Transamidination in the nephrectomized rat.

Reaction 1 is catalyzed by the enzyme arginine-glycine transamidinase (l), and Reaction 2 by guanidinoacetic acid methylpherase (2). Until recently, the only site of the enzyme transamidinase in mammals was thought to be the mammalian kidney (1, 3). In brief footnotes, Horner et al. (4) mentioned preliminary experiments which indicated that an enzyme which catalyzes the reversal of Reaction 1 is present in rat liver, and Walker (5) mentioned the detection by means of a canavanineornithine assay system of weak transamidinase activity in mammalian thymus and testes. The first data which show that a major extrarenal source of the enzyme exists in mammals were presented quite recently by Walker (6). This investigator detected, in vitro, transamidinase activity in the pancreas of the dog. Interestingly enough, the specific activity of the pancreatic enzyme, on a wet weight basis, was approximately 5 times higher than that of dog kidney. In the present report, evidence is presented that creatine can be synthesized by rats that have been bilaterally nephrectomized, thus the existence of an extrarenal site(s) of transamidination in this animal is indicated. Even more important, the results show that extrarenal transamidinase activity may be of considerable significance in the synthesis of creatine by the rat.