On Monday, we were talking about the neuroscience of food. It came up in discussion that we have menthol “sensors” in our lungs. This struck me as more than a little odd, so I looked it up. Apparently, no one really knows why we have menthol “sensors” in our lungs. (Even weirder, these are also located in the bladder and prostate. No one knows why they are there either.) Since I was kind of bored during Structured, and was avoiding writing up my chem lab, so I decided to do a bit more digging.
The menthol “sensor” is know as the Transient Receptor Potential Cation Channel subfamily M member 8. (For simplicity’s sake, we will just shorten that to TRPM8.) Basically, TRPM8 is a protein comprised of L-Methionine. L-Methionine is C5H11NO2S. This link is to ChemSpider, which shows what the molecule will look like.
TRPM8 is activated by temperatures below 25 degrees Celsius, eucalyptol, as well as menthol. (So, the sensation of cold we get from smelling Eucalyptus oil or chewing mint gum is really just a neuron misfiring.) TRPM8 is located in the membrane of cells and is a multi-pass membrane protein, which just means that it spans the membrane more than once. (Biologists and chemists are very creative with their names.) When TRPM8 is activated, it allows positively charge ions, aka cations, to pass through the membrane and into the cell.
Mentha (in other words, the mint family) is the only genus of plants to create menthol. That is one of their defining traits. Mentha grows well in soils with a pH of 6.5-8.5, or from neutral to moderately alkaline. It is not particularly significant that it can grow in neutral soil, since pretty much every plant can grow well in neutral soil. However, it is significant that it can grow in more alkaline soils. In alkaline soils, certain nutrients such as calcium, magnesium, sodium, basically the alkaline earth metals and alkali metals, are more readily available to the plant. Plants that do well in these soils need these nutrients. It is logical to conjecture that the nutrients they need become part of their internal structure. Mentha plants have a higher concentration of cation nutrients in them.
So, somewhere way down the evolutionary track, one of our ancestors was able to smell menthol. Since it smelled nice to them, they were more likely to eat the mentha plant. As menthol activates the TPRM8, and the plant is broken down through digestion, the cations are released and absorbed into the cells. Thus, they were more likely to live and pass on their genetic matter.
Why we have TPRM8 in our bladders is a question for another day…..