Exchange of the endogenous carnitine of ox heart mitochondria with external carnitine and its possible relevance to the mechanism of fatty acyl transport into mitochondria

Incubation of the mitochondria at 5°C with L [¹⁴C]carnitine results in uptake of label without change in mitochondrial carnitine content. The radioactivity is not lost on subsequent washing or incubation in the sucrose medium for 30 min at 5°C. Inclusion of L carnitine or acetyl L carnitine in the incubation decreases the radioactivity, but not the carnitine content, of the mitochondria. Exposure of the labelled mitochondria to bromoacetyl L carnitine prevents this decrease in radioactivity. Incubation of mitochondria containing L [¹⁴C]carnitine with D carnitine or deoxycarnitine (4 trimethyl amino butyrate, butyrobetaine) causes parallel decreases in radioactivity and enzymically assayed L carnitine. These results suggest that heart mitochondria may contain a system catalysing a carnitine carnitine (or, presumably, carnitine acylcarnitine) exchange across the inner membrane, analogous, for example, to that causing ATP ADP exchange. The relevance of the carnitine acyltransferases to the exchange system is not clear; the efficacy of D carnitine and deoxycarnitine precludes acyl transfer reactions, but both these substances can bind to the transferases.