Evidence for an enantioselective pumiliotoxin 7-hydroxylase in dendrobatid poison frogs of the genus Dendrobates

Abstract

Dendrobatid poison frogs readily accumulate alkaloids from diet into skin, where such compounds serve as a chemical defense against predators. Arthropods seem to be the source of decahydroquinolines (DHQs), several izidines, coccinellines, spiropyrrolizidines, pumiliotoxins (PTXs), and allopumiliotoxins (aPTXs). A DHQ iso-223F, and PTX (+)-251D were fed to poison frogs of the dendrobatid genera Dendrobates, Epipedobates, and Phyllobates. The two alkaloids were accumulated in skin unchanged except for the three species of Dendrobates, where ≈80% of accumulated PTX (+)-251D was stereoselectively hydroxylated to aPTX (+)-267A. The unnatural enantiomer PTX (-)-251D was accumulated efficiently when fed to Dendrobates auratus, but was not hydroxylated. The enantiomers of PTX 251D and their desmethyl analogs were synthesized from N-Boc-protected (-)- and (+)-proline methyl esters. Both PTX (+)-251D and aPTX (+)-267A proved to be potent convulsants in mice, with (+)-267A being ≈5-fold more toxic than (+)-251D. Both alkaloids were hyperalgesic at the site of injection. The unnatural PTX (-)-251D caused no overt effect in mice. Thus, the evolutionary development of a pumiliotoxin 7-hydroxylase would have provided frogs of the genus Dendrobates with a means of enhancing the antipredator potency of ingested PTXs.