Glycoproteins
can be simply defined as proteins that have carbohydrate moiety covalently
attached to their peptide portion. The glycoproteins as a group have multiple
and complex function and are found as enzymes, hormones, blood group substances
and as constituents of extracellular membranes. These are organic compounds,
composed of both a protein and carbohydrate monosaccharides, usually hexose,
hexosamine, fucose and sialic acid, joined together covalently linked to
polypeptide chain.
Myelin is a specialized multilamellar
membrane characterized ultrastructurally by the presence of a major dense line
and an intraperiod line. Central nervous- system-myelin membrane contains
several major Coomassie Blue-staining proteins (Agrawal & Hartman, 1979)
and a major myelin-associated glycoprotein (Quarles et al., 1972, 1973). Myelin contains several major glycoproteins in
addition to the 120000- and the 27000-mol.wt.glycoproteins originally described
by Quarles and his co-workers (Quarles et
al., 1972, 1979). Four additional major glycoproteins (mol.wts. 95 000, 88000,
43000 and 38000) and many minor glycoproteins are revealed only after the
treatment of isolated myelin membrane with neuraminidase before oxidation with
galactose oxidase and subsequent reduction with NaB3H4.
A
large number of glycoproteins that contain a terminal sialic acid residue were
detected by incubating the myelin membrane first with neuraminidase followed by
galactose oxidase by Mena et al.,
1978). This approach has been used successfully for the detection of
glycoproteins in several biological membranes (Gahmberg & Hakomori, 1973,
1975; Chui & Babitch, 1979; Mena & Moore, 1977). Several glycoproteins
are present in myelin, among which are MAG and MOG. Other proteins have also
been identified, some of which have enzyme activities. 
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