Abstract
Abstract Adenosine aminohydrolase from calf intestinal mucosa was found capable of hydrolyzing several chemically unrelated substituents on position 6 of purine ribonucleoside. Km values increased in the following order: methylamino, hydroxylamino, amino, bromo, ethylamino, iodo, and chloro. The order of the rate of hydrolysis was amino, hydroxylamino, chloro, bromo, iodo, methylamino, and ethylamino. An amino group on position 2 of 6-halogenated purine ribonucleosides activated hydrolysis of the halide on position 6. Other position 6 substitutions gave rise to competitive inhibitors or inert nucleosides. Adenine nucleosides substituted on position 9 were either substrates or completely inert. The enzyme does not appear to have specific requirements for the 2'- or 3'-hydroxyl groups of adenosine but exhibits a marked requirement for the 5'-hydroxyl group. Apparently, the requirements for enzymatic binding of nucleosides are different from the requirements for the catalytic reaction. A comparison of kinetic parameters for adenosine aminohydrolase from calf intestinal mucosa and Aspergillus oryzae points to a fundamental difference between the two enzymes.
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