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. 1973 Jun;133(2):369–382. doi: 10.1042/bj1330369

A comparative study of the effects of chemical modification on the immunochemical and optical properties of human plasma low-density lipoprotein(s) and apoproteins

Antonio M Gotto Jr 1,*,†, Robert I Levy 1, Samuel E Lux 1,‡, Maria E Birnbaumer 1,§, Donald S Fredrickson 1
PMCID: PMC1177707  PMID: 4353238

Abstract

1. The structure of human plasma low-density lipoprotein(s) [LD lipoprotein(s)] was investigated by several immunological and optical techniques. The effects of delipidation and of chemical modification by 3-carboxypropionylation, acetylation, diazotization and amidination were examined. A sensitive double-antibody radioimmunoassay for human LD lipoproteins is presented and is used to assess the extent of immunochemical modification. A computer best-fit analysis is used to analyse circular-dichorism (c.d.) spectra. These methods permit comparisons of the relative effects of chemical modification and delipidation of LD lipoprotein under similar experimental conditions. 2. 3-Carboxypropionylation, acetylation or diazotization produces qualitative and quantitative changes in the immunochemical properties of LD lipoprotein. Amidination causes minor changes detected by radioimmunoassay but not by double-diffusion experiments. In general, the order of effectiveness in displacing 125I-labelled LD lipoprotein is amidinated>diazo or acetyl>3-carboxypropionyl derivatives. 3. The order of the extent of conformational alteration induced in apoLD lipoprotein and LD lipoprotein, as judged by c.d. analyses, was 3-carboxypropionylation>diazotization>acetylation or amidination. 4. Delipidation of LD lipoprotein results in immunological alterations that are qualitatively detected by antisera to LD lipoprotein. Four of five antisera to apoLD lipoprotein form precipitin lines of identity between native LD lipoprotein and apoLD lipoprotein in double-diffusion experiments. An anti-(apoLD lipoprotein) serum that forms precipitin lines of complete identity between LD lipoprotein and apoLD lipoprotein reacts differently with these two antigens in radioimmunoassay. ApoLD lipoprotein is only one-fourth to one-half as effective as LD lipoprotein, on a protein basis, in the displacement of 125I-labelled LD lipoprotein from this anti-(apoLD lipoprotein). 5. Conformational analysis indicates that apoLD lipoprotein retains a high proportion of the structural integrity of the native lipoprotein. Delipidation induces a small decrease in the content of β-structure and a small increase in disordered structure, without greatly affecting the α-helical content. Chemical modification produces more severe conformational changes of apoLD lipoprotein than of LD lipoprotein. Computer analysis of the c.d. spectra of apoLD lipoprotein indicates that addition of high concentrations of sodium decyl sulphate abolishes most of the β-conformation concomitant with increases in α-helical and disordered structure. 6. There is parallelism between the alteration of the charge of LD lipoprotein and apoLD lipoprotein and the extent of immunochemical and conformational changes.

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Selected References

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