Sunday, 9 May 2010
Saturday, 24 October 2009
Sunday, 18 October 2009
of Carcharodon megalodon. The Cryptozoology Review 3 (2): 14-24.
Tuesday, 29 September 2009
The predominant pharmacologically active components in scorpion venoms are small polypeptide molecules, usually basic in nature. Scorpion venoms and their component elements have been studied for over 35 years; but lately the focus in these studies has shifted from their pharmacological and electrophysiological properties to their molecular structures. This came about due to the realization that many classes of peptides seem to bind to characteristic spots on their targets, which for the most part are ion channels. The peptide toxins also show considerable identity in their arrangement of cysteine residues within the polypeptide chain. These cysteine residues are terribly important because disulphide bonds are one of the major contributors to conformational stability in small peptides.
The first pharmacological studies on scorpion toxins concentrated on their effects on mammalian model systems, so mice were the first ones to draw the short end of the hypodermic syringe, for a while. Peptides were consequently classified into the alpha- and beta-types, where each type bound to its own special site on voltage-gated sodium channels. Eventually, however, it was discovered that some peptides worked against ion channels in insects, and now Sarcophaga argyrostoma blowfly larvae were adopted as the victim model system of choice. All anti-insect toxins induced paralysis, but one class of toxins was found to induce a contraction paralysis, while the other induced a depressant paralysis.
For a while it seemed that all peptides could be neatly docketed in this way, but continuing studies unearthed peptide toxins that were primarily anti-mammal but showed some anti-insect activity too. Subsequently peptides were found that showed comparable anti-insect and anti-mammal activities, one of which – showing anti-insect as well as alpha-type and beta-type anti-mammal activities – was first reported in the paper (Loret et al., 1991) referenced below. This sparked off a line of thought on how the types of peptides found in scorpions in different parts of the world could be used to put forward theories about where scorpions first appeared and how they diverged into the large number of species we see today.
The reason this cross-reactivity of toxins against insect and mammal receptors was such a big deal was this: Before this was discovered, toxins were found to be easily classifiable not only on the basis of their primary peptide sequences and pharmacological activities, but also based on geography. Alpha-type anti-mammal toxins came only from Old World scorpions and beta-type anti-mammal toxins came only from New World scorpions. All anti-insect toxins had been purified from Old World scorpions only. Deviations from this geographical structure brought to light interesting ideas about venom evolution in scorpions.
For example, the toxin mentioned above which had a high effect on mammals but a low effect on insects was a beta-type toxin from Centruroides (a New World species) which was toxic to insects, but 50 times weaker than Old World anti-insect toxins (these studies happened in the 80s). This could indicate that anti-insect activity is just starting to evolve in New World venoms, but has already become established in Old World ones, indicating that the Old World venoms appeared first in their evolutionary history.
The paper referenced below is a report of a toxin from Androctonus australis Hector (pictured below), called AaH IT4. The toxin was purified by successive steps of chromatography, on gel filtration, DEAE-Sephadex and C8 HPLC columns. Toxicities were tested on S. argyrostoma larvae and male C57/BL6 mice, and the ED50 values (for larvae) and LD50 values (for mice) were recorded. Binding assays using 125I-iodinated toxins on synaptosomal preparations from cockroaches and rats; radioimmunoassay assays to check for cross-reactions with rabbit antisera against known anti-insect, alpha-type and beta-type toxins; and circular dichroism analyses for structural data were carried out. In addition, sequence analysis and sequence alignment against various other scorpion venoms were carried out.
The experimental data showed that AaH IT4 competed with all three types of toxins for target-binding, and that it cross-reacted with the antibody against a beta-type toxin, indicating some structural similarity with the beta-toxin class. The dendrogram generated from the sequence alignment showed that AaH IT4 is more closely related to beta-type toxins than to either alpha-type or anti-insect toxins (which supports the result of the RIA experiment), although the divergence between AaH IT4 and the beta-type toxin lineage took place a long time ago. This an important point because beta-type toxins come from New World scorpions while AaH IT4 was purified from an Old World toxin, so any sequence similarity suggests that some relationship may exist between the two.
Another point to be made about AaH IT4 is that the sequence analysis shows an absence of the amino acid proline. Proline had previously been found in every purified and studied peptide scorpion toxin, and was suspected to play a role in the stability of their conformations, since proline is more conformationally restricted as compared to the other amino acids. One explanation for the binding of AaH IT4 to three different kinds of binding sites is that the absence of proline allows a certain amount of backbone flexibility which may allow the peptide to switch between conformational states that preferentially bind to each of the target sites.
The authors themselves hypothesize that since the evolution of insect-specific toxins is clearly advantageous to scorpions (for whom insects form a major part of the diet), it is possible that AaH IT4 represents the closest approximation available right now to some kind of ancestral toxin sequence/structure, which later diverged into anti-mammal and anti-insect varieties.
I think it’s worth mentioning, in addition, that the isolated AaH IT4 corresponds to 0.06% of the total protein in the venom, so it’s possible that the production of this particular venom component may have undergone some down-regulation over the years, as a result of the development of other, more specialised toxins.
Post-1991, of course, the issue of toxin classification has become even more complicated, and the venoms being studied now include those with anti-arthropod specificity, and the set of recognised targets have grown to include voltage- and ligand-gated potassium- as well as calcium-channels instead of just the initial emphasis on voltage-gated sodium-channels. Peptides with microbial activity have also been reported.
Loret, E., Martin-Eauclaire, M., Mansuelle, P., Sampieri, F., Granier, C., & Rochat, H. (1991). An anti-insect toxin purified from the scorpion Androctonus australis Hector also acts on the .alpha.- and .beta.-sites of the mammalian sodium channel: sequence and circular dichroism study Biochemistry, 30 (3), 633-640 DOI: 10.1021/bi00217a007
Saturday, 26 September 2009
Monday, 14 September 2009
2009 has been a year of deep reflection - a chance for Britain, as a nation, to commemorate the profound debts we owe to those who came before. A unique combination of anniversaries and events have stirred in us that sense of pride and gratitude which characterise the British experience. Earlier this year I stood with Presidents Sarkozy and Obama to honour the service and the sacrifice of the heroes who stormed the beaches of Normandy 65 years ago. And just last week, we marked the 70 years which have passed since the British government declared its willingness to take up arms against Fascism and declared the outbreak of World War Two. So I am both pleased and proud that, thanks to a coalition of computer scientists, historians and LGBT activists, we have this year a chance to mark and celebrate another contribution to Britain’s fight against the darkness of dictatorship; that of code-breaker Alan Turing.
Turing was a quite brilliant mathematician, most famous for his work on breaking the German Enigma codes. It is no exaggeration to say that, without his outstanding contribution, the history of World War Two could well have been very different. He truly was one of those individuals we can point to whose unique contribution helped to turn the tide of war. The debt of gratitude he is owed makes it all the more horrifying, therefore, that he was treated so inhumanely. In 1952, he was convicted of ‘gross indecency’ - in effect, tried for being gay. His sentence - and he was faced with the miserable choice of this or prison - was chemical castration by a series of injections of female hormones. He took his own life just two years later.
Thousands of people have come together to demand justice for Alan Turing and recognition of the appalling way he was treated. While Turing was dealt with under the law of the time and we can’t put the clock back, his treatment was of course utterly unfair and I am pleased to have the chance to say how deeply sorry I and we all are for what happened to him. Alan and the many thousands of other gay men who were convicted as he was convicted under homophobic laws were treated terribly. Over the years millions more lived in fear of conviction.
I am proud that those days are gone and that in the last 12 years this government has done so much to make life fairer and more equal for our LGBT community. This recognition of Alan’s status as one of Britain’s most famous victims of homophobia is another step towards equality and long overdue.
But even more than that, Alan deserves recognition for his contribution to humankind. For those of us born after 1945, into a Europe which is united, democratic and at peace, it is hard to imagine that our continent was once the theatre of mankind’s darkest hour. It is difficult to believe that in living memory, people could become so consumed by hate - by anti-Semitism, by homophobia, by xenophobia and other murderous prejudices - that the gas chambers and crematoria became a piece of the European landscape as surely as the galleries and universities and concert halls which had marked out the European civilisation for hundreds of years. It is thanks to men and women who were totally committed to fighting fascism, people like Alan Turing, that the horrors of the Holocaust and of total war are part of Europe’s history and not Europe’s present.
So on behalf of the British government, and all those who live freely thanks to Alan’s work I am very proud to say: we’re sorry, you deserved so much better.