Sodium-coupled and electrogenic transport of B-complex vitamin nicotinic acid by slc5a8, a member of the Na/glucose co-transporter gene family

SMCT (sodium-coupled monocarboxylate transporter; slc5a8) is a Na ⁺-coupled transporter for lactate, pyruvate and short-chain fatty acids. Similar to these already known substrates of SMCT, the water-soluble B-complex vitamin nicotinic acid also exists as a monocarboxylate anion (nicotinate) under physiological conditions. Therefore we evaluated the ability of SMCT to mediate the uptake of nicotinate. In mammalian cells, the cloned mouse SMCT (slc5a8) induced the uptake of nicotinate. The SMCT-induced uptake was Na⁺-dependent. The Michaelis constant for the uptake process was 296 ± 88 μM. The Na⁺-activation kinetics indicated that at least two Na⁺ ions are involved in the process. Among the various structural analogues tested, nicotinate was the most effective substrate. Nicotinamide and methylnicotinate were not recognized by the transporter. 2-Pyrazine carboxylate and isonicotinate interacted with the transporter to a moderate extent. SMCT-mediated uptake of nicotinate was inhibited by lactate and pyruvate. In the Xenopus laevis oocyte expression system, SMCT-mediated nicotinate transport was electrogenic, as evident from the nicotinate-induced inward currents under voltage-clamp conditions. Substrate-induced currents in this expression system corroborated the substrate specificity determined in the mammalian cell expression system. The kinetic parameters with regard to the affinity of the transporter for nicotinate and the Hill coefficient for Na ⁺ activation, determined by using the oocyte expression system, were also similar to those obtained from the mammalian cell expression system. We conclude that SMCT functions not only as a Na⁺-coupled transporter for short-chain fatty acids and lactate but also as a Na⁺-coupled transporter for the water-soluble vitamin nicotinic acid.