Aglycon specificity of fetal calf liver and ovine and porcine submaxillary gland α‐N‐acetylgalactosaminide α2→6 sialyltransferase

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Abstract
The specificity of fetal calf liver and ovine and porcine submaxillary gland N-acetylgalactosaminide α2→6 sialyltransferase was investigated with acceptors of low and high molecular weight containing O-glycosidically linked carbohydrate chains. It appeared that fetal calf liver microsomes were able to transfer sialic acid to C-6 of GalNAc in NeuAc(α2→3)Gal(β1→3)GalNAc-R, in which the aglycon could be protein as well as p-nitrophenol (Nph). The substrates Gal(β1→3)GalNAc-R and GalNAc-R were inactive as acceptor with this enzyme source. Ovine and porcine submaxillary gland microsomes were both active with GalNAc-protein, Gal(β1→3)GalNAc-protein, NeuAc(α2→3)Gal(β1→3)GalNAc-protein and NeuAc(α2→3)Gal(β1→3)GalNAcα-Nph, but not with GalNAcα-Nph and Gal(β1→3)GalNAcα-Nph. The N-acetylgalactosaminide α2→6 sialyltransferase which had been purified to homogeneity from porcine submaxillary gland [Sadler, J. E., Rearick, J. I. and Hill, R. L. (1979) J. Biol. Chem. 254, 5934–5941], was able to sialylate all three protein acceptors, but was virtually inactive with each of the three p-nitrophenyl glycosides. Our studies indicate that two N-acetylgalactosaminide α2→6 sialtransferases exist acting on O-glycosidically linked carbohydrate chains. The first enzyme, present in fetal calf liver, has a narrow specificity with regard to the oligosaccharide structure, but shows no specificity for the aglycon. Based on its specificity this enzyme can be designated as an [α-N-acetylneuraminosyl2→3-β-galactosyl 1→3]-α-N-acetylgalactosaminide α2→6 sialyltransferase. The second enzyme, present in porcine submaxillary gland, has an absolute requirement for protein as the aglycon. Once this condition is fulfilled, the enzyme is able to transfer sialic acid to each of the three oligosaccharide chains and thus the enzyme is an α-N-acetylgalactosaminylprotein α2→6 sialyltransferase. The data also seem to suggest that ovine and porcine submaxillary gland microsomes contain, in addition to the latter enzyme activity, the α2→6 sialyltransferase with the strict oligosaccharide specificity.

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