Isolation of a spirolactone norditerpenoid as a yeast Ca2+ signal transduction inhibitor from Kuji amber and evaluation of its effects on PPM1A activity

A different type of biologically active compound from Kuji amber (Late Cretaceous, Japan) before the K-Pg boundary [65 million years ago (Ma)] was isolated based on the growth-restoring activity of a mutant yeast involving Ca²⁺ signal transduction. It was identified as a spirolactone norditerpenoid, (4R*, 5S*, 8R*, 9R*, 10S*)-14,15,16,19-tetranor-labdan-13,9-olide (1) from spectral analyses with high-resolution electron ionization mass spectrometry (HREIMS), 1D and 2D nuclear magnetic resonance (NMR). Although the planar structure of 1 is known as an artificial derivative from marrubiin, it was isolated as a natural product from Kuji amber and its structure was elucidated for the first time. It had a growth-restoring activity against the mutant yeast through the direct or indirect inhibition of calcineurin activity [protein phosphatase, Mg²⁺/Mn²⁺-dependent 1A (PPM1A) activation]. Furthermore, the compound had potent inhibitory effect against the degranulation of rat basophilic leukemia 2H3 (RBL-2H3) cells.

Ca2+-Signal Transduction Inhibitors, Kujiol A and Kujigamberol B, Isolated from Kuji Amber Using a Mutant Yeast

A podocarpatriene and a labdatriene derivative, named kujiol A [13-methyl-8,11,13-podocarpatrien-19-ol (1)] and kujigamberol B [15,20-dinor-5,7,9-labdatrien-13-ol (2)], respectively, were isolated from Kuji amber through detection with the aid of their growth-restoring activity against a mutant yeast strain ( zds1Δ erg3Δ pdr1Δ pdr3Δ), which is known to be hypersensitive with respect to Ca²⁺-signal transduction. The structures were elucidated by spectroscopic data analysis. Compounds 1 and 2 are rare organic compounds from Late Cretaceous amber, and the mutant yeast used seems useful for elucidating a variety of new compounds from Kuji amber specimens, produced before the K-Pg boundary.