Coupled Endogenous Respiration in Brown Adipose Tissue Mitochondria

1 Carnitine + ATP were required for oxidation of endogenous substrates in brown adipose tissue mitochondria from newborn guinea‐pigs when the reaction medium lacked serum albumin. There was no evidence of phosphorylation linked to this oxidation. 2 When the reaction medium contained 2% serum albumin endogenous substrates were oxidized provided that phosphate and ADP were present. Carnitine did not influence the respiration. This oxidation was coupled to phosphorylation, P/O ratios between 2.4 and 2.8 being found. 3 Evidence is presented that the substrates responsible for the coupled endogenous respiration are mainly fatty acids, probably derived from an internal pool and activated in an internal mitochondrial compartment. 4 Brown adipose tissue mitochondria from newborn animals respiring on endogenous substrates could rapidly be brought into the controlled state (state 4) provided that high energy nucleotides were added to the reaction medium. In their absence, addition of ADP was followed by a slower and more gradual transition into state 4. 5 With brown adipose tissue from foetuses and weaned animals the endogenous respiration was under phosphate acceptor control in absence of high energy compounds. 6 The results suggest that the highly loose coupled state of brown adipose tissue mitochondria in thermogenic phases is related to lowered amounts of endogenous high energy phosphates.