In part 1, we looked at the evolution and structure of the bee sting. We know that the first venomous sting evolved in the common ancestor of all Aculeata around the late Triassic or early Jurassic. Given 200 million years (give or take) of evolution, it's not surprising that the contents of the venom varies between bees, ants, wasps and hornets. For example, bee stings are acidic (pH 5.0 - 5.5), whereas wasp stings are almost neutral (pH 6.8 - 6.9).
Anyway, we're not here to talk about wasps - this is a beekeeping blog. And today's entry is about the chemistry of bee venom. Those 200 million years of evolution have cooked up quite a cocktail of different substances, which work both in isolation and together to provide a precise pain experience when you get stung. Let's take a dive straight into the detail - looking at the content of honey bee (Apis mellifera) venom, here are the approximate percentages by dry weight:
Melittin (peptide, 40-50%)
Melittin is a peptide, which means that it is a molecule formed of a short chain of amino acids. This distinguishes it from a protein, which is a long amino acid chain. Peptides turn up a lot in biochemistry, as they perform all sorts of useful functions including breaking down proteins; some peptides are hormones that are involved in physiological regulation.Melittin is composed of 26 amino acids. It has three principle actions: firstly, it activates pain receptor cells. Secondly, it causes holes to appear in cell membranes (which basically damages the cells). The third action is to destroy red blood cells. All of these are bad, obviously, which is why melittin is such an effective venom component.
Phospholipase A (enzyme, 10-12%)
Specifically, phospholipase A2. This is an enzyme which breaks down phospholipids. In turn, phospholipids are a major component of cell membranes, so phospholipase A2 has the effect of damaging the out walls of cells. When the phospholipids break down, one of the components that is released is arachidonic acid. This is then oxygenated by other enzymes, to form eicosanoids. And eicosanoids signal the body's inflammation response.Hyaluronidase (enzyme, 1-2%)
Another enzyme. This one breaks down hyaluronic acid, which is a component of the tissue around the cells. When this tissue breaks down, it increases the permeability of cells, and allows molecules to disperse more quickly around the cells. In other words, hyaluronidase helps the other components of the venom to get around and into the cells faster.Apamin (peptide, 2-3%)
This one is another peptide. Fun fact - apamin was first isolated from the honey bee (Apis mellifera) - hence the name "apamin". Another fun fact is that apamin is the smallest known peptide nurotixin - and the only one small enough to pass from the blood into the brain. It acts on the SK channels, which are calcium-activated sodium channels inside neurons. The job of these channels is to regulate (i.e. slow down) repetitive firing of the neurons - in other words, their job is to make the pain reduce after the initial sting. But apamin blocks the SK channels, preventing them from slowing down the feeling of pain. So apamin has the job of keeping going the pain that is caused by melittin.Histamine (biogenic amine, 0.5-2%)
Histamine makes you itch. anybody who has an allergy will be familiar with the action of histamine - if you have hay fever you get an itchy nose, while a cat allergy will cause very itchy skin if a cat scratches you. It is also involved in the body's inflammatory response - inflammation has a number of effects, including making capillaries more permeable to white blood cells, which would normally be a way of fighting infection at a wound. The inclusion of histamine is rather clever, as it uses the body's defensive system against itself by increasing inflammation at the site of the sting.MCD Peptide (peptide, 2-3%)
Mast Cell Degranulating Peptide, to give it its full title. Mast cells are found in connective tissue (including just under the skin) and contain granules which themselves contain histamine and heparin. MCD peptide causes these granules to break down, releasing the heparin and histamine. Heparin is an anticoagulant. More histamine means even more itching and inflammation (see above).Noradrenaline (biogenic amine, 0.1-0.5%)
Also known as norepinephrine. Like adrenaline (which is chemically similar) noradrenaline increases heart rate and blood pressure, increases blood flow to the muscles, releases glucose for energy and can cause feelings of anxiety. In other words, its use in bee venom is to deliberately stimulate the body's fight-or-flight response - presumably to encourage the victim to run away!Serotonin (biogenic amine, trace amount)
At first sight it's odd to see serotonin here - it is, after all, one of the brain's "happy" chemicals, associated with good mood and feelings of well-being. However, a side-effect of serotonin injection is localised pain, and that's what the bee is shooting for here.Dopamine (biogenic amine, 0.2-1%)
Another chemical that is associated with happiness or pleasure. This is because of dopamine's effect on the brain's reward-motivation system. In fact, it's more correct to say that dopamine signals the perceived desirability or aversiveness of an event. In this case, being stung is something to be averted, and dopamine is there to remind your brain of that.Alarm Pheremones (trace amounts)
The function of the alarm pheremones is to increase the aggressiveness/defensiveness of other bees that are nearby. So, if a colony is attacked, the first (and bravest) bee to sting the attacker will cause alarm pheremones to be released when the venom is injected. This will encourage other bees to threaten, and then sting the attacker. The longer they hang around, the more they get stung. The wisest course of action is, of course, to run away!References
I referred to a couple of useful articles when researching this blog post.This is a piece of research that breaks down the components of honey bee venom by dry weight:
https://www.researchgate.net/publication/304012422_Bee_Venom_Production_Composition_Quality
And this is a nice infographic showing the different compositions of venom in the main groups of the Aculeata:
https://www.compoundchem.com/wp-content/uploads/2014/08/The-Chemical-Composition-of-Insect-Venoms-v2.pdf
No comments:
Post a Comment