Section 6
Chapter 5,734

Inter-relationships between proton electrochemical gradient, adenine-nucleotide phosphorylation potential and respiration, during substrate-level and oxidative phosphorylation by mitochondria from brown adipose tissue of cold-adapted guinea-pigs

Nicholls, D.G.; Bernson, V.S.

European Journal of Biochemistry 75(2): 601-612


ISSN/ISBN: 0014-2956
PMID: 18347
Accession: 005733777

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The interrelationships between respiratory rates, proton electrochemical gradients (.DELTA.hivin.mu.H+) and extra-mitochondrial adenine nucleotide phosphorylation potentials (.DELTA.Gp(out)) are examined during oxidative and substrate-level phosphorylation by mitochondria from the brown-adipose tissue of cold-adapted guinea pigs. In the absence of net ATP synthesis, .DELTA.Gp(out) is proportional to .DELTA.hivin.mu.H+ when the latter is varied from 230 mV to 190 mV, and is consistent with a stoichiometry of proton translocation for ATP synthesis (.fwdarw. H+/ATP) of 2.6. When there is a net production of ATP, .DELTA.Gp(out) falls below the level predicted from .DELTA.hivin.mu.H+. When ATP is generated by substrate-level phosphorylation, .DELTA.Gp(out) can be much greater than that predicted from .DELTA.hivin.mu.H+. In the absence of substrate-level phosphorylation, respiration is controlled by .DELTA.hivin.mu.H+, regardless of whether energy dissipation is varied by addition of proton translocators or by addition of extra-mitochondrial ATP-hydrolyzing systems. In contrast, the rate of respiration coupled to substrate-level phosphorylation appears to be controlled by the internal adenine nucleotide phosphorylation potential .DELTA.Gp(in). The rate of ATP synthesis by oxidative phosphorylation is dependent on .DELTA.Gp(out). In the absence of net ATP synthesis, both oxidative and substrate-level phosphorylation can maintain a .DELTA.Gp(out) in excess of 580 mV (56 kJ/mol), while in the presence of sufficient proton translocator to achieve uncontrolled respiration, a .DELTA.Gp(out) of 500 mV (48 kJ/mol) can be maintained by oxidative phosphorylation. Under conditions designed to approximate to those Pertaining in the brown adipocyte during nonshivering thermogenesis, oxidative phosphorylation alone appears to be adequate to maintain cellular ATP levels. It was not possible to confirm resports of a specific role which could be assigned to substrate-level phosphorylation in the regulation of energy dissipation by these mitochondria.

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