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

Dielectric behavior of water in biological solutions: studies on myoglobin, human low-density lipoprotein, and polyvinylpyrrolidone

The dielectric behavior of the aqueous solutions of three widely differing macromolecules has been investigated: myoglobin, polyvinylpyrrolidone (PVP), and human serum low-density lipoprotein (LDL). It was not possible to interpret unambiguously the dielectric properties of the PVP solution in terms of water structure. The best interpretation of the dielectric data on the myoglobin and LDL solutions was that, in both cases, the macromolecule attracts a layer of water of hydration one or two water molecules in width. For LDL, this corresponds to a hydration factor of only 0.05 g/g, whereas for myoglobin the figure is nearer 0.6 g/g. With myoglobin, part of the water of hydration exhibits its dispersion at frequencies of a few GHz, and the rest disperses at lower frequencies, perhaps as low as 10-12 MHz. The approximate constancy of the width of the hydration shell for two molecules as dissimilar in size as LDL and myoglobin confirms that the proportion of water existing as water of hydration in a biological solution depends critically on the size of the macromolecules as well as on their concentration.

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.