Human serum low density lipoprotein (d = 1.027-1.045) was delipidated with organic solvents and the apoprotein digested with thermolysin. The digest was fractionated by gel filtration and DEAE-cellulose chromatography. Two glycopeptides were obtained. One of the glycopeptides (GP-I) contained 2 residues of N-acetylglucosamine and 6 residues of mannose per mole of the glycopeptide, while the other contained 2 sialic acid, 5 mannose, 2 galactose, and 3 N-acetylglucosamine residues per mole of glycopeptide. The results of sequential enzymatic digestion with purified glycosidases, periodate oxidation, and partial acid hydrolysis lead us to propose the following sturctures for the two glycopeptides: (see article). These glycopeptides represent at least 50% of the carbohydrate moiety of LDL.
Some years ago, a series of h u m a n serum lipoproteins, distinguishable b y their h y d r a t e d densities and lipid-protein ratios were recognized and isolated b y ultracentrifugal techniques (1). Certain of these lipoproteins are invariably present in h u m a n serum and the concentrations of these and of others can be q u a n t i t a t i v e l y correlated with disease (2). N o difference in the lipoprotein distribution can be d e m o n s t r a t e d between serum and plasma. T h e purpose of this investigation was to obtain information a b o u t the i m m u n o c h e m i c a l specificity of some of the lipoproteins.
MethodsPreparation of Lipoproteinsl.--All lipoproteins, unless it is otherwise indicated, were isolated from pooled plasma. The plasma was obtained from citrated blood which was rejected by the San Francisco Blood Bank because of blood clots, positive serologic tests for syphilis, or because 3 weeks or more had elapsed since it was drawn. Between 8 and 16 pints of blood were used per pool.Low density lipoproteins SI6 and SI13 were isolated by the method of Lindgren et al. (3).The ultracentrifugal homogeneity of similar preparations has been discussed (5). The hydrated density of Ss6 lipoproteins is between 1.03 and 1.04 gm./ml, and that of Sjl3 lipoproteins between 1.015 and 1.03 gm./ml. In Fig. 1 ultracentrifuge records of lot" density lipoprotein preparations are presented3 These lipoproteins were used as immunizing antigens
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