The effects of oleic acid and the serum from two patients with Reye's syndrome (RS-serum) on the respiratory control ratio (RCR) of rat liver mitochondria were studied; the reaction site of each reagent was determined by using various substrates for the respiratory chain. The results were as follows: 1. Pre-incubation of rat liver mitochondria with oleic acid induced a decrease in RCR when succinate or ascorbate+cytochrome
c were employed as substrates. 2. Under the identical conditions, RS-serum induced a significant decrease in RCR with ascorbate+cytochrome
c. 3. When KCN was added to the assay medium, both oxygen consumption rate and RCR were decreased as expected in oxidizing ascorbate+cytochrome
c. But the effect of KCN was less obvious when pyruvate+L-malate were employed as substrates. The latter phenomenon might suggest that the rate-limiting step in the electron transport system from NADH to O
2 resided on the substrate side and not on the side between coenzyme Q and O
2.
Among the substrates (succinate, pyruvate+L-malate, ascorbate+N, N, N′, N′-tetramethyl-phenylenediamine, ascorbate+cytochrome
c) tested, the strongest effect was observed when ascorbate+cytochrome
c were employed as substrates. These results suggest that both oleic acid and RS-serum showed similar substrate-specific inhibition of RCR, predominantly at the site of cytochrome
c oxidase. Thus, a localized effect of oleic acid or RS-serum on the mitochondrial respiratory chain became apparent.
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