Formation of Glycosylated “Proteolipid-Like” Materials during Exogenous Protein Galactosylation by Human Body Fluids
Human cerebrospinal fluid (CSF) and serum transfer substantial amounts of galactose from UDP-galactose to appropriate exogenously-added proteins. With desialyzed, degalactosylated fetuin (DSG fetuin) as an acceptor,considerable transfer of galactose takes place to materials soluble in chloroform:methanol (2:1). In the case of serum, transfer of galactose occurs to an additional lipid fraction, viz. that soluble in chloroform:methanol:water (1:1:0. 3). When ovalbumin is used as the acceptor, no transfer occurs to the chloroform:methanol fractions with CSF as the enzyme source; with serum however, considerable transfer occurs to both the lipid fractions. In the absence of exogenous glycoprotein acceptor or Mn++, little or no transfer to the lipid fraction(s) is observed. When native, untreated fetuin is substituted for DSG-fetuin, no transfer to the lipoidal fraction is observed. The transfer activity also can be distinguished on the basis of the time course of the reactions. When serum is extracted with cold,organic solvents and then assayed for transfer activity, it was observed that the transfer of galactose to the lipid-soluble material is eliminated while transfer to protein is unaffected. When the lipid extract is added to the incubation mixtures, transfer activity is restored. The lipid fraction does not migrate in systems employed to separate glycosylated polyprenols. Mild acid hydrolysis in propanol fails to liberate the sugar completely from the lipid fraction. Several of the solubility properties of the lipid fraction were similar to those described for proteolipids. Uncharacteristic of the properties described for proteolipids was the observation that direct extraction of the lipoidal materials from the incubation mixture with chloroform:methanol proved unsuccessful. We feel that at present the galactosylated lipid material is best described as glycoproteolipid-like in nature and it is suggested that it is formed by the interaction between the body fluid lipids and galactosylated protein.
KeywordsLipid Fraction Incubation Mixture Transfer Activity Enzyme Source Methanol Fraction
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