MEMBRANE CONFORMATIONS AND THEIR RELATION TO CYTOXICITY OF ASIMICIN AND ITS ANALOGS

Certain plant species belonging to the family Annonaceae produce Annon aceous acetogenins, which are a unique class of long-chain fatty acid derivatives with potent cytotoxicity. Putative protein targets of the acetogenins aro membrane-associated proteins, including complex I. Asi micin and its analogues constitute a class of Annonaceous acetogenins containing two tetrahydrofuran (TMF) rings with hydrocarbon chains tet hered to each ring; an alpha,beta-unsaturated gamma-lactone ring is te rminal to one of the alkyl chains. The compounds examined in this stud y differ in the length of the alkyl chain between the THF rings and th e lactone ring, The positions of both the THF and the lactone rings wi thin liposomal membranes were determined by protein (H-1) nuclear magn etic resonance spectroscopy. The depth of membrane penetration of acet ogenins, coupled to membrane diffusion, controls the conformation of a cetogenins as they diffuse to an active site. Based on H-1 intermolecu lar nuclear Overhauser effects (NOEs). tile THF rings of all acetogeni ns studied reside near the polar interfacial head group region of the DMPC. This was corroborated by H-1 two-dimensional NOE spectroscopy an d, differential scanning calorimetry studies. The H-1 difference NOE s pectra indicated that the lactone rings of asimicin and parviflorin, t he latter of which has two fewer carbons in its alkyl chain, are locat ed below the glycerol backbone in the membrane. In contrast with asimi cin and parviflorin, the lactone ring of longimicin B, an asimicin ana logue with an alkyl chain four carbons shorter, resides close to the m idplane in the membrane. This was corroborated by manganese-induced br oadening studies. Since the THF rings are located near the center of t he acetogenin molecules and the lactone ring is terminal to a long alk yl chain, these observations indicate that an asimicin-type acetogenin can be in either sickle-shaped or U-shaped conformations, depending o n the length of the alkyl chain between the THF rings and the lactone ring, interestingly, longimicin B does nor exhibit significant cytotox icity, but parviflorin is as cytotoxic as asimicin. The cytotoxicity o f the asimicin-type of acetogenins would seem to be strongly related t o the membrane conformation. This is the first report elucidating the conformation of Annonaceous acetogenins in membranes.