Poisonous Plants That Will Kill You – Strophanthus and Ouabain Its been a while since my last post, but I will try to get back to work making interesting posts. Today I am introducing a new series to my blog, Poisonous Plants that will kill you, that I hope to post weekly or biweekly. So for todays plant I will introduce the genus Strophanthus and its deadly toxin Ouabain.

For a short history this plant traditionally grows in Southern Africa and produces a cardiac glycoside toxin, Ouabain. African tribes have known of the poison for a long time and would tip their arrows in it.

What I find most interesting and I hope my readers do as well is the biochemistry of this toxin and just how deadly it is. I mentioned ouabain is a cardiac glycoside which means it interacts with a cardiac protein. Ouabain is a deadly poison because it binds the sodium potassium pump on the surface of cardiac cells preventing contraction of the heart muscle. The Na+/K+ pump is an active transport enzyme that moves sodium to the outside of the cell in exchange for potassium. The enzyme like all enzymes perform their mechanism by binding the desired molecule, the enzyme then converts to its transition state before releasing the product and returning to its first state.

Enzyme + Substrate —> Transition State Enzyme with Substrate Bound –>  Enzyme + Product

So in the case of Ouabain the cardiac glycoside binds the transition state of the enzyme and inhibits the enzyme of returning to its normal state and releasing the product.

You may be asking why is this sodium potassium pump critical for the contraction of cardiac cells. The sodium concentration outside the cell is consistently higher than inside and moving sodium outside the cell requires ATP. The Na+/K+ pump doesn’t just bind singles of these elements but actually moves 3 sodiums out in exchange for 2 potassiums. The net effect of moving mass amounts of sodium out of the cell at this ration results in depolarization of the cell because the difference in charge from the less potassium. Another pump, a Ca/Na pump moves calcium out of the cell in exchange for the sodium and is responsible for stopping the contraction. High levels of calcium are critical to the contraction. Calcium from the sarcoplasmic reticulum is stored until depolarization and it can be released to be available to bind troponin for contraction. Thus inhibiting the sodium potassium ATPase with ouabain results in a cardiac cell unable to stop contraction due to the calcium levels remaining too high.

Cardiac glycosides such as ouabain have played a critical role in our understanding of the Na+/K+ pump and its role in the cell. Researchers have used ouabain in-vitro studies to understand this process, but the drug also has clinical potential. Conjestive heart failure occurs when the heart fails to pump sufficient supply of blood to the body. Ouabain and other cardiac glycosides have shown to be effective at low concentrations to improve the contractions of the heart.

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