Selective proteolytic digestion as a method for the modification of human HDL3 structure

Area of Science:

• Biochemistry
• Lipid Metabolism
• Protein Chemistry

Background:

• Human high-density lipoprotein (HDL) plays a crucial role in lipid transport.
• Understanding the structural organization of HDL, particularly the roles of apolipoproteins A-I and A-II, is essential for elucidating its function.
• Selective modification of HDL components can provide insights into its structural dynamics.

Purpose of the Study:

• To investigate the structural consequences of selectively digesting human HDL(3) using trypsin after masking lysine residues.
• To determine the impact of this digestion on the apoA-I and apoA-II components and their association with lipids.
• To explore the potential for regenerating modified HDL particles.

Main Methods:

• Masking lysine residues on human HDL(3) with 2,3,4,5-tetrahydrophthallic anhydride (THPA).
• Selective trypsin digestion of THPA-modified HDL(3).
• Separation of lipid-free and lipid-bound fractions.
• Immunological analysis (loss of immunoreactivity).
• Column chromatography and electron microscopy for particle size analysis.
• Amino acid composition analysis.
• Incubation of core particles with intact apoA-I.

Main Results:

• Trypsin digestion fragmented apoA-I but not apoA-II, yielding a lipid-bound core particle and a lipid-free apoA-I fraction.
• Approximately 50-80% of apoA-I was lipid-free; the core contained residual apoA-I and apoA-II with most of the lipid.
• ApoA-I lost all immunoreactivity, while apoA-II retained it.
• The core particle size was similar to untrypsinized HDL(3).
• Amino acid analysis indicated preferential lipid binding to hydrophobic domains of apoA-I, particularly tryptophan residues.
• Regeneration of HDL(3) with altered protein composition was achieved by adding intact apoA-I to core particles.

Conclusions:

• Selective proteolytic digestion is a viable method for modifying human HDL(3) structure.
• HDL(3) structure allows for some latitude in surface/volume ratio before particle reorganization.
• The findings suggest a potential mechanism for HDL to act as a storage for plasma C apolipoproteins.