Chemical synthesis and biochemical properties of peptide fragments of apolipoprotein alanine

Apolipoprotein alanine is an apolipoprotein isolated from very low density lipoproteins of human plasma. This protein contains 79 amino acids and binds phosphatidyl choline. Four fragments of this molecule corresponding to sequence positions 41-79 (I), 48-79 (II), 55-79 (III), and 61-79 (IV) have been synthesized by standard solid phase methods. The resulting peptides were cleaved from the resin and deblocked with liquid HF, then purified by chromatography on Sephadex G-50 and DEAE cellulose. Each purified peptide eluted as a single, symmetrical peak, exhibited a single band on polyacrylamide gel electrophoresis, and gave an amino acid analysis in good agreement with the theoretical value. Circular dichroism studies indicated that only fragments I and II became more helical in the presence of phosphatidyl choline and significantly inhibited the reactivation of delipidated β hydroxybutyrate dehydrogenase (EC 1.1.1.30), an enzyme that requires phosphatidyl choline for activity. When subjected to ultracentrifugation at density 1.064 g/ml in the presence of phosphatidyl choline, fragments I, II, III, and IV floated to the top of the tube to the extent of 85, 50, 13, and 9%, respecitvely. These results indicate that residues 55-79 do not contain the minimum determinants required for the binding of phospholipid. However, extension of the peptide's N terminus by 7 residues produces a molecule that does bind phosphatidyl choline.