UK-3A

A novel antifungal antibiotic, UK-3A, was obtained from the mycelial cake of Streptomyces sp. 517-02. UK-3A was very similar in structure to UK-2A, a structural relative of antimycin A. The antifungal spectrum of UK-3A was relatively broad (MICs for yeasts and filamentous fungi: 1.56-6.25 and 0.39-1.56 micrograms/ml, respectively). The cytotoxic activity of UK-3A was weak (IC50: 18-100 micrograms/ml).

UK-2A and UK-3A are structural relatives of antimycins, which were isolated as antifungal antibiotics with little cytotoxicity that demonstrated inhibition of respiratory activity. They halve the cellular respiration of yeast within 4-5 minutes and the intracellular ATP content within 2-5 minutes. Moreover, they inhibited the yeast mitochondrial respiration using beta-hydroxybutyrate and succinate as a respiratory substrate, but no inhibition was observed using ascorbate-reduced tetramethyl p-phenylenediamine as the substrate. The site of respiratory inhibition of UK-2A and UK-3A was thought to be the cytochrome bc1 complex in the mitochondrial electron transport chain of yeast cells. They also inhibited the mitochondrial respiration of rat liver. It has been suggested that intact animal cells might have some system to defend themselves from the actions of UK-2A and UK-3A.

Novel 2-hydroxynicotinoyl-serine-butyl esters have been synthesized. Three-step reactions from L-serine by esterification with n-butanol, amidation with 2-hydroxynicotinic acid and esterification with the corresponding carboxylic acids gave AD-1, AD-2 and AD-3. The toxicity level of esters were determined by Brine shrimp assay, and antibiotic activity was tested against Escherichia coli, Bacillus subtilis, Staphylococcus aureus and Candida albicans. AD-3 showed greater activity as a growth inhibitor of B. subtilis and S. aureus compared to Antimycin A(3).