Heme is not only just the binding site responsible for
oxygen transport by
hemoglobin, but it is also the prosthetic group of many different
heme-containing
enzymes, such as
cytochromes P450,
peroxidases,
catalase, and several
proteins involved in electron transfer.
Heme plays a key role in the mechanism of action of many different
antimalarial drugs. In degrading the host's
hemoglobin, the
malaria parasite Plasmodium and several other
heme-eating parasites are faced with this redox-active
metal complex.
Heme is able to induce the toxic reductive cascade of molecular
oxygen, which leads to the production of destructive
hydroxyl radicals. Plasmodium detoxifies
heme by converting it into a redox-inactive
iron(III)
polymer called
hemozoin.
Artemisinin, a natural
drug containing a biologically important
1,2,4-trioxane structure, is now the first-line treatment for multidrug-resistant
malaria. The
peroxide moiety in
artemisinin reacts in the presence of the flat, achiral
iron(II)-
heme; the mechanism does not reflect the classical "key and lock" paradigm for drugs. Instead, the reductive activation of the
peroxide function generates a short-lived
alkoxy radical, which quickly rearranges to a C-centered primary radical. This radical alkylates
heme via an intramolecular process to produce covalent
heme-
drug adducts. The accumulation of non-polymerizable redox-active
heme derivatives, a consequence of
heme alkylation, is thought to be toxic for the parasite. The alkylation of
heme by
artemisinin has been demonstrated in
malaria-infected mice, indicating that
heme is acting as the trigger and target of
artemisinin. The alkylation of
heme by
artemisinin is not limited to this natural compound: the mechanism is invoked for a large number of
antimalarial semisynthetic derivatives. Synthetic trioxanes or trioxolanes also alkylate
heme, and their alkylation ability correlates well with their
antimalarial efficacy. In addition, several reports have demonstrated the cytotoxicity of
artemisinin derivatives toward several tumor cell lines. Deoxy analogues were just one-fiftieth as active or less, showing the importance of the
peroxide bridge. The involvement of
heme in anticancer activity has thus also been proposed. The anticancer mechanism of endoperoxide-containing molecules, however, remains a challenging area, but one that offers promising rewards for research success. Although it is not a conventional
biological target,
heme is the master piece of the mechanism of action of
peroxide-containing
antimalarial drugs and could well serve as a target for future anticancer drugs.