Enzymatic Activity that Catalyzes Degradation of N⁶-(Δ²-Isopentenyl) adenosine

Abstract

N⁶-(Δ²-isopentenyl) adenosine (i⁶A) and its analogs N⁶-isopentenyl, isopentyl, 4-hydroxy-3-methylbut-2-trans-enyl, and 3-hydroxy-3-methylbutyl, are competitive inhibitors of the deamination of adenosine by an adenosine amino-hydrolase [EC 3. 5. 4. 2] isolated from chicken bone marrow. The K_i values fall in the range 1-3×10^-4. The same enzyme also catalyzes conversion of i⁶A and other N⁶-(alkyl substituted) analogs to inosine. The potent inhibitor of the aminohydrolase from calf intestinal mucosa, 6-amino-9-(1-hydroxy-2-octyl) purine, inhibits degradation of both adenosine and i⁶A by the bone marrow enzyme K_i 3.5×10^-7 and 1.0×10^-7, respectively.
An assay using 8-¹⁴C-i⁶A as substrate was used toassay various tissues for this enzyme activity. The major site of occurrence of this enzyme is associated with the blood system. The enzyme activity was detected in chicken, pig, rabbit, and human samples.
An assay based on gas chromatography was used to monitor the levels in vivo of N⁶-(substituted) adenosine derivatives in the blood of a rabbit. i⁶A is cleared fromthe blood in about 4 hr after administration of a single dose (intravenous, 150 mg/kg) while the analog, N⁶-(3-chlorobut-2-enyl) adenosine, that is not a substrate for the enzyme, is clearedmuch more slowly. These results suggest that this enzyme activity contributes to the clearance of i⁶A from humoral blood.