Characterization and quantitation of apolipoproteins A-I and E of normal and cholesterol-fed guinea pigs

We have characterized and quantified the two major plasma apoproteins of high density lipoproteins (HDL), apolipoproteins A-I (apoA-I) and E (apoE), of guinea pigs fed standard chow (normal) or chow supplemented with 1% cholesterol (cholesterol-fed). ApoA-I isolated from plasma HDL of the normal guinea pig exists in six polymorphic forms (pI 5.75-5.40). A similar isoform pattern of this apoprotein was present in nascent HDL isolated from perfused livers of normal and cholesterol-fed animals. This apoprotein contains cysteine and isoleucine and is slightly different in overall amino acid composition from apoA-I of human and rat, but activates lecithin:cholesterol acyltransferase from human plasma with an activation curve almost identical to that obtained with human apoA-I. ApoE present in nascent VLDL and HDL from perfused liver of normal animals contains three isoforms (pI 5.42-5.34). Following cholesterol feeding, the numbers of apoE isoforms from perfused livers were increased from three to five or more by shifting the major component (pI 5.42) to more acidic isoforms (pI 5.28-5.17). This shifting was mostly reversible when apoE was treated with neuraminidase, suggesting that cholesterol feeding leads to a modification of apoE by increasing its content of sialic acid. Similar changes of apoE isoforms were also observed in plasma lipoproteins as early as 10 days after cholesterol feeding. The amino acid compositions of four apoE isoform fractions isolated from plasma HDL of cholesterol-fed guinea pigs were similar to that of parent apoE. The plasma concentrations of apoA-I and apoE, measured by electroimmunoassay, were 6.2 +/- 2.0 and 2.2 +/- 0.5 mg/dl, respectively, in guinea pigs fed standard chow. In animals that had been fed 1% cholesterol, plasma levels of apoA-I slightly increased in 1 week and showed a twofold increase in 8-10 weeks. Plasma levels of apoE, on the other hand, sharply increased by 10-fold in 1 week and up to 22-fold in 8-10 weeks on the cholesterol diet.-Guo, L. S. S., R. L. Hamilton, J. P. Kane, C. J. Fielding, and G. C. Chen. Characterization and quantitation of apolipoproteins A-I and E of normal and cholesterol-fed guinea pigs.