IMMUNONEPHELOMETRIC QUANTITATION OF THE APOLIPOPROTEIN C-III IN HUMAN-PLASMA

A quantitative assay, based on endpoint immunonephlometry, was developed for human apolipoprotein C-II (Apo C-III) in plasma and lipoprotein fractions. The standard curve was constructed either with purified Apo C-III2 as a primary standard or with plasma as a secondary standard. It was linear between 50 and 400 ng Apo C-III per sample, corresponding to 1 .mu.l undiluted plasma. The intra- and interassay coefficients of variation (CV values) were 2.2 and 6.3%, respectively. The Apo C-III immunoreactivity was not influenced by detergents, denaturants nor by delipidation. The use of a nonionic detergent (Apovax, 0.1 g/l) avoided the need for organic solvent extraction for plasma containing up to 4 g of triglycerides/l by reducing the sample turbidity. As measured in 126 normolipidemic subjects, the plasma Apo C-III concentration was 0.118 .+-. 0.028 g/l (mean .+-. SD). Apo C-III concentrations were only slightly elevated in patients with Fredrickson type IIa hyperlipoproteinemia. The Apo C-III levels were .apprx. 3 times higher in type I, IIb, III and IV patients, while subjects with type V hyperlipemia had about a 5-fold increase in Apo C-III compared to the healthy patients. The plasma Apo C-III values were strongly correlated with the plasma triglyceride concentrations (r = 0.80, n = 201). The Apo C-III distribution among the various lipoprotein fractions showed a higher proportion of Apo C-III in VLDL in hypertriglyceridemic subjects compared to normolipemic subjects.