Discovery of Suomilide and Cyanopeptolin Analogues by ¹⁵ N Stable Isotope Labeling and Genome Mining of Cyanobacteria

Cyanobacteria have been a promising source for natural product drug discovery for decades using mainly activity-guided approaches. More recently, genome mining and bioinformatics-guided approaches have become increasingly utilized in bacterial drug discovery and have proven to be successful in revealing novel metabolites predicted by biosynthetic gene clusters. However, detection and identification of these predicted metabolites can be difficult. Connecting nitrogen-containing natural products with the corresponding gene clusters encoding them in cyanobacteria can be aided by cyanobacteria's ability to incorporate inorganic nitrogen into their secondary metabolites. Feeding cyanobacteria with ¹⁵N-labeled and ¹⁴N nitrate and comparing the resulting metabolic profiles via mass spectrometry allows for the identification of nitrogen-containing metabolites, which can then be connected to biosynthetic gene clusters. Investigation of <i>Nostoc</i> sp. UIC 10890 using this workflow identified 28 total gene clusters and five groups of masses incorporating ¹⁵N-labeled nitrogen. Two of these groups contained new compounds, and the structures were elucidated by NMR and MS/MS. Three new compounds suomilide F, cyanopeptolin UIC949, and cyanopeptolin UIC999 were shown to have inhibitory activity against different serine proteases.