Synthesis and Inhibitory Action of Novel Acetogenin Mimics with Bovine Heart Mitochondrial Complex I

Abstract

Studies on the inhibition mechanism of acetogenins, the most potent inhibitors of complex I, are useful to elucidate the structural and functional features of the terminal electron-transfer step of this enzyme. We synthesized acetogenin mimics that possess two alkyl tails without a gamma-lactone ring, named Deltalac-acetogenin, and examined their inhibitory action on bovine heart mitochondrial complex I. Unexpectedly, the Deltalac-acetogenin carrying two n-undecanyl groups (compound 3) elicited very potent inhibition comparable to that of bullatacin. The inhibitory potency of compound 3 markedly decreased with shortening the length of either or both alkyl tails, indicating that symmetric as well as hydrophobic properties of the inhibitor are important for the inhibition. Both acetylation and deoxygenation of either or both of two OH groups adjacent to the tetrahydrofuran (THF) rings resulted in a significant decrease in inhibitory potency. These structural dependencies of the inhibitory action of Deltalac-acetogenins are in marked contrast to those of ordinary acetogenins. Double-inhibitor titration of steady-state complex I activity showed that inhibition of compound 3 and bullatacin are not additive, though the inhibition site of both inhibitors is downstream of iron-sulfur cluster N2. Our results indicate that the mode of inhibitory action of Deltalac-acetogenins differs from that of ordinary acetogenins. Therefore, Deltalac-acetogenins can be regarded as a novel type of inhibitor acting on the terminal electron-transfer step of complex I.

Synthesis and Inhibition Mechanism of Δlac-Acetogenins, a Novel Type of Inhibitor of Bovine Heart Mitochondrial Complex I

“…It is noteworthy that the IC 50 values of stereoisomers of cis-solamin and corossoline, which are common acetogenins having one THF ring with various stereochemistries around the hydroxylated THF ring, are 1.5-2.0 nM under the same experimental conditions (13). These results along with the previous structure-activity study (8) indicate that structural factors required for the inhibitory action are entirely different between ∆lac-acetogenins and common acetogenins.…”

“…Further lengthening of the para substituent(s) beyond the n-butyl group resulted in a significant loss of the activity (compounds 9 and 10). Although this result seems to be somewhat peculiar, a similar tendency was observed for the original ∆lac-acetogenins, which have sufficiently long alkyl tails (8). An excessive increase in hydrophobicity of the tail may be rather adverse to the activity probably due to some sort of trapping in the hydrophobic lipid bilayer of the membrane.…”

“…The 1 H and 13 C NMR data of 2 were identical to those of 1 (8) (B) Compound 3. The synthetic procedures of compound 3 are outlined in Scheme 1B.…”

“…Although the reason the number of THF rings is not crucial is still unclear, concerning the stereochemistry, an exhaustive conformational space search analysis showed that some stable conformations of the adjacent bis-THF rings of stereoisomers are in fairly good superimposition due to the flexibility of this moiety (9). As we synthesized no stereoisomers of ∆lac-acetogenins in the previous study (8), the effect of the stereochemistry surrounding the hydroxylated THF rings on the inhibition remained unknown. Therefore, we synthesized compound 2 (enantiomer of 1), which has the S configuration at all chiral centers.…”

“…Double-inhibitor titration of steady-state complex I activity showed that the inhibition of compound 1 and bullatacin is not additive, suggesting that the binding sites of the two inhibitors are not identical (8). However, an electron paramagnetic resonance (EPR) spectroscopic study on the redox state of iron-sulfur clusters indicated that the inhibition site of compound 1 is downstream of the iron-sulfur cluster N2, as is the case for other ordinary complex I inhibitors (8). These results along with somewhat different profiles of structure-activity relationship between ∆lac-acetogenins and common acetogenins suggested that the mode of inhibitory action of ∆lac-acetogenins may be different from that of common acetogenins.…”