Guanine aminohydrolase in rat and mouse red cells: a potent inhibitor of guanylation of tRNA.

1. The red blood cells of mice and rats contained guanine aminohydrolase (EC 3.5.4.3). This enzyme was not present in rabbit, sheep or human red blood cells. 2. The enzyme from rat blood cells was separated into two activities by column chromatography on DEAE-cellulose. Both isozymes were labile but it was possible to show that the more abundant enzyme followed Michaelis-Menten kinetics, had an apparent Km of 4.0-10-6 M and was not activated by GTP nor inhibited by allantoin. 3. We believe, therefore, that guanine aminohydrolase was the protein in rat and mouse red blood cells that inhibited the enzyme (in rabbit reticulocytes) responsible for guanylation of tRNA.