The ADP-stimulated (State 3) respiration of myocardial mitochondria with glutamate-malate, glutamate-pyruvate, palmitylcarnitine and b-hydroxybutyrate as substrates declined in rats after the age of 20 months.There was no significant decline in pyruvate-malate, a-oxoglutarate, palmitylCoA, succinate and ascorbate cytochrome c oxidation. Skeletal muscle mitochondria from senescent animals showed a similar dccline i n glutamate-malate oxidation but not in palmityl-CoA, palmitylcarnitine, succinate and ascorbatecytochrome c oxidation. Thc controlled oxidation with ADP-limiting (State 4) and the ADP/O ratio were not affected. The results indicate an alteration in the subtle regulatory capacity for mitochondrial oxidation in senescent rats. It is suggested that the alteration may be in certain anion transport and associated functions across the mitochondrial membrane or dehydrogenase activity.

The effect of organic mercurials was tested on the following energy-linked reactions in bovine heart submitochondrial particles: (a) reduction of nicctinamide-adenine dinucleotide by succinate coupled to the aerobic oxidation of ascorbate-N,N,N',N'-tetramethyl-p-phenylenediamine, (b) nicotinamide nucleotide transhydrogenase reaction (reduced nicotinamideadenine dinucleotide -, nicotinamide-adenine dinucleotide phosphate) driven by the energy from the oxidation of ascorbate-tetramethyl-p-phenylenediamine, and (c) energy-driven intravesicular acidification measured by the bromothymol blue color change. Reaction a was inhibited about 90% by 10 PM or less of mersalyl or mercuriphenylsulfonate. The energy-linked transhydrogenase reaction (reaction b) was also highly sensitive to mercurials under conditions in which the energy-

Electron transport particles and purified H+-ATPase (F1-F0) vesicles from beef heart mitochondria have been treated with two classes of thiol reagent, viz. membrane-impermeable organomercurials and a homologous series of N-polymethylene carboxymaleimides (Mal-(CH2)x-COOH or AMx). The effect of such treatment on ATP-driven reactions (ATP-Pi exchange and proton translocation) has been examined and compared to the effects on rates of ATP hydrolysis. The organomercurials inhibited ATP-Pi exchange and one of them (p-chloromercuribenzoate) inhibited ATPase activity. Of the maleimide series (AMx), AM10 and AM11 inhibited both ATP-Pi exchange and ATP-driven membrane potential, but not ATPase activity. The other members of the series were essentially inactive. N-Ethylmaleimide was intermediate in its efficacy. Passive H+ conductance through the membrane sector F0 was 50% blocked by AM10, slightly blocked by AM2 and N-ethylmaleimide, and unaffected by the other members of the AMx series. The data imply that one -SH near the membrane surface and one -SH about 12 A from the surface are functional in proton translocation through the H+-ATPase.

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