Midgut mitochondrial transhydrogenase in wandering stage larvae of the tobacco hornworm, Manduca sexta

Midgut mitochondria from fifth larval instar Manduca sexta exhibited a transhydrogenase that catalyzes the following reversible reaction: NADPH + NAD⁺ [leftright arrow] NADP⁺ + NADH. The NADPH-forming transhydrogenation occurred as a nonenergy- and energy-linked activity. Energy for the latter was derived from the electron transport-dependent utilization of NADH or succinate, or from Mg++-dependent ATP hydrolysis by ATPase. The NADH-forming and all of the NADPH-forming reactions appeared optimal at pH 7.5, were stable to prolonged dialysis, and displayed thermal lability. N,N'-dicyclohexylcarbodiimide (DCCD) inhibited the NADPH [rightward arrow] NAD⁺ and energy-linked NADH [rightward arrow] NADP⁺ transhydrogenations, but not the nonenergy-linked NADH [rightward arrow] NADP⁺ reaction. Oligomycin only inhibited the ATP-dependent energy-linked activity. The NADH-forming, nonenergy-linked NADPH-forming, and the energy-linked NADPH-forming activities were membrane-associated in M. sexta mitochondria. This is the first demonstration of the reversibility of the M. sexta mitochondrial transhydrogenase and, more importantly, the occurrence of nonenergy-linked and energy-linked NADH [rightward arrow] NADP⁺ transhydrogenations. The potential relationship of the transhydrogenase to the mitochondrial, NADPH-utilizing ecdysone-20 monooxygenase of M. sexta is considered. Arch. Insect Biochem. Physiol. 69:118-126, 2008.