Purification of the lysosomal sialic acid transporter - Functional characteristics of a monocarboxylate transporter

Sialic acid and glucuronic acid are monocarboxylated monosaccharides, which are normally present in sugar side chains of glycoproteins, glycolipids, a nd glycosaminoglycans, After degradation of these compounds in lysosomes, t he free monosaccharides are released from the lysosome by a specific membra ne transport system. This transport system is deficient in the human heredi tary lysosomal sialic acid storage diseases (Salla disease and infantile si alic acid storage disease, OMIM 269920). The lysosomal sialic acid transpor ter from rat liver has now been purified to apparent homogeneity in a recon stitutively active form by a combination of hydroxyapatite, lectin, and ion exchange chromatography. A 57-kDa protein correlated with transport activi ty. The transporter recognized structurally different types of acidic monos accharides, Like sialic acid, glucuronic acid, and iduronic acid. Transport of glucuronic acid was inhibited by a number of aliphatic monocarboxylates (i.e. lactate, pyruvate, and valproate), substituted monocarboxylates, and several dicarboxylates. cis-Inhibition, trans-stimulation, and competitive inhibition experiments with radiolabeled glucuronic acid as well as radiol abeled L-lactate demonstrated that L-lactate is transported by the lysosoma l sialic acid transporter, L-Lactate transport was proton gradient-dependen t, saturable with a K-m of 0.4 mM, and mediated by a single mechanism. Thes e data show striking biochemical and structural similarities of the lysosom al sialic acid transporter with the known monocarboxylate transporters of t he plasma membrane (MCCT1, MCT2, MCT3, and Mev).