Studies on Serum Pre-Alpha-Lipoprotein. An Albumin-ApoA-I-Lysolecithin-Containing Lipoprotein Family (AAL)

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Abstract
By electrophoresis on agarose gel, the alpha-lipoproteins of serum separate into two distinct fractions, the most anodic of which is designated pre-alpha-lipoprotein. All neutral lipids and phospholipids regularly found in human serum were detected in pre-alpha-lipoprotein. Lysolecithin made up 33 ± 4.6% of the phospholipids present. By combination of column affinity chromatography and preparative electrophoresis, an additional fraction, designated AAL, was isolated from the pre-alpha-lipoprotein preparation. AAL contained albumin and Apo-A-I immunologically. In basic PAGE and in SDS-PAGE the AAL preparation contained one single band. After reduction of AAL with mercaptoethanol and a subsequent run in SDS-PAGE, two bands were obtained. One of the bands had the same molecular weight and electrophoretic mobility as isolated Apo-A I (mol.wt 28,400 Daltons), the other band had the same mol.wt and electrophoretic mobility as albumin monomer (mol.wt 67,000 Daltons). The mol.wt of the AAL complex consequently was 95,400 Daltons (67,000 + 28,400). The total amino acid composition of AAL was very similar to that of albumin, but differed significantly in having higher amounts of glycine, serine, and glutamic acid. Calculations of Metzger's difference index (DI), indicated a close similarity between AAL and an albumin-Apo-A-I complex. The results of the immunological studies, electrophoreses, and the mol.wt obtained were also compatible with the presence of an albumin-Apo-A I complex. Regular pre-alpha-lipoprotein could not be detected in three LCAT-deficient patients studied. Nor could the AAL complex be detected in these patients. In vitro, AAL had a significant binding capability for lysolecithin. It is suggested that pre-alpha-lipoprotein and AAL may play a role in plasma lysolecithin transport.