Heme synthesis, enzymes

Effect. Potential biomarkers of the subclinical effects of hydrogen sulfide are decreases in the activities of the heme synthesis enzymes, ALA-S and Haem-S (Jappinen and Tenhunen 1990). These effects have nothing to do with the mechanism of toxicity, however. Neurological indices are also used as biomarkers of effect for hydrogen sulfide (Gaitonde et al. 1987 Kilbum 1993 Stine et al. 1976 Tvedt et al. 1991b). 

C. Variegate porphyria patients present with the same, but generally milder, symptoms as AlP. The emission wavelength at 626 nm is characteristic of protoporphyrin IX the compound is responsible for the skin sensitivity not observed in AIR PCT can also be ruled out because the characteristic fluorescence emission for uroporphyrin is 615 nm. Homozygous deletion of PPO is unlikely because the patient s symptoms were triggered by stress in the form of caloric restriction. Patients with homozygous deficiency in the heme synthesis enzymes show early onset and more severe symptoms. 

We devised a screen for isolating mutants defective in iron-dependent regulation of heme biosynthesis that did not require prior knowledge of the mechanism or of the rate-limiting steps [83]. We speculated that if the pathway as a whole were regulated by iron, a mutant defective in that control would accumulate protoporphyrin under iron limitation. Mutants defective in the heme synthesis enzymes ferrochelatase [75] or protoporphyrinogen oxidase would likely have a similar phenotype, but porphyrin accumulation would likely be independent of iron in the structural gene mutants, and those strains would also be expected to be heme auxotrophs. 

FIGURE 17-5. Effects of lead on heme synthesis. Enzymes inhibited by lead are in italics. ALA aminolevulinic acid -ALAD = 5 amlnolevulinic acid dehydrase sucdnyl CoA = succinyl coenzyme A. 

The final step in heme synthesis involves the incorporation of ferrous iron into protoporphyrin in a reaction catalyzed by ferrochelatase (heme synthase), another mitochondrial enzyme (Figure 32-4). 

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. 

Oxygen activation is a central theme in biochemistry and is performed by a wide range of different iron and copper enzymes. In addition to our studies of the dinuclear non-heme iron enzymes MMO and RNR, we also studied oxygen activation in the mononuclear non-heme iron enzyme isopenicillin N synthase (IPNS). This enzyme uses O2 to transform its substrate ACV to the penicillin precursor isopenicillin N [53], a key step in the synthesis of the important P-lactam antibiotics penicillins and cephalosporins [54, 55],... 

A marked interference with heme synthesis results in a reduction of the hemoglobin concentration in blood. Decreased hemoglobin production, coupled with an increase in erythrocyte destruction, results in a hypochromic, normocytic anemia with associated reticulocytosis. Decreased hemoglobin and anemia have been observed in lead workers and in children with prolonged exposure at higher PbB levels than those noted as threshold levels for inhibition or stimulation of enzyme activities involved in heme synthesis (EPA 1986a). 

Answer D. Sideroblastic anemia in a person being treated for tuberculosis (with isoni-azid) is most likely due to vitamin deficiency. 6-Aminolevulinate synthase, the first enzyme in heme synthesis requires vitamin Bj (pyridoxine). 

Regulation of heme synthesis in erythroid cells occurs at enzymes catalyzing... 

These diseases arise from excessive expression of ALA synthase, coupled with deficiencies in other enzymes that operate downstream in the heme synthesis pathway. 

The answer is C. The patient s symptoms represent a composite of neurologic and gastrointestinal dysfunction, which are consistent with the anemia that is due to lead poisoning. Testing for lead would be appropriate for the patient, the other members of the household, and the house itself. Inorganic lead produces the microcytic anemia by inhibition of heme synthesis in erythropoietic cells of the bone marrow. All the other options represent enzymes of heme synthesis or degradation, but none of them are affected by lead. 

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... 

Although this may seem straightforward, in some cases, the response is only indirectly related and is therefore not a useful parameter of toxicity to use in a dose-response study. This may apply to situations where enzyme inhibition is a basic parameter but where it may not relate to the overall toxic effect. For example, inhibition by lead of aminolaevulinic acid dehydrase, an enzyme, which is involved in heme synthesis, can be readily demonstrated to be dose related, but is clearly not an appropriate indicator of lead-induced renal toxicity in vivo. 

As well as induction of the synthesis of the apoprotein portion of cytochrome P-450, there is also induction of the synthesis of the heme portion. Clearly, it is also necessary to have an increased amount of heme if there is an increase in the amount of the enzyme apoprotein being synthesized. Thus, the rate-limiting step in heme synthesis, the enzyme 5-aminolaevulinate synthetase, is inducible by both phenobarbital and TCDD. This is the result of transcriptional activation of the gene, which codes for the S-aminolaevulinate synthetase. It may be that the decrease in the heme pool, which results from incorporation of heme into the newly synthesized apoprotein, leads to derepression of the gene and hence increased mRNA synthesis. The gene repression could be heme-mediated, or heme may modulate P-450 genes. 

Reduced synthesis. The synthesis of enzymes may be decreased, resulting in a decrease in the in vivo activity. With cytochrome P-450 there are a number of ways in which this occurs. Thus, administration of the metal cobalt to animals will decrease levels of cytochromes P-450 by inhibiting both the synthesis and increasing the degradation of the enzyme. Thus, cobalt inhibits S-aminolaevulinic acid synthetase, the enzyme involved in heme synthesis. Cobalt will also increase the activity of heme oxygenase, which breaks down the heme portion to biliverdin. The compound 3-amino, 1, 2, 3-triazole decreases cytochromes P-450 levels by inhibiting porphyrin synthesis. 

Porphyrias are caused by inherited (or occasionally acquired) defects in heme synthesis, resulting in the accumulation and increased excretion of porphyrins or porphyrin precursors (see Summary Figure 21.7). With the exception of congenital erythropoietic porphyria, which is a genetically recessive disease, all porphyrias are inherited as autosomal dominant disorders. The mutations that cause the porphyrias are heterogenous (not all are at the same DNA locus), and nearly every affected family has its own mutation. Each porphyria results in the accumulation of a unique pattern of intermediates caused by the deficiency of an enzyme in the heme synthetic pathway. 

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. 

Nervous System. The nervous system is also a common target of toxic metals particularly, organic metal compounds (see Chapter 16). For example, methylmercury, because it is lipid soluble, readily crosses the blood-brain barrier and enters the nervous system. By contrast, inorganic mercury compounds, which are more water soluble, are less likely to enter the nervous system and are primarily nephrotoxicants. Likewise organic lead compounds are mainly neurotoxicants, whereas the first site of inorganic lead is enzyme inhibition (e.g., enzymes involved in heme synthesis). 

Fe/S clusters in regulatory enzymes have been proposed to act as sensors in such a manner that, upon detection of a measurand, the cluster disintegrates and activity stops. Putative examples are NO sensing by the [2Fe-2S] cluster in the terminal enzyme of heme synthesis, ferrochelatase [8], and 02 sensing by the [4Fe-4S] cluster in the regulatory enzyme of purine nucleotide biosynthesis, glutamine 5-phosphoribosyl-l-pyrophosphate amidotransferase [9], This is of course not a catalytic activity, since the cluster is destroyed in the action. 

The oxidative phosphorylation system contains over 80 polypeptides. Only 13 of them are encoded by mtDNA, which is contained within mitochondria, and all the other proteins that reside in the mitochondrion are nuclear gene products. Mitochondria depend on nuclear genes for the synthesis and assembly of the enzymes for mtDNA replication, transcription, translation, and repair (Tl). The proteins involved in heme synthesis, substrate oxidation by TCA cycle, degradation of fatty acids by /i-oxidalion, part of the urea cycle, and regulation of apoptosis that occurs in mitochondria are all made by the genes in nuclear DNA. 

---