. 

The hematopoietic system is affected by both short- and long-term arsenic exposure. Arsenic is known to cause a wide variety of hematological abnormalities like anemia, absolute neutropenia, leucopenia, thrombocytopenia, and relative eosinophilia - more common than absolute esino-philia, basophilic stippling, increased bone marrow vascularity, and rouleau formation (Rezuke et al, 1991). These effects may be due to a direct hemolytic or cytotoxic effect on the blood cells and a suppression of erythropoiesis. The mechanism of hemolysis involves depletion of intracellular GSH, resulting in the oxidation of hemoglobin (Saha et al, 1999). Arsenic exposure is also known to influence the activity of several enzymes of heme biosynthesis. Arsenic produces a decrease in ferrochelatase, and decrease in COPRO-OX and increase in hepatic 5-aminolevulinic acid synthetase activity (Woods and Southern, 1989). Subchronic... 

Finally, it is widely known that Pb impairs the formation of red blood cells. The mechanism involved in the impairment is that Pb inhibits both 5-aminolevulinic acid dehydratase (ALA-D) (Hernberg et al. 1970) and ferrochelatase (Tephly et al. 1978). These are two key enzymes involved in heme biosynthesis. ALA-D catalyzes the conversion of 5-aminolevulinic acid into porphobilinogen (PBG), whereas ferrochelatase is responsible for catalyzing the incorporation of Fe2+ into protoporphyrin IX to form heme (Figure 9.1). Lead inhibition of the two enzymes appears to be due to its interaction with Zn and Fe required in the process. 

The tris(mercaptoimidazolyl) ligand TmAr has been employed to emulate the coordination environment of 5-aminolevulinate dehydratase (ALAD). Several ZnX(TmAr) have been described145-147 that also have helped in the knowledge of the mechanism of action of ALAD. To study the lead poisoning that is associated with lead interaction with ALAD, PbX(TmAr) complexes have been also reported.148... 

The biosynthesis of porphyrin involves the formation of porphobilinogen from two molecules of S-aminolevulinic acid. The precise mechanism for this biosynthesis is as yet unknown. A possible mechanism starts with the formation of an imine between the enzyme that catalyzes the reaction and one of the molecules of S-aminolevulinic acid. An aldol-type condensation occurs between the imine and a free molecule of d-aminolevulinic acid. Nucleophilic attack by the amino group on the imine closes the ring. The enzyme is then eliminated, and removal of a proton creates the aromatic ring. 

J.C. Kennedy, S.L. Marcus, R.H. Pottier (1996). Photodynamic therapy (PDT) and photodiagnosis (PD) using endogenous photosensitization induced by 5-aminolevulinic acid (ALA) mechanisms and clinical results. J. Clin. Laser Med. Surg., 14, 289-304. 

Chlorophyll synthesis in plants is regulated at steps converting glutamate into S-aminolevulinate (ALA). The stromal protein glutamate 1-semialdehyde (GSA) aminotransferase catalyzes the last reaction in the synthesis of ALA. We identified and sequenced a c-DNA clone encoding the GSA-aminotransferase in order to study the expression of this enzyme and its catalytic mechanism of transamination. 

Up to the levei of protoporphyrin IX, C. biosynthesis is the same as that of the Porphyrins (see), bearing in mind that different mechanisms exist for the biosynthesis of S-aminolevulinate, depending on the organism. Conversion of protoporphyrin IX into Cm is shown in Fig. 3. The final steps of biosynthesis appear to take place in situ in the thylakoid membrane. [Biosynthesis of Heme and Chlorophylls, H. A. Dailey (ed.) McGraw Hill, 1990 S.B. Brown etal. J. Photo-chertu Photobiol., B Biology S (1990) 3-23]... 

We shall attempt in this review to summarize some facts and hypotheses concerning the mechanisms by which heme and chlorophyll are controlled at the first step of the biosynthetic chain—namely, at the level of the formation of (5-aminolevulinic acid (ALA). No complete literature review is intended. A few of the more comprehensive recent references are given in the first part of the bibliography [1-12]. 

Mechanical damage provides one model for a diseased skin state [112, 119, 120]. Mechanical damage and stratum comeum removal by tape stripping can iuCTease transepidermal water loss (TEWL) to a level likely attained in various dermatoses and is the most conunonly utilized method of barrier perturbation due to its simplicity. Abrasion by means of a brash bristle or a needle drawn across the skin s surface has been shown, by transmission electron microscopy, to loosen the top layers of the skin [117]. Abrasion may also enhance and control the delivery of vitamin C, 5-aminolevulinic add, vaccines, and biopharmaceuticals [121-124]. However, mechanical damage has infrequently been anployed either in vitro or in vivo human skin as a means to study absorption enhaneement. 

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. 

---