Antifungal activity of radicicol against Mucor flavus IFO 9560

The antifungal activity of radicicol against Mucor flavus IFO 9560 was inve stigated, Radicicol induced bursting of spores during germination and morph ological changes of the mycelial tip such as overbranching and swelling dur ing exponential growth. In addition, radicicol showed a dose-dependent inhi bitory effect on spore germination. Radicicol also inhibited the incorporat ion of radioactive precursors into DNA, RNA, protein, and chitin fractions by 20-30%, but not into the lipid fraction. There were no inhibitory effect s on either endogenous or exogenous cellular respiration, Moreover, leakage of UV-absorbing, phenol sulfate-positive, or folin reagent-positive materi als from the mycelia was not observed at an early stage of growth inhibitio n. On the other hand, kinetic studies of chitin synthase in the untreated m ycelia revealed that radicicol noncompetitively inhibited the enzyme at K-i of 87 mu M. Furthermore, upon incubation of the normal mycelia with radici col in 50 mM KH2PP4-NaOH buffer (pH 6.5) containing 10 mM MgCl2, chitin syn thase from the mycelia was inactivated gradually at first, and completely a fter 24-h incubation. These results suggested that radicicol exhibits the a ntifungal activity by disturbing cen wall biosynthesis through the inactiva tion of chitin synthase. However, at an early stage of growth inhibition, r adicicol was thought to affect cellular function including nucleic acid and protein syntheses, in addition to the reversible noncompetitive inhibition of chitin synthase.