Glycolipid acceptor specificity of a human Gal beta(1-3/1-4) GlcNAc alpha 2,3-sialyltransferase.

A human Gal beta(1-3/1-4)GlcNAc alpha 2,3-sialyltransferase, called ST-4, is a sialyltransferase involved in the in vivo biosynthesis of sialyl Lewis X (NeuNAc alpha 2-3Gal beta 1-4(Fuc alpha 1-3)GlcNAc) determinant. The ST-4 enzyme could utilize nLc4Cer (Gal beta 1-4GlcNAc beta 1-3Gal beta 1-4Glc beta 1-1'Cer) containing type 2 sugar chain, Lc4Cer (Gal beta 1-3GlcNAc beta 1-3Gal beta 1-4Glc beta 1-1'Cer) containing type 1 sugar chain, Gg4Cer (Gal beta 1-3GalNAc beta 1-3Gal beta 1-4Glc beta 1-1'Cer), and LacCer as glycolipid acceptor substrates, but not other neutral glycolipids (GalCer, GlcCer, Gb3Cer, Gg3Cer, Gb4Cer) and gangliosides (GM1a, GM2, GM3, GD1a, GD1b, and GT1b) as substrates. The order of sialic acid incorporation into glycolipids for the enzyme was nLc4Cer > Gg4Cer > Lc4Cer > LacCer. The apparent Km values of ST-4 for nLc4Cer and Gg4Cer were 0.47 and 2.5 mM, respectively. Thus, the ST-4 could efficiently utilize both nLc4Cer and Gg4Cer as glycolipid acceptor substrates in vitro, suggesting that the substrate specificity of the enzyme may be similar to that of a glycolipid sialyltransferase (SAT-3), which is defined as the enzyme that uses both nLc4Cer and Gg4Cer as glycolipid acceptor substrates.