Type III hyperlipoproteinemia typically is associated with homozygosity for 
apolipoprotein (apo) E2(Arg158----Cys). Dominant expression of type III 
hyperlipoproteinemia associated with apoE phenotype E3/3 is caused by 
heterozygosity for a human apoE variant, apoE3(Cys112----Arg, Arg142----Cys). 
However, this apoE3 variant was not separable from the normal apoE3 in these 
patients' plasma because the two proteins have identical amino acid composition, 
charge, and molecular weight. Therefore, to determine the functional 
characteristics of this protein, we used recombinant DNA techniques to produce 
this apoE variant in bacteria. We also produced a non-naturally occurring 
variant, apoE(Arg142----Cys), that had only the cysteine substituted at residue 
142. These two apoE variants were purified from cell lysates of the transfected 
Escherichia coli by ultracentrifugal flotation in the presence of phospholipid, 
by gel filtration chromatography, and by heparin-Sepharose chromatography. Both 
Cys142 apoE variants bound to lipoprotein receptors on human fibroblasts with 
only about 20% of normal binding activity. Therefore, cysteine at residue 142, 
not arginine at residue 112, is responsible for the decreased receptor binding 
activity of the variants. Cysteamine treatment and removal of the 
carboxyl-terminal domain had little effect on the binding activity, whereas both 
modulate the receptor binding activity of apoE2(Arg158----Cys). The mutation at 
residue 142 decreased the binding activity of apoE to both heparin and the 
monoclonal antibody 1D7 (this antibody inhibits receptor binding of apoE), 
whereas apoE2(Arg158----Cys), which is associated with recessive expression of 
type III hyperlipoproteinemia, binds normally to both. The Arg112, Cys142 
variant predominantes 3:1 over normal apoE3 in the very low density lipoproteins 
of plasma from an affected subject, as assessed by differential reactivity with 
the antibody 1D7. The unique combination of functional properties of the Arg112, 
Cys142 variant provides a possible explanation for its association with dominant 
expression of type III hyperlipoproteinemia.