ON RATE-DETERMINING STEP IN ACTION OF ADENOSINE DEAMINASE

Takadiastase adenosine deaminase and calf duodenal adenosine deaminase show no appreciable primary deuterium isotope effect on the kinetics of hydrolysis of adenosine. The product inosine is a competitive inhibitor of both enzymes. The alternate product inosine monophosphate is a competitive inhibitor of the enzyme from takadiastase. The product ammonia is a noncompetitive inhibitor of the enzyme from takadiastase, with Kiapp = 3.9 × 10-2 M for ammonia as the free base. Ammonia at concentrations as high as 1 M gives no appreciable inhibition of the enzyme from calf duodenum. These results, in conjunction with earlier data, support a mechanism involving rate-limiting isomerization of the enzyme-substrate complex and are consistent with rate-limiting formation of a tetrahedral intermediate. © 1969, American Chemical Society. All rights reserved.