Uric Acid Monohydrate Nanocrystals: An Adaptable Platform for Nitrogen and Salt Management in Reptiles

Both avian and nonavian reptiles excrete excess nitrogen in solid form─colloquially termed "urates"─as an evolutionary adaptation that aids in water conservation. Yet, there are many open questions regarding the composition, structure, and assembly of these biogenic materials. Here, analyses of urate excretions from ball python (<i>Python regius</i>) and 20 other reptile species reveal a clever and highly adaptable system employed to handle both nitrogenous waste and salts. Primitive species excrete urates consisting of 1-10 μm microspheres of turbostratic uric acid monohydrate (UAM) nanocrystals. The nanocrystals' high surface area and ionizable nature provides a platform to coeliminate substoichiometric concentrations of various salts through surface-ion pairing. In contrast, the granular urates produced by species from more advanced snake lineages are phase mixtures consisting of predominantly ammonium urate hydrate (AUH) and smaller amounts of other crystalline forms. Identification of microspheres as a minor but highly soluble component of these excretions suggests their likely role as reactive precursors to AUH, a hypothesis supported by in vitro experiments. Importantly, this points to a previously unrecognized physiologic function of uric acid, namely the ability to sequester ammonia by transforming it into a solid. The potential implications of this function in other species are discussed.