Heme metabolites

Yet another mode of defense against oxidant stress in mammalian cells is by transcriptional induction of heme oxygenase, which acts to generate heme metabolites with anti-oxidant properties (29). 

Heme Metabolites in Blood and Urine in Relation to Lead Toxicity and Their Determination... 

Probably no disease has killed more humans throughout history than malaria. Malaria is transmitted by the bite of an Anopheles mosquito. There are four different parasites that can infect humans, but Plasmodium falciparum is the most virulent. The parasite first migrates to the liver where after several replications the parasites are released into the bloodstream. The parasite then invades the red blood cells and feeds off the hemoglobin. Survival of the parasite relies on the ability of the parasite to package the toxic heme metabolites formed during hemoglobin metabolism in nontoxic hemazoin particles. The parasite causes anemia, fevers, chills, veno-occlusive disorders, liver damage, and death. 

Glycine participates in the biosynthesis of heme, purines, and creatine and is conjugated to bile acids and to the urinary metabolites of many drugs. 

Detection by LDMS and structural elucidation of other secondary metabolite products, generated in the host during the onset of the parasite disease, is discussed. These molecules may serve as additional biomarkers for rapid malaria diagnosis by LDMS. For instance, choline phosphate (CP) is identified as the source of several low-mass ions observed in parasite-infected blood samples in addition to heme biomarker ions. The CP levels track the sample parasitemia levels. This biomarker can provide additional specificity and sensitivity when compared to malaria detection based on heme ion signals alone. Furthermore the observed elevated CP levels are discussed in the context of Plasmodium metabolism during its intra-erythrocytic life cycle. These data can... 

LDMS is particularly well suited for the analysis of porphyrins.35-39 The heme molecule—a 22 rc-electron conjugated protoporphyrin system (Figure 8.1)—is an efficient photo-absorber in the visible and near UV (with an absorption maximum—the Soret band—near 400nm). This feature, concurrently with its low ionization potential, warrants that direct LDMS will possess extremely low limits for heme detection. The uses of IR or UV LDMS for structural characterization of natural porphyrins and their metabolites, synthetic monomeric porphyrins (e.g., used in photodynamic therapy), porphyrin polymers, and multimeric arrays, have been well documented.41148 In addition fast atom bombardment MS has been used to characterize purified hemozoin, isolated from the spleens and livers of Plasmodium yoelii infected mice.49... 

Adults injected ip for 3 days at 80 mg/kg BW daily then killed 24 h after last injection Heme destruction in the P-448-containing reconstituted monooxygenase system reactive epoxide, or possibly nonepoxide, intermediate metabolites may participate in cytochrome P-448 destruction 24... 

Given that hydroxylamine reacts rapidly with heme proteins and other oxidants to produce NO [53], the hydrolysis of hydroxyurea to hydroxylamine also provides an alternative mechanism of NO formation from hydroxyurea, potentially compatible with the observed clinical increases in NO metabolites during hydroxyurea therapy. Incubation of hydroxyurea with human blood in the presence of urease results in the formation of HbNO [122]. This reaction also produces metHb and the NO metabolites nitrite and nitrate and time course studies show that the HbNO forms quickly and reaches a peak after 15 min [122]. Consistent with earlier reports, the incubation ofhy-droxyurea (10 mM) and blood in the absence of urease or with heat-denatured urease fails to produce HbNO over 2 h and suggests that HbNO formation occurs through the reactions of hemoglobin and hydroxylamine, formed by the urease-mediated hydrolysis of hydroxyurea [122]. Significantly, these results confirm that the kinetics of HbNO formation from the direct reactions of hydroxyurea with any blood component occur too slowly to account for the observed in vivo increase in HbNO and focus future work on the hydrolytic metabolism of hydroxyurea. 

Ortiz de Montellano PR, Komives EA. Branchpoint for heme alkylation and metabolite formation in the oxidation of arylacetylenes by cytochrome P-450. J Biol Chem 1985 260(6) 3330-3336. 

Different antimalarials selectively kill the parasite s different developmental forms. The mechanism of action is known for some of them pyrimethamine and dapsone inhibit dihydrofolate reductase (p. 273), as does chlorguanide (proguanil) via its active metabolite. The sulfonamide sulfadoxine inhibits synthesis of dihydrofolic acid (p. 272). Chlo-roquine and quinine accumulate within the acidic vacuoles of blood schizonts and inhibit polymerization of heme, the latter substance being toxic for the schizonts. 

More recently, and based on the same concept, O Neill and coworkers have prepared a series of related systems (80a-c) based on Vennerstrom s adamantyl trioxolane unit". These derivatives, which are synthesized in only three steps from adamantan-2-one, have activity in the low nanomolar region (<3 nM versus K1 P. falciparum). Like the prototypes synthesized by the groups of Meunier" and Singh", these compounds can be formulated as water-soluble salts and it is anticipated that these agents may have the capacity to hit the parasite by two distinctive mechanisms. Indeed, any chemical or metabolic degradation of the endoperoxide bridge in these compounds will result in metabolites that may still have the ability to function as inhibitors of heme polymerization provided that they do not become covalently attached to proteins in the bioactivation process. 

We often rationalize drug interactions as reflecting the reversible competition of two substrates for an active site. However, it is becoming increasingly clear that other mechanisms of inhibition are operational in vivo. For example, some mechanism-based inhibitors are activated during metabolism and form a complex with the heme of CYP3A, known as a metabolite... 

Reactive metabolites of xenobiotics may differ in reactivity, and therefore have varying impact on enzymatic activities in terms of proximity to their origin. For example, some intermediates are highly reactive and directly inhibit the enzyme that leads to their formation. These substances are commonly referred to as suicide inhibitors, for obvious reasons. Some suicide inhibitors, such as piperonyl butoxide (PBO), a pesticide synergist) are common inhibitors of certain CYP isozymes. PBO amplifies the toxicity of certain insecticides by inhibiting the insect s CYP enzymes that are involved in its degradation. It is metabolized to a highly reactive carbene, which forms an inhibitor complex with the heme iron of CYP, as shown in Scheme 3.6. 

The heme iron in the peroxidase is oxidized by the peroxide from III+ to V4- in compound I. The compound I is reduced by two sequential one-electron transfer processes giving rise to the original enzyme. A substrate-free radical is in turn generated. This may have toxicological implications. Thus the myeloperoxidase in the bone marrow may catalyze the metabolic activation of phenol or other metabolites of benzene. This may underlie the toxicity of benzene to the bone marrow, which causes aplastic anemia (see below and chap. 6). The myeloperoxidase found in neutrophils and monocytes may be involved in the metabolism and activation of a number of drugs such as isoniazid, clozapine, procainamide, and hydralazine (see below). In in vitro systems, the products of the activation were found to be cytotoxic in vitro. 

The cytochrome P-450 destroyed may be a specific isoenzyme, as is the case with carbon tetrachloride and allylisopropylacetamide (Table 5.28). Indeed, with carbon tetrachloride the isoenzyme destroyed is the one, which is responsible for the metabolic activation (CYP1A2). With allylisopropylacetamide, it is the phenobarbital-inducible form of the enzyme, which is preferentially destroyed as can be seen from Table 5.25. It seems that it is the heme moiety, which is destroyed by the formation of covalent adducts between the reactive metabolite, such as the trichloromethyl radical formed from carbon tetrachloride (see chap. 7), and the porphyrin ring. 

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