Human Na+ -coupled citrate transporter: primary structure, genomic organization, and transport function

Abstract

This paper describes the cloning and functional characterization of the human Na(+)-coupled citrate transporter (NaCT). The cloned human NaCT shows 77% sequence identity with rat NaCT. The nact gene is located on human chromosome 17 at p12-13. NaCT mRNA is expressed most predominantly in the liver, with moderate expression detectable in the brain and testis. When functionally expressed in mammalian cells, human NaCT mediates the Na(+)-coupled transport of citrate. Studies with several monocarboxylates, dicarboxylates, and tricarboxylates show that the transporter is selective for citrate with comparatively several-fold lower affinity for other intermediates of citric acid cycle. The Michelis-Menten constant for citrate is approximately 650 microM. The activation of citrate transport by Na(+) is sigmoidal, suggesting involvement of multiple Na(+) ions in the activation process. The transport process is electrogenic. This represents the first plasma membrane transporter in humans that mediates the preferential entry of citrate into cells. Citrate occupies a pivotal position in many important biochemical pathways. Among various citric acid cycle intermediates, citrate is present at the highest concentrations in human blood. The selectivity of NaCT towards citrate and its predominant expression in the liver suggest that this transporter may facilitate the utilization of circulating citrate for the generation of metabolic energy and for the synthesis of fatty acids and cholesterol.