Biosynthesis of lactosylceramide and paragloboside by human lactose synthase A protein.

Lactosylceramide and paragloboside were synthesized from their precursor glycolipids and UDP-galactose by lactose synthase A protein [UDP-Gal : GlcNAc beta-4-galactosyltransferase, EC 2.4.1.22] purified to homogeneity from human plasma. The partially purified human liver enzyme and an extract from human lymphoblastoid cells also exhibited the above activities. Rabbit antibody against the purified human plasma lactose synthase A protein neutralized the glycolipid synthesis activity as well as the activity for lactose synthesis by the enzyme preparations from plasma, liver and lymphoblastoid cells. These results suggest that lactose synthase A protein existing in plasma, liver and lymphoblastoid cells can synthesize not only lactose but also lactosylceramide and paragloboside in vitro. The enzyme could play a role in the synthesis of these two glycolipids in vivo.