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The purpose of this study was to determine the concentrations and composition of apoB-containing lipoprotein families in whole plasma and major lipoprotein density classes of a selected group of Native Americans from northeastern Oklahoma with non-insulin-dependent diabetes mellitus. The measurement of lipoprotein density classes showed that the total lipoprotein mass of very-low-density lipoproteins was significantly higher and that of high-density lipoproteins significantly lower in diabetic patients than nondiabetic control subjects regardless of their plasma triglyceride levels. The VLDLs were enriched with TG, free cholesterol, and apolipoproteins C-III and E. HDLs were enriched with TG but depleted of apoC-III and apoE. There was no change in the levels of TG-enriched low-density lipoproteins. Fractionation of VLDL by sequential immunoprecipitation with antisera to apoE and apoC-III established that increased concentrations of this density class in diabetic patients are due to elevated levels of TG-rich lipoprotein LP-B:C and lipoprotein LP-B:C:E. The levels of LP-B:C particles were increased more than the levels of LP-B:C:E particles. The LDLs were characterized by a slight increase in TG-enriched lipoprotein B and no change in the levels of LP-B:C and LP-B:C:E. There was no difference between diabetic patients with or without vascular disease in the levels of LP-B and LP-B:C:E. However, patients with vascular disease had higher concentrations of LP-B:C particles in VLDL and whole plasma than patients without vascular disease.(ABSTRACT TRUNCATED AT 250 WORDS)