Oxyconformity in the intertidal worm Sipunculus nudus: the mitochondrial background and energetic consequences

The energetic consequences of strict oxyconformity in the intertidal worm S . nudus were studied by characterizing the Po-2 dependence of respiration i n mitochondria isolated from the body wall tissue. Mitochondrial respiratio n rose in a Po-2 range between 2.8 and 31.3 kPa from a mean of 56.5 to 223. 9 nmol O mg protein(-1) h(-1). Respiration was sensitive to both salicylhyd roxamic acid (SHAM) and KCN. Po-2 dependence remained unchanged with satura ting and nonsaturating substrate levels (malate, glutamate and ADP). A conc omitant decrease of the ATP/O ratio revealed a lower ATP yield of aerobic m etabolism at elevated Po-2. Obviously, oxyconforming respiration implies pr ogressive uncoupling of mitochondria. The decrease in ATP/O ratios at highe r Po-2 was completely reversible. Addition of 90.9 mu mol H2O2 l(-1) did no t inhibit ATP synthesis. Both observations suggest that oxidative injury di d not contribute to oxyconformity. The contribution of the rates of mitocho ndrial ROS production and proton leakiness to mitochondrial oxygen consumpt ion and uncoupling was investigated by using oligomycin as a specific inhib itor of the ATP synthase. The maximum contribution of oligomycin independen t respiration to state 3 respiration remained below 6% and showed a minor, insignificant increase at elevated Po-2, at a slope significantly lower tha n the increment of state 3 respiration. Therefore, Po-2 dependent mitochond rial proton leakage or ROS production cannot explain oxyconformity. In conc lusion Po-2 dependent state 3 respiration likely relates to the progressive contribution of an alternative oxidase (cytochrome o), which is characteri zed by a low affinity to oxygen and an ATP/O ratio similar to the branched respiratory system of bacteria. The molecular nature of the alternative oxi dase in lower invertebrates is still obscure. (C) 2001 Elsevier Science Inc . All rights reserved.