At the end of November, NASA announced “an astrobiology finding that will impact the search for evidence of extraterrestrial life“. The “finding” was experimental evidence suggesting discovery in Mono Lake of “A Bacterium That Can Grow by Using Arsenic Instead of Phosphorus“.
Humans have intestinal phosphate transport proteins that efficiently move phosphate into the body. Phosphorus and arsenic are chemically similar, so the human body also takes in arsenic from the environment.
In a simple model of phosphorus homeostasis, excess phosphorus is removed from the body by the kidneys. However, there is active research into the physiological mechanisms used to sense how much phosphorus is in the body and how such sensory information can be used to allow for efficient regulation of phosphorus levels in the body.
In laboratory mice, sodium-dependent phosphate cotransporter NPT2b (SLC34A2) plays a major role in the intestinal uptake of phosphorus. In mice that were engineered to lack NPT2b, there was a compensatory increase the Npt2a phosphate transport protein in kidney tubules that might help such mutant mice retain phosphorus in the body. Npt2a/Npt2c double knockout mice show severe hypophosphatemia (low blood phosphate level). Mutations in the human Npt2c homolog (SLC34A3) cause hypophosphatemia (for example, see). Other intestinal transport proteins such as Pit1 and Pit2 also exist.
Some bacteria do make use of arsenic in their energy metabolism and there is interest in the idea that some arsenic-tolerant life forms might use arsenic as a substitute for phosphorus in their macromolecules such as DNA. It is expected that a DNA backbone containing arsenic would be less stable than DNA with phosphorus. In humans, arsenic is a carcinogen that causes DNA strand breaks. I would be interesting if there are some bacteria that have physiological mechanisms for efficiently repairing arsenic-induced DNA damage.
When aresnic gets inside the human (see) body it is methylated. Experiments with lab mice suggest that methylated derivatives of arsenic can be removed from the body by aquaglyceroporin and multidrug resistance 1 (MDR1) transport proteins.
1) Recent advances in the renal-skeletal-gut axis that controls phosphate homeostasis
2) Parallel Universes? Arsenic and Phosphate – The Blogosphere, the Traditional Media, and the Scientific Literature