Abnormal in Vivo Metabolism of Apolipoprotein E4 in Humans

Apolipoprotein E (apoE) is important in modulating the catabolism of remnants of triglyceride-rich lipoprotein particles. It is a polymorphic protein with the three common alleles coding for apoE2, apoE3, and apoE4. ApoE3 is considered the normal isoform, while apoE4 is associated both with hypercholesterolemia and type V hyperlipoproteinemia. We quantitated the kinetics of metabolism of apoE4 in 19 normolipidemic apoE3 homozygotes and 1 normolipidemic apoE4 homozygote, and compared this with the metabolism of apoE3 in 12 normolipidemic apoE3 homozygotes. In the apoE3 homozygous subjects, apoE4 was catabolized twice as fast as apoE3, with a mean plasma residence time of 0.37 +/- 0.01 d (+/- SEM) and 0.73 +/- 0.05 (P less than 0.001), respectively. When plasma was fractionated into the lipoprotein subclasses, the greatest amount of labeled apoE4 was present on very low density lipoproteins, while the largest fraction of labeled apoE3 was associated with high density lipoproteins. The plasma apoE concentration was decreased in an apoE4 homozygote compared with the apoE3 homozygotes (3.11 mg/dl vs. 4.83 +/- 0.35 mg/dl). The reduced apoE4 concentration was entirely due to a decreased apoE4 residence time in the apoE4 homozygote (0.36 d vs. 0.73 +/- 0.05 d for apoE3 in apoE3 homozygotes). These results indicate that apoE4 is kinetically different than apoE3, and suggest that the presence of apoE4 in hypercholesterolemic and type V hyperlipoproteinemic individuals may play an important pathophysiological role in the development of these dyslipoproteinemias.