COMPOSITION AND PROPERTIES OF SOLUBLE LIPOPROTEINS IN RELATION TO STRUCTURE

Abstract

Lipoproteins from diverse sources have total lipid contents extending over an essentially continuous range but do show strong evidence of a transition in their composition in the region where protein, phospholipid, and neutral lipid are present in essentially 1:1:1 proportions. Those with protein contents below 33% (LPL) have neutral lipid–phospholipid ratios ranging from 1:1 to 10:1; but this ratio remains at about 1:1 in all those with protein contents above 33% (HPL). The protein content of the LPL type decreases with increasing particle size and is sufficient to provide only a surface film. If the protein is present in this form, the particle must have a lipid core; if it is present in more globular form, much of the particle surface must be phospholipid, and such structures resemble microemulsions or micelles. The composition and properties of the HPL type, however, are consistent with their having a structural integrity approaching that of a protein molecule. For example, their molecular weight appears to be independent of their lipid content, and egg yolk HPL exhibits pH-dependent reversible dissociation into two subunits comparable to that observed in certain proteins. Differences in the N-terminal residues (serum lipoproteins) and amino acid composition (yolk lipoproteins) of the proteins of most LPL and HPL types indicate that, if their interconversion does occur, it would involve more drastic changes than either alterations in structure or a simple loss or gain of lipid.