Urate deposits in the gecarcinid land crab Gecarcoidea natalis are synthesised de novo from excess dietary nitrogen

The urate content of the gecarcinid land crab Gecarcoidea natalis was correlated to the amount of nitrogen assimilated. Crabs fed a high-nitrogen diet (ad libitum amounts of soy beans and fig leaves, Ficus macrophylla) for 6 weeks assimilated approximately 23 times more nitrogen (33.9±5.6mmolkg-1day-1) than animals fed a diet low in nitrogen (fig leaves alone) (1.5±0.7mmolkg-1day-1). Animals maintained on a high-nitrogen diet accumulated urate (67.1±29.4mmolkg-1drymass), while animals fed the low-nitrogen diet did not accumulate significant amounts of urate compared with the control animals killed at the beginning of the dietary period. The urate deposits clearly originate from the excess dietary nitrogen ingested on the high-nitrogen diet. The intake of preformed dietary purine was low (0.028±0.005mmolkg-1drymass) and at most could only account for 0.04% of the urate accumulated by crabs fed the high-nitrogen diet. This indicates that the urate was synthesised de novo. When crabs were fed a high-nitrogen diet supplemented with [15N]glycine, the 15N heavy isotope was incorporated into urate. This provided direct evidence that the urate was synthesised de novo.