Previously, we demonstrated that the human E apoprotein existed in three forms (E-2, E-3, and E-4), and that the three forms differed from one another by cysteine-arginine interchanges at two substitution sites (A and B). The E-2, E-3, and E-4 apo-E contain cysteine/cysteine, cysteine/arginine, and arginine/arginine at sites A/B, respectively. Subjects with Type I11 hyperlipoproteinemia, a genetic disease associated with defective plasma lipoprotein clearance, possess the E-2 form of apo-E. It was postulated that the substitution of cysteine for arginine at site B in the E-2 might be responsible for an impaired interaction of Type I11 apo-E with cell surface receptors. To test this possibility, the binding activities of the various forms of apo-E to the receptors on human fibroblasts were compared. The E-3 and E-4 apo-E readily bound to the receptors; however, the E-2 apo-E-binding activity was defective. Consideration was given to the possibility that a positively charged residue at site B, as occurs in both E-3 and E-4, was important for normal binding activity. To investigate this, the cysteine residues of the E-2 apo-E were converted by cysteamine treatment to a positively charged lysine analogue. This resulted in a marked increase in the binding activity of the E-2 apo-E. These studies demonstrated that the defective binding of the E-2 apo-E from Type I11 hyperlipoproteinemic subjects was due, at least in part, to the cysteine-arginine interchange at site B, and they suggested the importance of a positively charged residue at this position in the sequence to mediate normal apolipoprotein-receptor interaction.

We recently demonstrated that three forms of the human E apoprotein exist (E-2, E-3, and E-4), each with a distinct amino acid sequence (1). The three distinct forms of apo-E’result from differences in their amino acid sequences, involving an interchange of arginine and cysteine at two sites, A and B, in the protein. The E-3 apo-E has a cysteine residue at site A and an arginine residue at site B, whereas the E-2 apo-E has cysteine residues at both sites. The E-4 apo-E, on the other hand, is characterized by an absence of cysteine and the apparent existence of arginine residues at both sites A and B (1). The interchange of the arginine and cysteine residues is sufficient to explain the known charge differences observed