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Characterization of A-I-containing lipoproteins in subjects with A-I Milano variant
M C Cheung1, A V Nichols, P J Blanche
1Department of Medicine, School of Medicine, University of Washington, Seattle.
Abstract:
The A-I Milano variant of apolipoprotein A-I (A-IM), by virtue of its Arg-173----Cys substitution, is capable of forming a disulfide bond with the 77-amino-acid apolipoprotein A-II polypeptide (A-IIS) as well as with itself to produce dimers, A-IM/A-IIS and A-IM/A-IM, respectively. A-I-containing lipoproteins (Lp): particles with A-II (Lp(A-I with A-11)) and particles without A-II (Lp(A-I without A-II)) in the plasma of two nonhyperlipidemic A-IM carriers were investigated to determine the effect of A-IM on these lipoproteins. Despite the existence of abnormal apolipoprotein dimers and the unusually low HDL cholesterol (17 and 14 mg/dl), A-I (67 and 75 mg/dl), and A-II (18 and 18 mg/dl) levels in the two carriers, the plasma A-I of the carriers was distributed between Lp(A-I with A-II) and Lp(A-I without A-II) in a proportion comparable to that observed in normals. As expected, A-IM/A-IIS mixed dimer was found in carrier Lp(A-I with A-II). However, A-IM/A-IM dimer was located almost exclusively in carrier Lp(A-I without A-II). Chemical (dimethylsuberimidate) crosslinking of the protein moieties of the major subpopulations of Lp(A-I with A-II) and Lp(A-I without A-II) of normal and A-IM carriers showed that Lp(A-I with A-II), which is located predominantly in the 7.8-9.7 nm interval ((HDL2a + 3a + 3b)gge), had an apparent protein molecular weight equivalent to two molecules of A-I and one to two molecules of A-II per particle. Most of the Lp(A-I without A-II) particles, located predominantly in the size intervals of 9.7-12.9 nm (designated (HDL2b)gge) and 8.2-8.8 nm (HDL3a)gge) had protein moieties exhibiting a molecular weight equivalence predominantly of four and three molecules of A-I, respectively. A small quantity of particles with apparent protein content of two molecules of A-I in the 7.2-8.2 nm interval ((HDL3b + 3c)gge) was also detected. These studies showed that in nonhyperlipidemic A-IM carriers, the occurrence of apolipoprotein dimers had not markedly affected the protein stoichiometry of Lp(A-I with A-II) and Lp(A-I without A-II).
Insights
The A-I Milano variant (A-IM) forms abnormal apolipoprotein dimers, yet lipoprotein composition remains similar to normal individuals. This suggests A-IM does not significantly alter lipoprotein structure despite altered protein levels.
Area of Science:
- Biochemistry
- Lipid Metabolism
- Molecular Biology
Background:
- Apolipoprotein A-I (A-I) is a major component of high-density lipoproteins (HDL).
- The A-I Milano (A-IM) variant possesses a unique cysteine substitution enabling abnormal disulfide bond formation.
- Lipoproteins containing A-I (Lp(A-I)) exist with or without apolipoprotein A-II (A-II).
Purpose of the Study:
- To investigate the impact of the A-I Milano variant on lipoprotein composition.
- To determine how A-IM affects the distribution and structure of Lp(A-I) particles.
- To analyze the stoichiometry of apolipoproteins within different Lp(A-I) subpopulations in A-IM carriers.
Main Methods:
- Analysis of plasma lipoproteins from nonhyperlipidemic A-IM carriers.
- Characterization of apolipoprotein dimers (A-IM/A-IIS and A-IM/A-IM).
- Chemical crosslinking (dimethylsuberimidate) to determine protein molecular weight of lipoprotein particles.
Main Results:
- A-IM carriers exhibited low HDL, A-I, and A-II levels but maintained normal distribution of A-I between Lp(A-I with A-II) and Lp(A-I without A-II).
- A-IM/A-IIS dimers were found in Lp(A-I with A-II), while A-IM/A-IM dimers were primarily in Lp(A-I without A-II).
- The protein stoichiometry of Lp(A-I with A-II) and Lp(A-I without A-II) was largely unaffected by the presence of apolipoprotein dimers.
Conclusions:
- The A-I Milano variant's ability to form disulfide-linked dimers does not significantly alter the overall protein composition of major lipoprotein subclasses.
- Lipoprotein structure and A-I distribution remain relatively preserved in nonhyperlipidemic individuals carrying the A-IM variant.
- Further research may elucidate the functional implications of these findings on cardiovascular health.