Scent of Death

Chemists think about poisons. And students seem to perk up when I mention examples of such substances in class. I enjoyed learning more about them by reading Most Delicious Poison, written by evolutionary biologist Noah Whiteman. There is significant overlap to the book I just read about flavor molecules since they are secondary metabolites produced by plants. Just like many poisons! Whiteman’s book is a little more dense, but just as interesting. Today’s post features Chapter 6, “Abiding Alkaloids”. It begins with the scent of death.

 

When an organism dies, it’s a feast for microbes acting as decomposers. Many of the compounds released that your nose knows are amines – they have nitrogen in them. Cadaverine and putrescine are two examples that you’d guess are stinky. Skatole is a less obvious name, but it has the odor of feces at higher concentrations, and oddly has a flowery smell (including jasmine) at low concentrations. A perfume that has an intoxicating smell? Whiteman reminded me about the root word ‘toxic’. To be intoxicated used to mean that you got poisoned. Now it means you’re drunk or high.

 

Since nitrogenous compounds are alkaline, these compounds are also called alkaloids. I was surprised to learn that “our (living) bodies produce endogenous cadaverine, putrescine, and spermidine because these molecules serve critical roles in our cells. Spermidine is particularly interesting. Adding it to the diets of laboratory animals extended their life spans by 15 to 30 percent. In human cells bathed in spermidine, aging also slowed… One effect of spermidine is that it helps keep our genes switched off. As cells age, more and more genes get turned on willy-nilly… Spermidine may also enhance the removal of damaged cells, including those containing beta-amyloid plaques…”

 

Whiteman goes on to discuss the co-evolution of plants and insects and the role of specific secondary metabolites that act as signals and poisons. Plants evolve defense mechanisms against hungry insects. The insects evolve resistance to those poisons. Stinky plants such as the famous corpse flower trick specific insects to visit and in doing so help to distribute pollen. This is how niches develop. Whiteman believes this is why “there are so many toxic plants and toxin-specialized herbivores… the chemical war of nature… has also given rise to much of the pharmacopeia that we use and abuse.”

 

Skatole is an example of an indole. Related molecules with an indole chemical skeleton include a range of mind-altering drugs. Whiteman discusses DMT (N,N-dimethyltryptamine) found in a variety of tropical plants, and psilocybin found in magic mushrooms. Interestingly, our neurotransmitters have similar chemical structures. Serotonin looks very much like psilocin. Lysergic acid or LSD is also an indole, albeit a more complicated looking one with four fused rings. I also learned that “when a chain of psilocin molecules become bonded to one another… [they] act much like tannins which also turn blue when oxidized.” It reminded me about the quest to find inks and paint pigments!

 

I can’t help but also mention tubocurarine, the poison known as curare, which also has quite the complicated chemical structure. By the way, there are beautiful illustrations of plants paired with the chemical structures of molecules in this book. Indigenous hunters in the tropics tipped their arrows or blow-darts with curare. It’s a silent and effective killer, paralyzing the prey by blocking the activity of the neurotransmitter acetylcholine. (Once upon a time, I studied the enzyme responsible for breaking down acetylcholine.) Tubocurarine, I learned, was also “the first muscle relaxant used in general anesthesia”. Whiteman discusses this alongside cocaine and other drugs from plants with names such as mandrake, devil’s snare, moonflower, in Chapter 8, “Devil’s Breath and Silent Death”.

 

There’s so much more I could discuss from this book. Instead I’ll end by saying that if I ever teach a medicinal chemistry class (a doubtful prospect given I’m not an organic chemist), I will be drawing inspiration from Most Delicious Poison. I particularly enjoyed the later chapters on “The Herbivore’s Dilemma” and “The Spice of Life” (which discusses flavors) but I encourage you to read Whiteman’s book if any of this interests you!