The substrate specificity of the enzyme endo-alpha-N-acetyl-D-galactosaminidase from Diplococcus pneumonia.

The substrate specificity of the enzyme endo-alpha-N-acetyl-D-galactosaminidase from Diplococcus pneumonia was re-examined using bovine submaxillary mucin and remodelled antifreeze glycoprotein as substrates. Incubation with desialylated bovine submaxillary mucin, which contains six O-linked core types, indicated that the disaccharide Gal beta1-3GalNAc, which is present in very small amount, was the only glycan released, while the disaccharide GlcNAc beta1-3GalNAc, which is the major structure present, and other disaccharides, were not released. To test whether the core disaccharide Gal beta1-3GalNAc with sialic acid linked alpha2-3 to the Gal or linked alpha2-6 to the GalNAc was released, the enzyme was incubated with remodelled antifreeze glycoprotein containing (1) [3H]NeuAc alpha2-3Gal beta1-3GalNAc and (2) Gal beta1-3[[14C]NeuAc alpha2-6]GalNAc as substrates. No NeuAc-containing trisaccharide was released. These results serve to clarify the doubts of many researchers regarding the activity of this enzyme on some newly-described core types and on sialylated substrates.