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When lipoprotein a was isolated in the presence of the proteolytic inhibitor Trasylol, its apoprotein exhibited one dominant band corresponding to a molecular weight of about 1.2 million when analyzed by electrophoresis on 3.25% sodium dodecyl sulfate-polyacrylamide gels. After chemical reduction, this band was missing but was replaced by two bands, one corresponding to a molecular weight of about 490,000 and the other to a molecular weight of about 645,000. Before treatment with reducing agents, the apolipoprotein a and apolipoprotein B immunoreactivities were detectable in the same electrophoretic band, but after reduction the apolipoprotein a was demonstrated to be separate from the apolipoprotein B. These results suggest that the apoprotein of lipoprotein a is composed of two subunits which are similar in molecular weight and are held together by one or more disulfide bonds. One subunit possesses apolipoprotein a and the other apolipoprotein B immunoreactivity. The secondary structure of the apoprotein components within lipoprotein a has been studied by circular dichroism and found to differ significantly from the secondary structure of the apoproteins in low density lipoproteins and high density lipoproteins. About 30% alpha-helical structure was measured in lipoprotein a compared to 48% in low density lipoproteins and 70% in high density lipoproteins. Lipoprotein a exhibited a much higher percentage of disordered structure than either of the other two lipoproteins.
Gaubatz et al. (Fri,) studied this question.