The Surface‐Exposed, Trypsin‐Accessible Segments of Apolipoprotein B in the Low‐Density Lipoprotein of Human Serum

Limited tryptic digestion of native low‐density lipoprotein (LDL) (ϱ 1.024‐ 1.045 g/ml) liberates some 20 ‐ 25% of its surface‐exposed protein as soluble, lipid‐free peptides (T‐peptides). The T‐peptides were resolved into twelve subfractions (peaks 1 ‐ 12) by gel filtration chromatography on Bio‐Gel P4 in 5 % formic acid, and classified into four major groups on the basis of criteria of size, amino acid composition and predominant C‐terminal residue. The first group, peaks 1‐4, (∼ 30% of total peptide protein; M, 2600‐6000), displayed lysine as preponderant C‐terminal (Lys : Arg, 3.7‐7.3:1), were acidic in nature (basic:acidic residues, 0.37‐0.61:1), and more hydrophobic than holo‐apo‐LDL. Peaks 1 and 2 contained glycopeptides (four or less by fingerprinting), the monosaccharide composition of which resembled that of LDL glycopeptides of type I1 [Swaminathan, N. & Aladjem, F. (1976) Biochemistry, 15 , 1516‐15211. The second group, peaks 5 and 6, (∼ 30% of the total; M r , 1500‐2000), were also apparently derived predominantly by cleavage at lysyl peptidyl bonds (Lys:Arg, 4.6‐4.9:1), but were distinct in their higher proportion of basic residues (acidic:basic, 0.83‐1.05:1). The third group (peaks 7 and 8) contained peptides approx. eight residues in length, with equal proportions (∼ 10%) of arginine and lysine. In contrast, the smaller fragments (2‐5 residues) of peaks 9‐12 exhibited arginine as major C‐terminal residue (Lys:Arg, mean 0.45:1), as well as similar amounts of acidic and basic residues. Examination of the quantitative distribution of lysine and arginine showed 80% of arginines to be present in fragments of eight residues or less, with the majority (45 %) in peptides of 2‐5 residues. In contrast, ∼ 70% of total lysines were located in fragments of z 10‐20 residues; only ∼ 10% of lysines were in small (2‐5 residue) peptides. Arginine appears therefore to occur most often at short (2‐5 residues) and lysine at longer (six or more residues) distances from the next lysine or arginine in trypsin‐accessible LDL protein. Clusters of a minimum of two positive charges, often featuring arginine, can thus be envisaged. The resemblance of the amino acid profiles of the major peptide subfractions (peaks 1‐8) to that of apolipoprotein B further supports the contention that this apoprotein possesses a repetitive‐type sequence. Finally, despite evaluation of peptide heterogeneity by fingerprinting, the inferred presence of incompletely‐cleaved tryptic peptides in peaks 3, 4 and 5 precludes attribution of a minimal unique amino acid sequence to apolipoprotein B.