Studies on an acidic glycoprotein-containing fraction isolated from guinea-pig serum

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Abstract
A fraction of acidic glyco-proteins (fraction I) has been isolated from guinea pig serum by means of chromatography with carboxymethylcellulose. It represents about 15% of the protein-bound hexose. Fraction I is made up almost entirely of [alpha]-globulins. A large part of it consists of material formed of components which are very similar in physical properties. It resolves into a number of bands on starch-gel electrophoresis at pH 3.4 and two or three immunologically distinct components have been detected. The remainder of fraction I is composed of about ten components with some physical properties similar to those of the constituents forming the major part of fraction I. A large part of fraction I sediments at a similar rate in the ultracentrifuge. An estimate of the molecular weight has been made of the material and a value for fraction I as a whole has also been obtained. Most of the components of fraction I are immunologically distinct from one another and show immunological indentity with substances prosent in guinea-pig serum. The total carbohydrate content of fraction I is about 19% made up principally of galactose, mannose, hexosamine and sialic acid. Traces of uronic acid and possibly fucose also occur. A study has been made of the conditions required to release various sugar residues by treatment with acid. A method which involves heating with a mixture of Dowex 50 and 0.25 N-hydro-chloric acid has been used to bring about the liberation of all sugars other than sialic acid. Under appropriate conditions, up to 80% of the sialic acid of fraction I can be released by the action of neuramini-dase, indicating that most or all of the sialic acid residues occupy terminal positions. The composition of the perchloric acid-soluble protein fraction of guinea-pig serum has been investigated with particular reference to the importance of components of fraction I as constituents of the fraction. Up to six components of fraction I were detected in this fraction, but together they account for only about 20% of it. The remainder of the fraction is made up of a number of other serum proteins: these are [alpha]- and [beta]-globulins, at least a number of which are glycoproteins. The carbohydrate composition of the fraction is rather similar to that of fraction I.