PLANT CYTOTOXICITY OF NEBULARINE (PURINE RIBOSIDE)

The plant cytotoxicity of nebularine, a purine riboside antibiotic produced by the fungus Lepista (Clitocybe) nebularis, has been examined. The compound inhibits the growth of seedlings of various species and causes mitotic aberrations in root tips. Because of the potent effect of this purine derivative on cell division, the possibility that it is a cytokinin antagonist was examined experimentally, In both Amaranthus and leaf senescence bioassays, nebularine strongly antagonises the effects of exogenous 2.5 mu M cytokinins. An eight-fold excess of the antibiotic over kinetin or zeatin resulted in a 60% inhibition in the Amaranthus assay. The inhibitory effect of 2.5 mu M nebularine was alleviated by increasing the concentration of kinetin. Similar effects were seen in the leaf senescence assay. During the study nebularine (20 mu M) was observed to produce an 85% inhibition of xanthine oxidase; kinetic analysis of the effect showed that nebularine is a non-competitive inhibitor of this enzyme.