Revision of the Unstable Picrotoxinin Hydrolysis Product

Abstract

The plant metabolite picrotoxinin (PXN) is a widely used tool in neuroscience for the identification of GABAergic signaling. Its hydrolysis in weakly alkaline media has been observed for over a century and the structure of the unstable hydrolysis intermediate was assigned by analogy to the degradation product picrotoxic acid. Here we show this assignment to be in error and we revise the structure of the hydrolysis product by spectroscopic characterization in situ. Counterintuitively, hydrolysis occurs at a lactone that remains closed in the major isolable degradation product, which accounts for the longstanding mistake in the literature.

Methylation Confers Accessibility, Stability and Selectivity to Picrotoxinin

“…13b We observed the initial (kinetic) site of hydrolysis with 1 equiv. NaOH to be C11, 26 and exposure to excess base led to complex decomposition. 27 We hypothesized that this latter decomposition derived from irreversible C15 hydrolysis -a "point of no return" due to concomitant epoxide opening or cyclohexane relaxation, with no precedent for chemical or biochemical reversion.…”

“…Due to the translational potential of these observations, we also investigated the relative rates of hydrolysis observed in mouse plasma. C5 methylation led to a doubling of the extrapolated half-life in plasma (Figure 3e), likely a mixture of reversible C11 hydrolysis 26 and irreversible C15 hydrolysis. 13b No prior studies have identified PXN modifications that enhance metabolic stability.…”