Category II solutions to three blocked reactions in urea metabolism. Flux was restored through the blocked reaction AGPRim by the addition of two resolving reactions (EC 188.8.131.52 and EC 184.108.40.206) which couple AGPRim to the blocked reactions ACGSm and ACODA through the consumption and production of their dead-end metabolites, N-acetylglutamate and N-acetylornithine, respectively. We found no evidence that could rule out this gap filling hypotheses. The S1 solutions to the blocked reactions ACGSm and ACODA were identical and involved inter-conversion of the dead-end metabolites of these reactions by glutamate N-acetyltransferase (EC 220.127.116.11). As opposed to the solution for AGPRim, we did not find any indications that reactions similar to EC 18.104.22.168 are found in humans.