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Thermotropic changes in the surface structure of lipoprotein B from human-plasma low-density lipoproteins. A spin-label study.

The lipoprotein B fraction of human-plasma low-density lipoproteins was labeled with (a) spin-label analogues of stearic acid I(m/n), the N-oxyl-4′,4′-dimethyloxazolidine derivatives of 5-oxo-, 12-oxo-, and 16-oxostearic acid, I(12/3), I(5/10), and I(1/14), respectively, and (b) a spin-label analog of maleimide (3-maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxyl) bound covalently to the protein moiety. Electron spin resonance spectra were measured in the temperature range from 4 °C to 50 °C. The evaluation of the spectra yielded the following results. 
 
 
 
1 
The stearic acid spin labels I(m/n) show a reversible transition in their motional freedom at temperatures between 25 and 30 °C. This effect is stronger for labels I(1/14) and I(5/10) than for I(12/3). The polarity of their environment decreases in a sigmoidal way between 15 and 40 °C for labels I(12/3) and I(5/10). No polarity changes are observed with label I(1/14). 
 
2 
Below the transition the motional and polarity characteristics of I(m/n) are dominated by lipidprotein interaction, whereas above 30 °C the spectral parameters approach the situation of a model system of phosphatidylcholine-cholesterol. 
 
3 
The protein-bound spin label shows two spectral components arising from weakly and tightly immobilized binding sites, the ratio of the former to the latter increasing strongly between 25 and 30 °C concomitantly with a decrease in the separation of the two signals. The combined results indicate reversible changes in the fine structure involving both protein and polar lipids at the surface of lipoprotein B which correlate to the thermotropic transition of the apolar lipids in the particle core.