Catalyzes the oxidation of cytokinins, a family of N(6)-substituted adenine derivatives that are plant hormones, where the substituent is an isopentenyl group. Substrate preference is 2-(2-hydroxyethylamino)-9-methyl-N(6)-isopentenyladenine >> isopentenyladenine > cis-zeatin = isopentenyladenosine = zeatin >> zeatin riboside. A + H2O + N(6)-dimethylallyladenine = 3-methyl-2-butenal + adenine + AH2 Optimum pH is 6.5. Monomer. Might be located on membranes.