The composition and metabolism of high density lipoprotein subfractions

The composition and metabolism of high density lipoprotein (HDL) subfractions were investigated in seven nomal individuals. Mean HDL2 (d, 1.063–1.125 g/ml) composition (by weight) was 43% protein, 28% phospholipid, 23% cholesterol, and 6% triglyceride, and mean HDL3 (d, 1.125–1.21 g/ml) composition was 58% protein, 22% phospholipid, 14% cholesterol, and 5% triglyceride. The mean apoA‐I; apoA‐II weight ratio was 4.75 for HDL2 and 3.65 for HDL3.HDL2 protein was proportionally slightly richer in C apolipoproteins and higher molecular weight constituents (including apoE) than HDL3. Kinetic studies utilizing radiolabeled HDLA (d, 1.09–1.21 g/ml), HDL2, and HDL3 demonstrated rapid exchange of apoA‐I and apoA‐II radioactivity among HDL subfractions, similar fractional rates of catabolism of apoA‐I and apoA‐II within HDL, and similar radioactivity decay within HDL subfractions. Mean plasma residence time was 5.74 days for radiolabeled HDL2 and 5.70 days for radiolabeled HDL3. Differences in HDL protein mass among individuals were largely due to alterations in catabolism, and in general both HDL2 and HDL3 were catabolized via a plasma and a nonplasma pathway. Data from simultaneous radiolabeled very low density lipoprotein and HDL studies in 2 individuals are consistent with the concept that apoC‐II and apoC‐III are catabolized at a different rate than are apoA‐I and apoA‐II within the HDL density range.