title: Alterations of oxidative phosphorylation reactions in mitochondria isolated from hypothyroid rat liver.
abstract: Several parameters of oxidative phosphorylation reaction were studied in vitro in isolated liver mitochondria from hypothyroid rats. When succinate was employed as a respiratory substrate for mitochondria incubated in a mannitol/sucrose/phosphate buffer, and measurements were performed during initial additions of ADP, the magnitude of state 3 and state 4 respiration was not different between mitochondria from the hypothyroid and those from the control rats. During the course of repetitive additions of ADP and consequently of sequential transitions from state 4 to state 3 and back to state 4, mitochondria from hypothyroid animals showed a gradual decline in the rate of both state 3 and state 4 respiration whereas those from normal animals did not. The total succinate dehydrogenase activity was not different between the two types of mitochondria, and the decline in state 3 and state 4 respiration was not accompanied by any change in the apparent Km for ADP or in the corrected ADP/O ratio [Biochem. Biophys. Res. Commun. (1973) 53, 988]. The amount of oxygen consumed during the state-4----3----4 transition was lower in the hypothyroid than in the control mitochondria. These alterations were reversed if the hypothyroid animals were injected with thyroxine intraperitoneally (2 micrograms/100 g body weight) for 3 weeks before isolation of mitochondria. These results indicate that the fall of respiratory activity in hypothyroidism may result from the decrease not only of respiratory activities of state 3 and state 4, but also of the energy spent in the process of the state-4----3----4 transition, while the coupling efficiency per se remains normal. These properties become manifest when mitochondria respond to pulses of ADP load, a situation likely to occur in situ.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000818:Animals; D004734:Energy Metabolism; D007037:Hypothyroidism / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D008930:Mitochondria, Liver / Q000201:enzymology / Q000378:metabolism*; D010085:Oxidative Phosphorylation; D010101:Oxygen Consumption; D051381:Rats; D011919:Rats, Inbred Strains; D013385:Succinate Dehydrogenase / Q000378:metabolism
chemical_list: D000244:Adenosine Diphosphate; D013385:Succinate Dehydrogenase