Apoprotein A-I measured by radial immunodiffusion in heparin-MnCl2 supernates.

We compared measurements of apoprotein A-I (apoA-I) in plasma and in heparin-MnCl2 supernates analyzed by radial immunodiffusion. The apoA-I values were similar when the samples were fresh [n = 41, mean (SD), mg/L: plasma, 1393.4(349); heparin-MnCl2 supernates, 1364.9(332), p less than 0.01], but were greater than 8% lower in heparin-MnCl2 supernates after storage for seven days at 4 degrees C [plasma, 1348.0(351); heparin-MnCl2 supernates, 1237.6(342), p less than 0.001]. Neither heparin nor MnCl2 interfered directly with the immunodiffusion assay, and treating samples with tetramethylurea and urea to maximize the exposure of apoA-I did not prevent the decrease. MnCl2 (46 mmol/L) added to isolated HDL (d 1.063-1.21) decreased apoA-I values by 5.6% when measured immediately and by 16.7% after storage at 4 degrees C for seven days. High-density lipoprotein cholesterol values were unchanged by MnCl2. The results indicated that apoA-I was more stable in plasma than in heparin-MnCl2 supernates, probably because of an interaction between apoA-I and MnCl2.