Purpurascenines A–C, Azepino-Indole Alkaloids from Cortinarius purpurascens: Isolation, Biosynthesis, and Activity Studies on the 5‑HT_2A Receptor

Three previously undescribed azepino-indole alkaloids, named purpurascenines A–C (1–3), together with the new-to-nature 7-hydroxytryptophan (4) as well as two known compounds, adenosine (5) and riboflavin (6), were isolated from fruiting bodies of Cortinarius purpurascens Fr. (Cortinariaceae). The structures of 1–3 were elucidated based on spectroscopic analyses and ECD calculations. Furthermore, the biosynthesis of purpurascenine A (1) was investigated by in vivo experiments using ¹³C-labeled sodium pyruvate, alanine, and sodium acetate incubated with fruiting bodies of C. purpurascens. The incorporation of ¹³C into 1 was analyzed using 1D NMR and HRESIMS methods. With [3-¹³C]-pyruvate, a dramatic enrichment of ¹³C was observed, and hence a biosynthetic route via a direct Pictet–Spengler reaction between α-keto acids and 7-hydroxytryptophan (4) is suggested for the biosynthesis of purpurascenines A–C (1–3). Compound 1 exhibits no antiproliferative or cytotoxic effects against human prostate (PC-3), colorectal (HCT-116), and breast (MCF-7) cancer cells. An in silico docking study confirmed the hypothesis that purpurascenine A (1) could bind to the 5-HT_2A serotonin receptor’s active site. A new functional 5-HT_2A receptor activation assay showed no functional agonistic but some antagonistic effects of 1 against the 5-HT-dependent 5-HT_2A activation and likely antagonistic effects on putative constitutive activity of the 5-HT_2A receptor.