Two subpopulations of intermediate density lipoprotein and their relationship to plasma triglyceride and cholesterol levels

We observed the appearance of two intermediate density lipoprotein (IDL) subfractions on gradient gel electrophoresis of lipoproteins in the density range 1.006–1.030 g/ml and estimated their approximate concentrations in plasma in subjects with a wide range of lipid levels, from 0.55 to 28.0 mmol/l plasma triglyceride and 3.75–10.0 mmol/l cholesterol. The larger species, IDL-I (31.7±0.7 nm, mean±SD), showed little variation in size in normal and moderate hyperlipidaemic individuals. The estimated concentration of IDL-I was positively related to plasma triglyceride (r=0.63, P=0.0004) and low density lipoprotein (LDL) cholesterol (r=0.68, P=0.0003). These findings are consistent with the view that IDL-I is a metabolic intermediate between very low density lipoprotein (VLDL) and LDL. The smaller subfraction, IDL-II (25.7±2.4 nm) was virtually the only true species observed in subjects with plasma triglyceride <1.0 mmol/l and its estimated concentration fell as plasma triglyceride increased (r=−0.58, P=0.0002). IDL-II particle size changed in concert with LDL particle size (r=0.61, P<0.0001), suggesting that they were influenced by common metabolic factors. These observations provide further support for the hypothesis outlined by Musliner et al. [1] that IDL-I was part of the delipidation chain from VLDL to LDL, whereas IDL-II arose from a separate source, possibly directly released from the liver. Hence the two subpopulations of IDL differ in their relationship to plasma triglyceride and cholesterol levels.