Mechanism of action of the antifugal agent polyoxin D

The antibiotic polyoxin D was shown to inhibit the incorporation of (14)C-glucosamine into cell wall chitin in Neurospora crassa at levels which were comparable with those required for inhibition of fungal growth. At the same time, the antibiotic increased the accumulation of a nucleotide, which was identified as uridine diphosphate (UDP)-N-acetylglucosamine, indicating inhibition of chitin synthesis. Chitin synthetase (UDP-N-acetylglucosamine: chitin N-acetylglucosaminyl transferase, EC 2.4.1.16) of N. crassa was found to be strongly inhibited by polyoxin D, as determined by the transfer of (14)C-N-acetylglucosamine from (14)C-UDP-N-acetylglucosamine to the particulate fraction. The inhibition was competitive with respect to UDP-N-acetylglucosamine and specific for chitin synthetase. The K(i) for polyoxin D in the reaction was 1.40 x 10(-6)m, and the K(m) for UDP-N-acetylglucosamine was 1.43 x 10(-3)m. The formation of osmotically sensitive, protoplast-like structures, when the fungus Cochliobolus miyabeanus was grown in the presence of polyoxin D, also suggested that the primary site of action of polyoxin D was in the formation of cell wall structures.