The effects of temperature and some inhibitors on the carnitine exchange system of heart mitochondria.

1. 11-Trimethylamino-undecanoyl-L-carnitine (Ki = 56 muM) and 6-trimethylaminohexanoyl-L-carnitine (Ki = 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 on the carnitine exchange. 2. The carnitineout in equilibrium carnitinein exchange has a temperature coefficient (Q10) of 8.5 and an activation energy of 176 kJ/mol which is constant from 0 degrees - 18 degrees C. The Km for L-carnitine (5.3 mM) is also constant over this temperature range. 3. 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. It is probably mediated by the same system as the very much faster exchange with external carnitine.