The Effects of Temperature and Some Inhibitors an the Carnitine Exchange System of Heart Mitochondria

11‐Trimethylamino‐undecanoyl‐l‐carnitine (K; = 56 μM) and 6‐trimethylaminohexanoyl‐l‐carnitine (K₁, = 1.3 mM) are competitive inhibitors of the ox heart mitochondrial carnitine exchange system. The latter is itself a poor substrate, with a V about 0.7 of that for carnitine. These compounds have been'used as ‘stop inhibitors’ in a study of the effect of temperature an the carnitine exchangc. The carnitine_out↔ H carnitine_in exchange has a temperature coefficient (Q₁₀) of 8.5 and an activation energy of 176 kJ/mol which is constant from 0 °‐18 °C. The K_m for L‐carnitine (5.3 mM) is also constant over this temperature range. The slow efflux (leak) of mitochondrial L.‐[¹⁴C]carnitine in the absence of external substrate has a similar temperature dependence and is also inhibited by 11‐trimethylamino‐undecanoyl‐L‐carnitine. lt is probably mediated by the same system as the very much Taster exchange with external carnitine