It’s important to find balance in life, and that’s true for nitrogen as well.
Incoming!
When your body breaks down proteins in the liver, it’s left with wastes containing nitrogen as a byproduct. Ammonia is the simplest form, but it is quite toxic, so it needs to go somewhere or get converted into something else as soon as possible.
If you’re a fish, you just let that ammonia dissolve into the water around you. If you’re a mammal, you convert it to urea, which uses up a little extra energy but isn’t as toxic as ammonia, and then the kidneys filter it out and it leaves the body in urine.
If you’re a bird or a reptile, you convert that ammonia to uric acid and excrete it directly as the pasty white urate portion of droppings. (It’s uric acid if there’s a free proton involved, or urate if something else like sodium is attached. For our purposes, consider urates and uric acid to be the same thing.) Creating uric acid uses up more energy than creating urea, but it means birds and reptiles don’t have to use as much water to get rid of their nitrogenous wastes.
But wait, there’s more!
Mammals also produce uric acid. You might have vague memories of A-T C-G DNA pairs from your biology class. Adenosine and guanine are purines. When those purines are broken down, uric acid is produced instead of urea.
But mammals don’t excrete urates, so where does it go? The answer is that uric acid is dissolved in the blood until the kidneys grab it and it goes out in the urine.
Except when it all goes wrong
What happens if the kidneys don’t filter out the uric acid? If the kidneys don’t do their thing, the uric acid builds up in the blood until it crystallizes in the joints. This painful condition is called gout. For people prone to getting gout, one way to avoid it is to eat a diet low in purines. Decreasing the purines coming into the body leads to decreased uric acid production by the liver, which hopefully avoids an attack of gout. This is such a common problem that you can come up with a list of high-purine foods to avoid with one google search.
What happens if the blood carrying nutrients absorbed by the intestine goes around the liver instead of going through it? Normally everything goes through the liver first, so any ammonia can be converted to urea before it has a chance to cause problems. But sometimes the blood vessels don’t develop correctly or go wonky later in life, and then there’s a higher concentration of ammonia plus uric acid in the blood and urine, and those two compounds like to combine together in the bladder to form ammonium urate stones. Bladder stones are no fun.
(Anecdote time! There was a cute older Pomeranian at the practice I worked in before vet school. His chart had notes in large letters from when he was a neutered as a puppy warning that this dog took a really, really long time to recover from anesthesia. Then while I was in vet school, this dog came in to the practice have bladder stones removed on a day when I happened to be visiting. The stones turned out to be ammonium urate. And then when I was a senior student, this dog came to the vet school to have surgery to fix the blood vessel that was shunting blood around his liver. That was one of those “yep, it all adds up” moments, though that dog almost made it his entire life without having his liver shunt diagnosed or corrected.)
Want to know more?
https://www.diffen.com/difference/Purines_vs_Pyrimidines
https://opentextbc.ca/biology/chapter/22-4-nitrogenous-wastes/
Textbook of Veterinary Internal Medicine, Ettinger & Feldman, 5th edition. (A book made from paper. How quaint!)
During Fun Facts Friday, I talk about something I think is interesting. Do you have questions? Suggestions for a future post? Add a comment below!
