Biosynthesis of a Globoside-type Glycosphingolipid by a β-N-Acetylgalactosaminyltransferase from Embryonic Chicken Brain

A β-N-acetylgalactosaminyltransferase activity has been detected in 11-day-old embryonic chicken brain which catalyzes the transfer of N-acetylgalactosamine from UDP-N-acetyl[14C]galactosamine to a triglycosylceramide (or, GloboTri-cer; O-α-galactosyl(1→4)-O-β-galactosyl(1→4)-O- β-glucosyl(1→1)-ceramide) to form the globoside-type tetraglycosylceramide, GalNAc → Gal → Gal → Glc → ceramide. The enzyme is membrane bound and requires sodium taurocholate and Mn2+ for optimal activity. The optimum pH value for the incorporation of N-acetylgalactosamine was 6.5 in 2-(N-morpholino)ethanesulfonic acid buffer. The Km values were 1.7 x 10-3 m and 2.0 x 10-4 m for GloboTri-cer and UDP-GalNAc, respectively. The radioactive product of the reaction was isolated, purified, and characterized as globoside-type tetraglycosylceramide. The terminal [14C]GalNAc was cleaved 78% by the action of jack bean β-N-acetylgalactosaminidase. The 14C product formed a precipitin line with anti-globoside serum, and the line fused with that of globoside derived from porcine heart or erythrocytes.