Characterization of the high-affinity monocarboxylate transporter MCT2 in Xenopus laevis oocytes

Observations on lactate transport in brain cells and cardiac myocytes indic ate the presence of a high-affinity monocarboxylate transporter. The rat mo nocarboxylate transporter isoform MCT2 was analysed by expression in Xenopu s laevis oocytes and the results were compared with the known characteristi cs of lactate transport in heart and brain. Monocarboxylate transport via M CT2 was driven by the H+ gradient over the plasma membrane. Uptake of lacta te strongly increased with decreasing pH, showing half-maximal stimulation at pH 7.2. A wide variety of monocarboxylates and ketone bodies, including lactate, pyruvate, beta-hydroxybutyrate, acetoacetate, 2-oxoiso-valerate an d 2-oxoisohexanoate, were substrates of MCT2. All substrates had a high aff inity for MCT2, For lactate a K-m value of 0.74 +/- 0.07 mM was determined at pH 7.0. For the other substrates, K-i values between 100 mu M and 1 mM w ere measured for inhibition of lactate transport, which is about one-tenth of the corresponding values for the ubiquitously expressed monocarboxylate transporter isoform MCT1. Monocarboxylate transport via MCT2 could be inhib ited by alpha-cyano-4-hydroxycinnamate, anion-channel inhibitors and flavon oids. It is suggested that cells which express MCT2 preferentially use lact ate and ketone bodies as energy sources.