The association of intercellular structural glycoproteins with lipids in canine cartilage

Two noncollagenous insoluble structural glycoprotein fractions (A and G), from uncalcified canine puppy rib cartilage, each contained about 8% of extractable lipid. This percentage of lipid is higher than that found in any of the other tissue fractions obtained by the preparative procedure used. The composition of the lipid extracted from the structural glycoproteins is quite distinct from the lipid in the guanidine . HCl extract of cartilage, the lipid of cartilage matrix vesicles, and that of other known lipid membranes. On ultracentrifugation, solubilized A or G, obtained by using 50 mM dithiothreitol (DTT) in 5 M guanidine . HCl followed by dialysis, showed a floating fraction (density 1.14 - 1.16 g/mL) which was decreased by prior delipidation and increased by lipidation. Chromatography of [3H]palmitate, [14C]cholesterol, or [14C]phosphatidylcholine on Sepharose 2B showed that their elution behaviour is altered in the presence of solubilized A or G. All the labelled lipids cochromatograph, but only with A or G protein in the high molecular weight region. After boiling the low density ultracentrifugation fraction with 1% sodium dodecyl sulfate (SDS), 8 M urea, and 50 mM DTT, disc gel electrophoresis showed that it contained the same subunits as the high density fraction. It is concluded that lipids can readily form a complex with a specific fraction of A or G. This fraction may be a combination of several undissociated subunits or an undenatured form of proteins. The complex is stable enough to be of possible significance in the metabolism of connective tissues. No antigenic relationships have been found between the cartilage structural glycoproteins or cartilage extracts and the plasma lipoproteins LDL, VLDL, and Lp(a).