. Biological structure and function; proceedings. Biochemistry; Cytology. I20 BRIXTON CHANCE succinate would deprive DPNH of the oxidizing capacity of the chain and lead to a greater degree of pyridine-nucleotide reduction. However, many features of the reaction called our attention to a need for closer study. For example, the rate at which succinate produced increased reduction of pyridine nucleotide appeared to be slow compared with that at which K 50 sec -M Spectrophotometric troce "^ 340-374m//, log Iq/I = 0 010. 7mM succinate Aerobic —>• .—"^ mitochondria ' 330/xM 0-5fxW\ / P
. Biological structure and function; proceedings. Biochemistry; Cytology. I20 BRIXTON CHANCE succinate would deprive DPNH of the oxidizing capacity of the chain and lead to a greater degree of pyridine-nucleotide reduction. However, many features of the reaction called our attention to a need for closer study. For example, the rate at which succinate produced increased reduction of pyridine nucleotide appeared to be slow compared with that at which K 50 sec -M Spectrophotometric troce "^ 340-374m//, log Iq/I = 0 010. 7mM succinate Aerobic —>• .—"^ mitochondria ' 330/xM 0-5fxW\ / PNH/sec Platinum microelectrode trace Pyridine nucleotide red uction I Fig. 2. Illustrating increased reduction of pyridine nucleotide in a suspension of rat-liver mitochondria caused by addition of 7 mM succinate. Absorbancy changes measured spectrophotometrically by double-beam spectrophotometer and respira- tion by vibrating platinum microelectrode. Downward deflection upon addition of reagent indicates increased light absorption at 340 m/x relative to 374 m/x. Diagram indicates final concentrations of reagents added, respiratory rate in / and increment of oxygen taken up during phosphorylation of 330 [jlm ADP. Rates of pyridine-nucleotide reduction also given in / The metabolic states of mitochondria are indicated by numerals 1-3-4. Rat-liver mitochondria diluted in isotonic salt meciium to concentration of approximately 2 mg. protein/ ml. at pH 7-4, temperature 25' (Expt. 332-2). succinate could intercept oxidizing equivalents from the respiratory chain and was no more rapid than the rate at which pyridine nucleotide could be reduced by DPNH-linked substrates. Another puzzling feature of the re- action was that it w^as slowed by addition of very low concentrations of uncoupling agents and completely inhibited by larger concentrations [3]. This too seemed inconsistent with a simple competitive reaction which should also occur at higher respi
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