title: Secondary structure and thermal behavior of trypsin-treated low-density lipoproteins from human serum, studied by circular dichroism.
abstract: Low-density lipoproteins (LDL) were prepared from the serum of normolipidemic men on normal diets with or without supplemental beta-carotene. LDL were subjected to limited hydrolysis (5 h at 37 degrees C) with trypsin (enzyme:protein, 1:40 w/w), and their digested products separated by gel filtration. The trypsin-treated LDL contained about 80% of the original protein and essentially all of the original lipids of native LDL. The circular dichroic spectrum of trypsin-treated LDL below 240 nm resembled that of native LDL, except that the magnitudes of the ellipticity were smaller, corresponding to 25 and 33% helical content, respectively. The lower content of helix in trypsin-treated LDL suggests that certain helical regions in apolipoprotein B are sensitive to tryptic attack; however, a major portion of the helical structure of the apolipoprotein is resistant. The thermal stability of helix in trypsin-treated LDL resembled that of native LDL, suggesting that removal of the trypsin-accessible regions of the apolipoprotein has little or no effect on the forces stabilizing the remaining helices. Data on the induced circular dichroism of beta-carotene, an intrinsic probe of the neutral lipid core, showed a reduced transition temperature for cholesteryl esters after trypsin treatment. This finding suggests that the trypsin-accessible regions of apolipoprotein B may influence the fluidity of the core.
mesh_terms: D000328:Adult; D002338:Carotenoids / Q000378:metabolism; D055598:Chemical Phenomena; D002621:Chemistry; D002850:Chromatography, Gel; D002942:Circular Dichroism; D004032:Diet; D006358:Hot Temperature; D006801:Humans; D006868:Hydrolysis; D008077:Lipoproteins, LDL / Q000097:blood*; D008297:Male; D008560:Membrane Fluidity; D013329:Structure-Activity Relationship; D014357:Trypsin; D019207:beta Carotene
chemical_list: D008077:Lipoproteins, LDL; D019207:beta Carotene; D002338:Carotenoids; D014357:Trypsin