Toxic free heme

While hemoglobin proteolysis yields needed amino acids, it also releases toxic free heme. Fig. (1) Upon proteolysis, heme is released into the digestive vacuole where the iron of the heme moiety is oxidized from the predominantly ferrous (+2) state to the ferric (+3) state. Estimates suggest... 

CO entered the doorstep of Biology in 1949 when it was identified as a natural metabolite in the exhaled air of healthy humans [1]. Until then, CO was only a feared toxin with some industrial applications in the synthesis of raw materials and H2. Still uncomfortable with the dubious reputation of this newcomer, its acceptance within the new community was slow. It took 20 years for Tenhunen et al. to issue its biological birth certificate as the result of the catabolism of toxic, free heme together with Fe and biliver-din [2]. It took another 20 years to identify its role as a molecular mediator and recognize it as another member of the necessarily small club of biologically active diatomic molecules, where NO was the Molecule of the Year 1992. Recently, this club was joined by a third noxious member, H2S, and the three are now collectively called gasotransmitters [3]. 

Balia, G., Vercelloti, G.M. and Muller-Eberhard, U. (1991). Exposure of endothelial cells to free heme potentiates damage mediated by granulocytes and toxic oxygen species. Lab. Invest. 64, 648-655. 

The previous section dealt with the mechanisms behind the bioactivation of 1,2,4-trioxanes and endoperoxides. In this section we will examine briefly the suggested targets of the artemisinins. Since the original proposal by Meshnick and coworkers it is still believed by many researchers in the field that heme liberated from the haemoglobin proteolysis process is the species responsible for the bioactivation of the endoperoxide bridge to potentially toxic free radicals in the food vacuole of the parasite (see above). 

Chloroquine is a rapidly acting blood schizonti-cide with some gametocytocidal activity. It is used with primaquine for Plasmodium vivax and Plasmodium ovale infections. It has been widely used prophylactically by traveler s to endemic areas. Its mechanism of action is unclear. It is believed to hinder the metabolism of hemoglobin in the parasite. Presumably chloroquine prevents the formation in the plasmodia of polymers out of free heme which then builds up and becomes toxic. Resistance occurs as a consequence of the expression of a membrane phospho-glycoprotein pump in the plasmodia which is able to expel chloroquine from the parasite. Plasmodium falciparum is the most likely to become resistant. 

Chloroquine probably acts by concentrating in parasite food vacuoles, preventing the biocrystallization of the hemoglobin breakdown product, heme, into hemozoin, and thus eliciting parasite toxicity due to the buildup of free heme. 

During hemoglobin degradation, free heme, ferriprotoporphyrin-IX or Fe(II) PPIX (Fig. 2) is released in the digestive vacuole. The toxicity of heme to the parasite has been demonstrated [27-30] it is supposed to cause the disruption of metabolic functions by means of peroxidation of membranes and inhibitions of enzymes via the generation of oxidative free radicals [31],... 

Interaction of marine isonitriles derivatives with heme was shown to inhibit the transformation of heme into / -hematin and then hemozoin, a polymer produced by Plasmodium in order to neutralize the toxic (detergent-like) free heme produced in the food vacuole. In addition, isonitriles were shown to prevent both the peroxidative and glutathione-mediated destruction of heme under conditions that mimic the environment within the malaria parasite. In summary, isonitriles, similarly to quinoline antimalarials [38], exert their antiplasmodial activity by preventing heme detoxification. 

So how might heme be transported through the heme A biosynthetic pathway There are three limiting possibilities. The first possibility is via simple diffusion. Given the toxic nature of free hemes 23, 24), however, this scenario... 

Another critical aspect of heme A biosynthesis involves the regulation of the heme A biosynthetic pathway. Because free hemes are toxic to cells (25, 24), there is tremendous evolutionary pressure to maintain tight control over the heme A biosynthetic pathway to ensure that sufficient, but not excessive, quantities of heme A are available. How is the flux of heme through the... 

Rogers, B. Yakopson, V Teng, Z. Guo, Y Regan, R. F. Heme oxygenase-2 knockout neurons are less vulnerable to hemoglobin toxicity. Free Radical Biol. Med. 2003,35, 872-881. 

Figure 11.7 Heme groups in different binding situations (a) free heme which is cell toxic (b) In hemoglobin, the heme is bound to a protein backbone (R). Hemoglobin is ordered in a quaternary structure, meaning that four globular a-helical subunits arrange in a tetrahedral shape, (c) Heme is detoxified in hemozoin crystals, which are insoluble.

In a complementary study, a hemozoin crystal was investigated in the same way [79]. Hemozoin is synthesized when the malarial parasite Plasmodiumfalciparum digests the protein and the free heme, which is toxic for the parasite, is leftover. The crystallization of heme yields hemozoin (see Figure 11.7c), which is insoluble in the cell. In this way, the parasite gets rid of the toxic heme while insoluble crystals of hemozoin with a size of a few hundred nanometers are formed. This happens by dimerization of two heme molecules via one of their carboxyl moieties. The other carboxyl group coordinates with the central iron atom (see Figure 11.7c). 

Iron. Iron plays a vital role during development and growth and is an important factor in many metabolic reactions, including protein synthesis as a co factor of both heme and nonheme enzymes, and in the development of neuronal processes. However, free iron, particularly Fe2+, is highly toxic by virtue of its ability to trigger cellular deleterious effects, including the Fenton reaction, which generates free radical species and lipid peroxidation (Ch. 32). 

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. 

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