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Articles

Active Cytokinins

Photoaffinity Labeling Agents to Detect Binding ^1,2

Ruth Y. Schmitz^b, F. Hardy Moore, III^b,6 and Folke Skoog^b,5

^a Roger Adams Laboratory, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801, Institute of Plant Development, Birge Hall, University of Wisconsin, Madison, Wisconsin 53706

Four series of azidopurines have been synthesized and tested for cytokinin activity in the tobacco callus bioassay: 2- and 8-azido-N⁶-benzyladenines, -N⁶-(²-isopentenyl)adenines, and -zeatins, and N⁶-(2- and 4-azidobenzyl)adenines. The compounds having 2-azido substitution on the adenine ring are as active as the corresponding parent compounds, while those with 8-azido substitution are about 10 or more times as active. The 8-azidozeatin, which is the most active cytokinin observed, exhibited higher than minimal detectable activity at 1.2 x 10^–5 micromolar, the lowest concentration tested. The shape of the growth curve indicates that even a concentration as low as 5 x 10^–6 micromolar would probably be effective. By comparison, the lowest active concentration ever reported for zeatin has been 5 x 10^–5 micromolar, representing a sensitivity rarely attained.

All of the azido compounds have been submitted to photolysis in aqueous ethanol, and the photoproducts have been detected and identified by low and high resolution mass spectrometry. They are rationalized as products of abstraction and insertion reactions of the intermediate nitrenes. The potential of the major released products as cytokinins was also assessed by bioassay. 2-Azido-N⁶-(²-isopentenyl)adenine competed with [¹⁴C]kinetin for the cytokinin-binding protein isolated from wheat germ. When the azido compound was photolysed in the presence of this protein, its attachment effectively blocked the binding of [¹⁴C]kinetin.

⁴ Present address: Armour Research Center, Armour-Dial, Inc., Scottsdale, Arizona 85260.

⁵ To whom requests for reprints should be sent.

⁶ Present address: Roger Adams Laboratory, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801.

¹ This work was supported at the University of Illinois by Research Grant GM-05829 from the National Institutes of Health and by an unrestricted grant from the Hoffmann-La Roche Foundation and at the University of Wisconsin by National Science Foundation Research Grant BMS 72-02226 and by the Research Committee of the Graduate School with funds from the Wisconsin Alumni Research Foundation. The high resolution mass spectrometer and data processing equipment at the University of Illinois employed in the present study were provided by National Institutes of Health Grants CA 11388 and GM 16864, from the National Cancer Institute, and the National Institute of General Medical Sciences.

² Presented in part at the Symposium on Cytokinins, Meeting of the American and Canadian Societies of Plant Physiologists, Madison, Wisconsin, August 16-20, 1977.