Effects of plant flavonoids on Manduca sexta (tobacco hornworm) fifth larval instar midgut and fat body mitochondrial transhydrogenase

The reversible, membrane-associated transhydrogenase that catalyzes hydride-ion transfer between NADP(H) and NAD(H) was evaluated and compared to the corresponding NADH oxidase and succinate dehydrogenase activities in midgut and fat body mitochondria from fifth larval instar Manduca sexta. The developmentally significant NADPH-forming transhydrogenation occurs as a nonenergy- or energy-linked activity with energy for the latter derived from either electron transport-dependent NADH or succinate utilization, or ATP hydrolysis by Mg++-dependent ATPase. In general, the plant flavonoids examined (chyrsin, juglone, morine, quercetin, and myricetin) affected all reactions in a dose-dependent fashion. Differences in the responses to the flavonoids were apparent, with the most notable being inhibition of midgut, but stimulation of fat body transhydrogenase by morin, and myricetin as also noted for NADH oxidase and succinate dehydrogenase. Although quercetin inhibited or stimulated transhydrogenase activity depending on the origin of mitochondria, it was without effect on either midgut or fat body NADH oxidase or succinate dehydrogenase. Observed sonication-dependent increases in flavonoid inhibition may well reflect an alteration in membrane configuration, resulting in increased exposure of the enzyme systems to the flavonoids. The effects of flavonoids on the transhydrogenation, NADH oxidase, and succinate dehydrogenase reactions suggest that compounds of this nature may prove valuable in the control of insect populations by affecting these mitochondrial enzyme components.