Analysis of the Primary Structure of the Strongly Hydrophobic Brain Myelin Proteolipid Apoprotein (Lipophilin). Isolation and Amino Acid Sequence Determination of Proteolytic Fragments

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Abstract
Proteolipid apoprotein (lipophilin) and DM-20 protein [myelin protein with molecular mass 20 kDa [kilodalton]] from bovine brain white matter proved to be identical in polyacrylamide gel electrophoresis and automated Edman degradation of the N-terminal end over 20 cycles. Lipophilin can by hydrolysed by trypsin [EC 3.4.21.4], thermolysin [EC 3.4.24.4], chymotrypsin [EC 3.4.21.1] and subtilisin [EC 3.4.21.14]. A new, effective and rapid high-performance liquid chromatographic separation method for hydrophilic and hydrophobic polypeptides was described according to molecular mass on an analytical and preparative scale. Three large and several small peptides were isolated from the tryptic and thermolysinolytic hydrolysate and purified by combined molecular sieve and high-performance chromatographic separation and purification for automated Edman degradation. Amino acid residues (40) of the large tryptic fragment and sequences of 43 and 22 amino acids of 2 thermolysinolytic fragments were determined. These 3 polyppetides are partial structures of the 14 kDa large tryptophan fragment I or the cyanogen bromide fragment I (18-19 kDa). Thermolysin also releases a polypeptide from incompletely reductively carboxymethylated lipophilin which is cleaved into the large thermolysin fragment mentioned, 22 residues of which were analyzed and a 14 amino acids long sequence of tryptophan fragment IV, described in the previous paper. Reductively carboxymethylated lipophilin, the lysine side chains of which were blocked with maleic anhydride, can be cleaved at arginine specific sites. Bio-Gel P-150 and high-performance chromatographic purification yielded a polypeptide, which upon performic acid oxidation was split into a 15 kDa and a 7.8 kDa polypeptide. The 15 kDa polypeptide resembles the N-terminal end as proven by 31 cycles in Edman degradation. The 7.8 kDa polypeptide corresponds to the 72 amino acid C-terminal sequence, which equals cyanogen bromide fragments II, III and IV and embraces tryptophan fragment IV.

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