The Estimation of Urea and Amino Acid Nitrogen in Animal Tissues.*

In the course of some work on protein metabolism it became advisable to estimate urea in animal tissues. Although we were drawn somewhat to the urease method we felt compelled for two reasons to employ other technique. In the first place, it was quite apparent that owing to the relatively large amount of ammonia in the tissue samples, the urea estimations would have to be accompanied by determinations of the preformed ammonia. Here we found ourselves presented by several additional difficulties, not the least of which was considerable uncertainty about the source of the tissue ammonia and the rate of its formation. From the work of Gad-Andresen (1) and of Hoagland and colleagues (2, 3) and by inference from the known lability of the blood ammonia precursors (4-10) we considered it likely that the preformed ammonia of the tissues would not be estimated easily. There was also the possibility that an unknown fraction of the tissue ammonia might arise, post mortem, from urea, leading obviously to a decrease in the latter. The observations of GadAndresen (1) on ammonia formation in muscle, although unconfirmed in some quarters (2, 3), justified the entertainment of such a fear. Finally, the recent work of Addis (11) throws grave doubt upon the permissibility of the urease method in the presence of arginase. Of the remaining analytical methods, we regarded that of Fosse (12) as the most hopeful; although for simplicity in technique we wished to employ the same deproteinizing agent in the estimation