The lipid-free protein (apo-HDL2) of immunochemically pure human serum HDL2 (d 1.063-1.125) was separated by Sephadex G-200 chromatography in 8 M urea into four components: I, III, IV, and V with a per cent weight distribution of 5, 65, 22, and 8, and an apparent molecular weight of (103) 52.0-55.0, 25.8-28.0, 16.4-17.6, and 11.2-11.8, respectively (calibrated Sephadex columns, and disc gel electrophoresis in sodium dodecyl sulfate). Equilibrium ultracentrifugation analysis corroborated the molecular weight data obtained by chromatography and electrophoresis. However, under appropriate conditions of speed and protein concentration, III exhibited a smaller molecular weight component of approximately 17,000. Fraction II was minor and only occasionally seen. Fraction I appeared as an aggregate of the other components. Fractions III, IV, and V differed in immunological and spectral (circular dichroism and ultraviolet absorption spectroscopy) properties, electrophoretic mobility (disc gel electrophoresis), and amino acid composition. These three fractions all showed heterogeneity by disc gel electrofocusing. The results suggest that apo-HDL2 contains three distinct classes of polypeptides which are totally (IV and V) or partially (III) dissociated in urea and are probably made up of subcomponents. The observed similarity in band pattern (disc gel electrophoresis in the presence or absence of urea) between HDL2 and apo-HDL2 was taken to support the hypothesis that HDL2 is made up of lipoprotein subspecies with distinct peptide moieties. © 1969, American Chemical Society. All rights reserved.
