Characterization of the human plasmalemmal carnitine transporter in cultured skin fibroblasts.

Carnitine is an essential cofactor for long-chain fatty acid oxidation. We characterized the human carnitine transporter in vitro in a cultured skin fibroblast model both at the previously established Km concentration of carnitine uptake in fibroblasts (5 mumol/liter) and at 0.05% Km (0.25 mumol/liter). A rapid exponential dose-dependent decrease in mean percentage of carnitine uptake was demonstrated with increasing concentrations of nigericin, but no significant decrease was found with equimolar amounts of valinomycin. This would suggest that the Na+ gradient is integral to carnitine transport function. Interference of the Na+ (out-in) gradient by nigericin may be secondary to cytoplasmic acidification by this K+ proton ionophore. The rate of uptake was fully saturated at an extracellular Na+ concentration of 150 mmol/liter. Replacement of 150 mmol/liter extracellular Na+ with Li+ resulted in an 80 and a 50% reduction, and replacement with K+ and Rb+ ions resulted in a 100 and an 85 to 90% reduction in carnitine uptake, respectively, at carnitine concentrations of 0.25 and 5 mumol/liter, underlining the specific requirement for the Na+ ion. The effects of different site-specific respiratory chain toxins, namely, rotenone (complex I), antimycin A (complex III), and potassium cyanide (KCN) (complex IV) on carnitine uptake was also examined. A rapid exponential dose-dependent decrease in mean percentage of carnitine uptake with increasing concentrations of inhibitors was demonstrated. These data suggest either a metabolic energy requirement of the carnitine transporter or interference of the Na+ (out-in) gradient by a proton gradient (in-out) secondary to the accumulation of intracellular H+ ions, due to the action of the respiratory chain toxins, further suggesting that the transporter is sensitive to and inhibited by intracellular H+ ions. The effects of several sulfhydryl-binding agents, namely 2,4-dinitrofluorobenzene, N-ethylmaleimide, and mersalyl acid, were examined, and a significant inhibition of carnitine uptake was demonstrated, suggesting that free sulfhydryl groups are also integral to the import function of the human fibroblast transporter.