薬科学Heronamide C is a 20-membered polyene macrolactam isolated from a marine-derived Streptomyces sp. and shows potent antifungal activity by targeting membrane phospholipids. Testing the stability of heronamide C under UV irradiation (365 nm) revealed that heronamide C is converted mainly to heronamide B and a minor product, heronamide A, as determined by LC-MS analysis. In addition, the antifungal activity of heronamides A and B as tested against fission yeast was much weaker than that of heronamide C. Taking these results together, further structure-activity relationship studies of the heronamides are helpful for developing a chemical probe and a promising lead compound for antifungal drugs. Keywords: Heronamides, Polyene, Macrolactam, a Marine-derived Streptomyces, Antifungal activity, Liquid chromatography-mass spectrometry, Natural product chemistry, Chemical probe, Chemical biology海洋放線菌由来の20員環ポリエンマクロラクタム化合物で抗真菌作用を持つヘロナミドCの紫外線照射（365nm）による安定性試験を行った。また、ヘロナミドCと紫外線照射によって生成したヘロナミドA、Bの抗真菌作用を調べた。その結果、ヘロナミドCは紫外線照射によってヘロナミドA、B（主変換体）へと変換され、それらはヘロナミドCほどの抗真菌活性を示さないことが明らかになった。これらの結果から、ヘロナミドCが有する20員環ポリエンマクロラクタム骨格は抗真菌作用に重要であることが示唆され、さらなる構造活性相関研究はケミカルプローブ開発や新規抗真菌剤の開発に役立つことが期待される。