Marinoic Acid, a Novel Bufadienolide-Related Substance in the Skin of the Giant Toad, Bufo marinus.

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Abstract
We found a novel substance, 3β-hydroxy-11, 12-seco-5β, 14β-bufa-20, 22-dienolide-11, 14-olide-12-oic acid (1), which we called marinoic acid, in the skin of the toad, Bufo marinus. The structure was established from NMR and MS data. Like bufadienolides, marinoic acid contained an A/B ring structure in the cis configuration and a D/α-pyrone ring structure, but the structure of the C ring differed considerably from that of bufadienolides. Marinoic acid exhibited biological activity, as demonstrated by inhibition of Na+, K+-ATPase enzymatic activity, and by inhibition of [3H]ouabain binding to the digitalis receptor site on Na+, K+-ATPase, although marinoic acid was a less effective inhibitor than typical bufadienolides. Although marinoic acid cannot be classified as a bufadienolide, its chemical structure and its Na+, K+-ATPase inhibitory activity suggest that it is bufadienolide-related.

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