Monitoring of the exchange-diffusion of carnitine, acetylcarnitine and ADP by measuring the influx of radioactive substrates into mitochondria or their efflux, as commonly employed, underestimated their true transport. Higher transport rates were realized when the imports were monitored by analysing, in the entire incubation medium, formation of metabolites that could proceed only after the substrate import. A recycling of substrate present in an inner microenvironment near the translocase and in the external medium appeared to be responsible for these results. Microcompartmentation of carnitine was observable also at 30 degrees C. These findings strengthen the concept that a sharing of a microcompartment between transporters and enzymes metabolizing the entered substrates occurs and appears to offer a kinetic advantage for the reactions involved. The possibility that different segments of metabolism involving the same substrate may proceed at different loci within the matrix and thus be amenable to independent controls is also indicated by these findings.

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