This method selectively labels glycoproteins on the cell surface. Tritiated borohydride reduction may follow chemical oxidation of vicinal hydroxyl groups by sodium periodate or enzymatic oxidation by galactose oxidase (GAO).** The periodate oxidation is more effi cient. It generates aldehydes on the terminal sialic acid residues of glycoproteins. The enzymatic oxidation is milder, and by virtue of the molecular size of the enzymes, more likely to be limited to the cell surface. Since most mammalian glycoproteins have a penultimate galac tose orN-acetylgalactosamine residue followed by sialic acid, the latter is removed by simultaneous incubation in neuraminidase, rendering the galactose and galactosamine residues available to the enzymatic activity of GAO that oxidizes these residues at the carbon-6 position. The method is generally applicable and may be used in conjunction with methods described in Part B of this chapter and in Chapter 2 .See alsoChapter 4 , Part D in this volume.
