An x-ray small-angle-scattering study of the structure of trypsin treated low density lipoprotein from human serum

Secondary structure and thermal behavior of trypsin-treated low-density lipoproteins from human serum, studied by circular dichroism

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.

Tryptic modification of human serum low-density lipoprotein leads to altered rates of turnover and intravascular transformation in the guinea pig

The metabolism of human serum low-density lipoprotein (LDL) and its trypsin-treated counterpart have been compared in the guinea pig in vivo. Removal of surface-exposed protein from the lipoprotein particle in this way resulted in significant modification of its metabolism in guinea pigs in vivo. Limited trypsinisation of LDL permitted removal of 20-25% of its protein moiety; trypsinised LDL was deficient in lysine and arginine residues (25-30% of each removed). The modified particle retained its basic structural features, such as internal molecular architecture, but displayed an elevated net negative surface charge and diminished immunological reactivity. Following intravascular injection of iodinated LDL (131I) and trypsinised LDL (125I) into the same animal, the two lipoproteins displayed biexponential decays; the rate constants for the plasma turnover of LDL and trypsinised LDL were significantly different (P less than 0.05), trypsinised LDL exhibiting a slower disappearance from the circulation. Density-gradient ultracentrifugation revealed marked elevation in the modal densities of both LDL and trypsinised LDL upon metabolism in vivo, although the rate of increase was greater for trypsinised LDL than LDL in each case (average increment 0.022 g/ml and 0.014 g/ml at 24 h respectively). The diminished plasma clearance of trypsinised LDL as compared to the native human and guinea pig LDL indicates that sites required for the cellular recognition and uptake of the LDL particle reside in its surface-exposed, trypsin-accessible protein. Furthermore, such protein appears to play a central role in regulating the intravascular processes by which the lipid content of LDL is diminished, and by which it is transformed to a particle of higher density.

A change observation of oriented diffraction from low-density serum lipoprotein

The X-ray diffraction patterns have been recorded from concentrated preparations of low-density serum lipoprotein (LDL). In one case a specimen gave rise to oriented diffraction: the 36 A ring, which has previously been attributed to the cholesteryl esters in LDL, shows markedly oriented intensity; and of the other small-angle rings, which do not show oriented peak intensity, one nonetheless shows significant anisometry. The possibility cannot be ruled out that an artifactual structure has been created; however, it is calculated that the structure will, if exposed in an unoriented preparation, give rise to 36 A diffraction not obviously distinguishable from the native pattern. If valid for the native LDL particle, these observation suggest that the cholesteryl esters in the core of the particle may be arranged in simple planar, or else cylindrically concentric, layers and that the overall shape of the particle is not a uniform sphere.

A comparative dielectric study of human serum low density lipoprotein before and after partial digestion by trypsin

The relative permittivity of aqueous solutions of human serum low density lipoprotein (LDL) and partially trypsin digested lipoprotein (T-LDL) has been determined for various concentrations at 20 degrees C over the frequency range 0.15-100 MHz. Comparison of the dielectric dispersion curves for the digested lipoprotein with those for the native preparation revealed a larger low-frequency dielectric increment, which may be attributed to an increase in the number of counterions moving over the surface of the molecule. An explanation of this observation is an elevation of 70% in the net negative charge on the surface of the trypsin-treated particle as compared to its native counterpart.