Cytotoxic Effect of 2-Hydroxyjuglone, a Metabolite in the Branched Pathway of Melanin Biosynthesis in Pyricularia oryzae

Abstract

Scytalone and 2-hydroxyjuglone (2-HJ) are metabolites which greatly accumulate in the culture of Pyricularia oryzae due to the blocking of allo-melanin biosynthesis by such chemicals as tricyclazole or CGA 49104 (pyroquilon). Scytalone was found to be less effective than 2-HJ in reducing the viability of P. oryzae. Thus, effects of 2-HJ on the fungal respiration system and macromolecular syntheses were investigated by using the spheroplasts of P. oryzae. Incorporation of ¹⁴C-precursors into acid-precipitable material as well as respiration was inhibited by the compound non-specifically. In contrast, no such inhibitory effects were observed with CGA 49104. While inhibition by 2-HJ was particularly significant in RNA, protein and lipid syntheses, uptake of ¹⁴C-uridine or ¹⁴C-phenylalanine by spheroplasts was also inhibited by 2-HJ quite markedly, as was seen in the uptake of ¹⁴C-glucose. These results suggest that 2-HJ acts on essential biochemical processes and on membrane systems in P. oryzae. Since the fungal appressoria accumulate significant amount of 2-HJ during their maturation in the presence of tricyclazole or pyroquilon, such a metabolite may possibly play a part in the deterioration of the blast fungus pathogenicity.