Employing "phosphorylating" submitochondrial particles as the source of pyridine nucleotide transhydrogenase, the occurrence of an energy-linked NADH----NADP+ transhydrogenation in the adult cestode Hymenolepis diminuta was demonstrated. The isolated particles displayed rotenone-sensitive NADH utilization and the reversible transhydrogenase, with the NADPH----NAD+ transhydrogenation being more prominent. Although not inhibiting the NADPH----NAD+ reaction, rotenone, but not oligomycin, inhibited the catalysis of NADH----NADP+ transhydrogenation. In the presence of rotenone, Mg2+ plus ATP stimulated by more than 3-fold NADH----NADP+ transhydrogenation. This stimulation was ATP specific and was abolished by EDTA or oligomycin. Succinate was essentially without effect on the NADH----NADP+ reaction. These data demonstrate the occurrence of an energy-linked transhydrogenation between NADH and NADP+ with energization resulting from either electron transport-dependent NADH oxidation or ATP utilization via the phosphorylating mechanism in accord with the preparation of "phosphorylating" particles. This is the first demonstration of an energy-linked transhydrogenation in the parasitic helminths and apparently in the invertebrates generally.

The catalysis of hydrogen peroxide accumulation by the mitochondrial, membrane-associated NADH oxidase and less active succinoxidase of adult Hymenolepis diminuta was confirmed. NADH-dependent peroxide formation by isolated mitochondrial membranes occurred at about half the coincident rates of NADH and oxygen utilization, whereas succinate-dependent peroxide formation accounted for approximately 40% of the oxygen consumed. These findings, coupled with evaluations of the oxidases, indicated that both systems use in common 2 mechanisms for oxygen reduction, 1 of which is peroxide-forming. Neither system was sensitive to cyanide, azide, or antimycin A. Rotenone inhibition of NADH oxidation resulted in equivalent decreases in oxygen consumption by the peroxide-forming and nonperoxide-forming mechanisms. In contrast, malonate inhibition occurred via disruption of the peroxide-forming mechanism. Fumarate stimulated membrane-catalyzed NADH oxidation, despite aerobic conditions, and this fumarate reductase was rotenone-sensitive. NADH- or succinate-dependent peroxide formation virtually was abolished and oxygen consumption was minimal in the presence of fumarate. Malonate also inhibited fumarate-dependent NADH oxidation and succinate-dependent peroxide formation/oxygen consumption. Collectively, these findings clearly indicate that NADH- or succinate-dependent hydrogen peroxide accumulation involves the malonate-sensitive fumarate reductase, in the absence of fumarate. A model of the H. diminuta electron transport system is presented.