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

## TL;DR

Reptiles use uric acid nanocrystals to manage nitrogen and salts, with different excretion strategies in primitive and advanced species.

## Contribution

Discovery of a new physiological role for uric acid in sequestering ammonia into a solid form.

## Key findings

- Primitive reptiles excrete uric acid monohydrate nanocrystals as microspheres for waste elimination.
- Advanced snakes produce ammonium urate hydrate, with uric acid microspheres acting as precursors.
- Uric acid can sequester ammonia, offering a novel function in nitrogen management.

## Abstract

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 (Python regius) 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.