Mitochondria from Hamster Brown‐Adipose Tissue

1 The oxidation of palmitoyl-l-carnitine by mitochondria from the brown adipose tissue of cold-adapted hamsters is highly dependent on the nature of the incubation medium. No respiration occurs in media containing 200 mM sucrose or 100 mM choline chloride, while 100 mM NaCl allows rapid respiration. 2 Media containing 100 mM KCl allow steadily increasing rates of palmitoyl-carnitine oxidation. High initial rates may be obtained by adding valinomycin, or by decreasing the concentration of KCl. 3 Respiration can proceed in media containing sucrose or choline chloride if they are made sufficiently hypotonic. Pyruvate and 2-oxoglutarate oxidation is also sensitive to tonicity, whereas l-glycerol-3-phosphate oxidation is unaffected. 4 A close correlation was found between the sucrose-impermeable space and the rate of palmitoyl-carnitine oxidation under all conditions examined, a low space determination being associated with respiratory inhibition. 5 Mitochondria stored in 250 mM sucrose have a very low sucrose impermeable space. Evidence is presented that this space must be increased by hypotonic conditions or by ion permeation before respiration can occur. Mitochondria stored in 100 mM KCl, 20 mM K-TES have a much larger sucrose-impermeable space and can immediately oxidize palmitoyl-carnitine. 6 The ion permeability properties of these mitochondria are discussed, together with the nature of the respiratory inhibition associated with matrix condensation.