INHIBITION OF RESPIRATION AT SITE-I BY ASIMICIN, AN INSECTICIDAL ACETOGENIN OF THE PAWPAW, ASIMINA-TRILOBA (ANNONACEAE)

The insecticidal mode of action of the acetogenin asimicin and a standardized acetogenin extract, FO20, isolated from Asimina triloba Dunal was investigated. Dietary FO20 reduced the rate of oxygen consumption by fourth instar Ostrinia nubilalis as measured with a constant volume manometer. Further examination of the respiratory effect of both FO20 and asimicin was carried out on mitochondria isolated from the midguts of fifth instar O. nubilalis and in which O2 was measured polarographically. The results of these studies indicate a significant reduction in state 3 respiration using malate and pyruvate as substrate, caused by asimicin (concentration for 50% INHIBITION = 0.55 nmol/mg protein) or FO20 (I50 = 1.69 ·g/mg protein). No reductions of ADP:O or respiratory control ratios were observed in partially inhibited mitochondria. Oxygen uptake was restored and state 3 respiration was not inhibited when succinate was the substrate. These results are evidence of a site of inhibition between NADH and ubiquinone (site 1) within the electron transport chain. © 1993 Academic Press.