Snapshot Issue 30 January 2007
Prior to a hunting expedition, the men of some South American tribes scrape the ‘juice’ off a frog’s skin and smear it onto fresh burns inflicted on their arms or chest. The net result is an hour’s vomiting, incontinence, a rapid heartbeat and intense sweating, followed by listlessness which lasts a day or more, to finally come to one’s senses feeling – as the biologist Peter Gorman jotted down in his field notebook – ‘quite godlike in my strength and [with an] acuteness of my senses’. What the hunters qualify as ‘hunting magic’.
A frog’s skin – like the skin of all amphibians – is covered with peptides of all sorts, which protect it from toxic company such as bacteria, protozoa, viruses and fungi. Dermaseptin is one of these numerous peptides and is secreted onto the skin of South American tree frogs of the Phyllomedusa genus. It kills off microbes by interfering with their membrane, probably by inserting itself into the phospholipid layer, thereby disturbing the lipid arrangement. As a consequence, the membrane’s permeability is modified and the microbe – if not killed – loses its ability to function properly.
An interest in dermaseptin – and dermaseptin-4 in particular – is increasing because researchers have discovered that it can bewilder four agents that are of medical interest: Herpes Simplex Virus Type I, HIV, Plasmodium falciparum which causes malaria… and sperm. In all cases, dermaseptin perturbs the membrane’s phospholipid arrangement. As a result, spermatozoa lose their faculty to wriggle, the Herpes virus cannot bind to the cells it would like to infect and Plasmodium falciparum cannot multiply. Though the action on HIV could be of the same nature, it seems that dermaseptin’s role in HIV infection is more indirect. Indeed, HIV needs epithelial lesions to reach the lymph nodes and cause infection. Such lesions are more often than not created by microbes. Dermaseptin would kill off the microbes, so that there would be less lesions and, hence, less chance of HIV infection.
Frogs have been a great source of antimicrobial peptides since 1969. It is still important to find novel ones, and to be able to tamper with them, not only because – with time – any toxic agent finds a way to bypass its enemy but also because the design of a good drug is one whose antimicrobial activity is great, and whose toxicity is weak. Malaria, like AIDS, kills millions of people every year. Dermaseptin is promising because not only could it be used as a means of vaginal contraception but it could also counter sexually transmitted diseases. Medical magic?
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