Apolipoprotein CII in type I hyperlipoproteinemia. A study in three cases

In three children with type I hyperlipoproteinemia, the relationships of plasma apo CII to TGs, TRLs, and LPL were assessed to better understand the pathophysiology of the disorder. Total plasma apo CII (by radioimmunoassay) in the three children (33.2, 20.4, and 21.4 mg/dl) was fourfold to sixfold higher than in normals (mean ± S.D., 5.0 ± 2.3) and in patients with primary hypertriglyceridemia with types IV and V lipoprotein phenotypes (8.5 ± 2.7 and 14.2 ± 1.8 mg/dl, respectively). In the three patients, less than 2.5% of total plasma apo CII was transported in the plasma fraction containing HDL (d > 1.006 gm/ml plasma fraction) compared to 53.0% ± 20.5, 21.3% ± 12.0, and 2.9% ± 0.2 in normals and patients with types IV and V, respectively. By an in vitro assay using milk LPL, the LPL-activating potency of apo CII in TRLs from the three children (1.4, 2.8, and 1.6 U/μg of apo CII) was comparable to that in normals (1.5 ± 0.5 U); higher than that observed in patients with types IV (1.0 ± 0.1 U); and higher than that in patients with type V lipoprotein phenotypes (0.9 ± 0.2 U) and normal PHLA. In two of three children with type I, the amount of apo CII in TRL protein (3.8% and 0.5%) was lower than in normals (10.0% ± 1.9) and within the fifth to ninety-fifth percentile confidence limits for the third child (13.7%). The molar ratios of TRL TG:apo CII in the children (1658:1, 15,372:1 and 3233:1) were higher than in normals (435 ± 147:1) or in patients with type IV (734 ± 382:1) or type V (1614 ± 273:1) hyperlipoproteinemia. In one affected child, dietary fat restriction lowered total plasma TG from 10,208 to 632 mg/dl; the TRL TG:apo CII ratio fell from 15,372:1 to 376:1; the initially elevated total plasma apo CII (20.4 mg/dl) remained essentially unchanged (20.0 mg/dl) after dietary fat restriction. The normal in vitro LPL-activating potency of the TRL apo CII in this child remained within the normal range during the period of fat restriction. In type I hyperlipoproteinemia, unlike other primary hypertriglyceridemias, deficient tissue LPL is not associated with inhibition of LPL-activating property of apo CII in TRL. The striking elevation of total plasma apo CII in children with type I hyperlipoproteinemia is apparently consonant with a physiological compensatory response to achieve TG homeostasis.