Thursday, January 29, 2015

Food to die for

By Evelyn Puspitasari

We’ve had this discussion before, food is good. I personally have always loved seafood. Cooked seafood? Yes. Raw fish, you ask? Yes. I’m always down for sashimi. Any day. Hit me up. Have you heard of fugu though? If you say no, shame on you. Fugu is delish. So, let’s talk about fugu. Fugu is what the Japanese call pufferfish. 

Yep, they're cute.
They are also known as blow fish. That is because when they feel endangered, they can.. blow up.. to a ball of spiky fish. 

Yes, that is a part of their defense mechanism. But that is not their only defense strategy, they also have tetrodotoxin (TTX) coursing through their body, with the highest concentrations in their ovary and liver. Contrary to popular believe, the toxin cannot affect you if you get poked by one of their spikes. It will affect you, however, if you eat the fish and it wasn’t properly prepared. Hence, only certified chefs are allowed to prepare pufferfish. In Japan, where the fish is considered as a delicacy, chef candidates have to go through 2-3 years of apprenticeship, followed by a final examination where they have to take a written test, a fish identification test, and a practical test of preparing and eating the fish. The Japanese take this really seriously, it’s no joke. 

Fugu has to be prepped while the fish is still alive. Not to worry though, chefs severe the spinal cord of the fish to keep the pain level to a minimum. Here's a video of proper methodology to prepare fugu. Do not do this at home. Unless you have a license, which you probably don't.

Despite all the precautions, accidents do happen. Last October in Brazil, eleven people had to be rushed to the hospital when they accidentally ate a puffer fish that was mixed up with the rest of their lunch. Most recently, a brother and sister showed up to a hospital complaining of symptoms that are related to TTX poisoning. Turns out, they were poisoned from the dried fish they bought in New York. Oh, did I mention TTX poisoning has no antidote? Luckily, the victims from both cases all recovered.. Despite having no antidote, victims have a chance to fully recover as long as they seek help in a timely manner. In this case, all they can do is to wait until the toxin has run its course, while being hooked up to a respirator that breathes for them. This is because TTX is a neurotoxin that will paralyze the muscles, causing the victim to have difficulty breathing, and without help will die from asphyxiation. Also, TTX victim remains conscious even when they can’t move or breathe. Like a motionless zombie. 

Why does TTX have these effects on its victims? How does it work? 

Well.. We know that neurons are responsible for communication in the form of electrical signals from one point to another. An action potential is required for a neuron to communicate with other neurons, muscles, or glands. There are three phases of action potential: depolarization, repolarization, and hyperpolarization.
Three phases of action potential
Generally speaking, the membrane potential of the organism must shift to be more depolarized than before, which means the membrane potential will become less negative. Followed by a rapid repolarization of membrane potential to a slightly more negative state compared to its original resting potential (hyperpolarization). The drop in membrane potential is what we call repolarization, that is triggered by the closing of specific ion channels and opening of different ion channels in the membrane. Hodgkin and Huxley, the dynamic duo, did a lot of study with the giant squid’s axon to find out that an action potential begins with an inflow of Na+ through voltage-gated channels that will depolarize the membrane, then an outflow of K+ through voltage-gated channels will hyperpolarize the membrane. 

Understanding the mechanism of action potential was a crucial step in figuring out the TTX mechanism. The next step was to determine the structure of the sodium ion channel which was done by a team in Japan led by Shosaku Numa who used Electrophorus electricus, the electric eels, as their model animal. Their accomplishment tells us that the sodium channel is made up of a single peptide chain with four homologous domains, each consisting of six transmembrane helices.
Narahashi, et. al. made a hypothesis that TTX selectively inhibits the sodium transport mechanism that results in the conduction of nerve and muscle. To prove their hypothesis, they treated giant axons of lobster with TTX at different concentrations of 1x10-7 to 5x10-9 g/mL. Voltage-clamping was performed and the result of their experiment shows that there is no increase in influx of Na+ that normally occurs during depolarization when the axon was exposed to TTX, whereas the outflux of K+ was not affected.
Turns out the binding site for TTX consists of amino acid residues that form a pore where Na+ ions come in. When TTX is present, it blocks the pore, thus preventing the entry of Na+ ions through the voltage-gated channel. Hence, resulting in the absence of action potential which means there will be no communication from neurons to nerve tissues or muscles! That is why the victims of TTX poisoning get zombified… 

Pufferfish themselves are obviously not affected by their own toxin. Simple substitutions of amino acid residues in their voltage-gated channels makes them to be highly resistant to their own toxin. Pretty cool defense mechanism, pufferfish. 

Nakamura, Y., S. Nakajima, and H. Grundgest. 1965. The action of tetrodotoxin on electrogenic components of squid giant axons. The Journal of General Physiology 48: 985-996.
Häusser, M. 2000. The Hodgkin-Huxley theory of the action potential. Nature Neuroscience 3: 1165-1165.
Narahashi,T., J.W. Moore, and W.R. Scott. 1964. Tetrodotoxin blockage of sodium conductance increase in lobster giant axons. The Journal of General Physiology 47: 965-974.
Soong, T.W. and B. Venkatesth. 2006. Adaptive evolution of tetrodotoxin resistance in animals. TRENDS in Genetics 22:621-626.
Venkatesh, B., S.Q. Lu, N. Dandona, S.L. See, S. Brenner, and T.W. Soong. 2005. Genetic basis of tetrodotoxin resistance in pufferfishes. Current Biology 15: 2069-2072.

Freeman, Scott. Biological Science. San Francisco: Benjamin Cummings, 2011. Print.

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