Joergensen 1985 Biochem J

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Jørgensen BM, Rasmussen HN, Rasmussen UF (1985) Ubiquinone reduction pattern in pigeon heart mitochondria. Identification of three distinct ubiquinone pools. Biochem J 229:621-9.

» PMID:4052014 Open Access

Joergensen BM, Rasmussen HN, Rasmussen UF (1985) Biochem J

Abstract: Intact pigeon heart mitochondria showed 10-30 % ubiquinone reduction in the absence of substrates. This reduction could not be ascribed to endogenous substrates, as judged by lack of effect of inhibitors and uncouplers and by the very low endogenous respiratory rate. Addition of NADH in the presence of antimycin caused further reduction of about 10 % ubiquinone, apparently coupled to the rotenone- and antimycin-sensitive exo-NADH oxidase system [Rasmussen (1969) FEBS Lett. 2, 157-162]. Citric acid cycle substrates reduced most of the remaining ubiquinone in the presence of antimycin; 15-20 % of the total ubiquinone content was still in the oxidized form under the most reducing conditions. Three pools of ubiquinone therefore appeared to be present in heart mitochondria: a metabolically inactive pool consisting of reduced as well as oxidized ubiquinone, a pool coupled to oxidation of added (cytoplasmic) NADH, and the well-known pool coupled to citric acid cycle oxidations. Ferricyanide selectively oxidized the ubiquinol reduced by added NADH, indicating that this pool is situated on the outer surface of the mitochondrial inner membrane. Ubiquinone reduction levels were determined with a new method, which is described in detail.

Selected quotes

  • The possible existence within the same membrane of different pools is of particular relevance to those models of the respiratory chain that involve transversediffusion of ubiquinone through the membrane (Mitchell, 1975; Kröger, 1976) [for reviews, see Gutman (1980); Trumpower (1981); Rich(1984)]. Compartmentation of ubiquinone in another membrane, the cytoplasmic membrane of Pseudomonas aeruginosa, has been suggested by Matsushita et al. (1980).
  • The present paper demonstrates the existence of three different ubiquinone pools in intact pigeon heart mitochondria. One pool,consisting of UQox, as well as UQred, is apparently metabolically inactive. Another pool is reduced by succinate and internal NADH and coupled to oxidative phosphorylation. The third pool exhibits the characteristics of being associated with the so-called exo-NADH oxidase system, which effects the oxidation of added NADH observed in muscle mitochondria (Rasmussen, 1969).
  • Ubiquinone was partially reduced in pigeon heart mitochondria in cubated aerobically in the absence of added substrates (Table 1, column A). Under these conditions, the degree of reduction was insensitive to rotenone, inhibiting the NADH:ubiquinone oxidoreductase, and to antimycin and cyanide, inhibiting the respiratory chain between UQred and 02. ADP or the uncoupler FCCP did not affect the degree of reduction either. These findings, together with the very low rate of endogenous respiration (about 1/1000 of the succinate+glutamate State-4 rate), exclude that endogenous substrates were responsible for this background reduction. The UQred present in untreated mitochondria was therefore most likely metabolically inactive.
  • Some UQox also appeared to be metabolically inactive because complete reduction was never observed, not even in cyanide- or antimycin-inhibited mitochondria supplied with malate+pyruvate+NADH or succinate+glutamate+NADH ingreat excess (Tables 1 and 4). Lack of complete ubiquinone reduction under similar reducing conditions was also observed with rat heart mitochondria (Kröger & Klingenberg, 1966).
  • The increase in UQred caused by addition of NADH to antimycin- or cyanide-inhibited mitochondria in the absence of other substrates was 9.7±3.3 % of UQtot (mean±S.D. for 27 experiments, 19 preparations). Ferricyanide .. does not penetrate the membrane .. the amount of UQred reacting with ferricyanide was equal to the amount of UQred formed by added NADH and independent of UQ reduction by malate+pyruvate.
  • .. succinate+glutamate reduced exo-UQox slowly in addition to the fast reduction of the rest of the metabolically active UQox. The leakage of electrons to the exo-UQ pool was slower from malate+pyruvate, i.e. from internal NADH, than from succinate+glutamate (Table 4, column C). This difference excludes that the leak was caused by direct interaction between UQ pools.
  • the rate of respiration of succinate+glutamate and the rate of oxidation of added NADH are additive.
  • The results presented in the present paper suggest the existence of three ubiquinone pools in heart mitochondria: one pool is metabolically inactive, another pool participates in the oxidation of succinate and internally generated NADH through the respiratory chain, and a third pool, called the exo-UQ pool, is detected by its reduction by added NADH.
  • The metabolically inactive ubiquinone is therefore most likely present in all mitochondria, where it may be confined to parts of the membrane that are devoid of catalysts. It may here exert structural or protective effects, as suggested by Kobayashi et al. (1980) and Landi et al. (1984). Freezing and thawing of the mitochondria cause oxidation of inactive UQred(cf. Kröger, 1978). This effect may be explained by membrane rearrangements bringing inactive ubiquinone and catalysts in contact.

Cited by

  • Komlodi et al (2021) Simultaneous measurement of respiration and redox state of the Coenzyme Q pool in mitochondrial preparations. Bioenerg Commun 2021.3 doi:10.26124/bec:2021-0003


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