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The mitochondrial fraction from the free-living nematode Panagrellus redivivus decarboxylates pyruvate to produce significant amounts of acetaldehyde. Acetaldehyde formation is stimulated by thiamin pyrophosphate, shows a sharp optimum at pH 6.8 and is greater under anaerobic than aerobic conditions. The pyruvate decarboxylase activity cofractionates with, and is probably a partial reaction of, the pyruvate dehydrogenase complex. Acetaldehyde production is modulated by NAD+, ATP and acetyl-CoA and is greatly stimulated by lipoic acid. The pyruvate decarboxylase system is extremely sensitive to thiol-group inhibitors and is inhibited by oxygen in the presence of pyruvate.

1. The fluorescence of aurovertin increases about 100-fold on binding to sub-mitochondrial particles. 2. The mitochondrial ATPase (F 1 ) binds one mole aurovertin/mole F 1 with a dissociation constant of 6·10 −8 M. 3. The fluorescence of mitochondrion-bound aurovertin is maximal during State-3 respiration and is partially quenched on anaerobiosis, addition of respiratory inhibitor, oligomycin or uncoupler, or transition to State 4. This quenching is still present when the binding site is saturated with aurovertin, showing that the quantum yield of fluorescence is lowered. 4. Aurovertin is bound co-operatively to State-3 mitochondria. 5. The curve relating inhibition of State-3 respiration to aurovertin concentration is more sharply sigmoidal than the binding curve. 6. An analysis of the binding and inhibition data leads to the conclusion that aurovertin induces a conformation change in the binding site on F 1 in two ways: (i) directly by acting as an allosteric effector of an oligomeric system, (ii) indirectly by inhibiting State-3 respiration which changes the allosteric constant of the oligomeric system. 7. The concentration of the aurovertin-binding site in both rat-liver and rat-heart mitochondria is about the same as that of the antimycin-binding and oligomycin-binding sites.

1. Introduction Yeast cells offer a unique possibility to study func- tion and biogenesis of components of the mitochon- drial membrane in view of the fact that the yeast mito- chondria are amenable to profound genetic or pheno- typic modifications. This advantage has led to several studies on adenine nucleotide translocation in yeast mitochondria which, however, have brought some con- troversial results [l-6]. The present study was under- taken with the aim of helping to clarify these con- troversies. The study confirms and extends the pre- vious data [ 1,2,7] showing that an ADP/ATP carrier is also present in mitochondria isolated from anaero- bically grown yeast and from a cytoplasmic respira- tory-deficient mutant. In addition, it provides further analysis of the genetically modified ADP/ATP carrier in yeast nuclear mutant [3]. 2. Material and methods Wild type yeast Saccharomyces cerevisiae DT XII, its cytoplasmic (p-) respiration-deficient mutant