Meat Science in an International Marketplace 58

Just as total body energy expenditure in animals can be classified into that which supports resting energy metabolism, work, growth, etc., cellular energy expenditure can similarly be classified. Our overall objective is to examine the metabolic origins of cellular energy expenditure, differentiating between metabolic states where cells are at relative rest, and where cellular energy expenditure is high. In most situations when energy expenditure is high, mitochondrial ATP production (oxidative phosphorylation) is coupled and efficient. Uncoupling refers to the dissociation of the oxidation of energy substrates, such as fatty acids, from the synthesis of ATP by mitochondria. Uncoupling can occur during states of high energy expenditure or during states of metabolic rest. In brown adipocytes, uncoupling protein 1 (UCP1) activity can cause very high rates of energy expenditure for the purpose of thermogenesis (heat production). UCP1 is found exclusively in brown adipocytes. While uncoupling also occurs in other cells of the body, it is of greatest importance during periods of relative metabolic rest. The latter form of uncoupling is referred to as mitochondrial proton leak, and accounts for roughly one quarter of the resting metabolic rate of the rat. The mechanisms of mitochondrial proton leak are not well understood. The recently identified uncoupling proteins may play some role, but may also have some other physiological functions. Our recent findings from transgenic mice with altered expression of UCPs will be reviewed. Proton leak activity scales roughly in proportion with metabolic rate in mammals of different body size, and is related to thyroid hormone status. Proposed functions for mitochondrial uncoupling include thermogenesis, control of oxidative phosphorylation efficiency and protection from reactive oxygen species. Support: NSERC of Canada.