Storage of human plasma samples leads to alterations in the lipoprotein distribution of apoC-III and apoE

Jeffrey S. Cohn, Claudia Rodriguez, Helene Jacques, Michel Tremblay, and Jean Davignon

Hyperlipidemia and Atherosclerosis Research Laboratory, Clinical Research Institute of Montreal, Montreal, Quebec H2W 1R7

Corresponding Author: cohnj{at}ircm.qc.ca

Frozen plasma samples from large multicentre studies have often been used to investigate the relationship between lipoprotein apoC-III and apoE levels and risk of coronary artery disease (CAD). Our experience however is that frozen storage can affect the lipoprotein distribution of apoC-III and apoE. This was investigated in the present study by measuring apoC-III and apoE by ELISA in HDL and apoB-containing lipoproteins of human plasma samples (n = 16) before and after two weeks of frozen storage (-20oC). HDL were separated by heparin-manganese precipitation (HMP) or by fast protein liquid chromatography (FPLC). Total plasma apoC-III and apoE levels were not affected by frozen storage. HDL-HMP apoC-III and apoE levels were significantly higher in frozen v fresh samples: 7.7 ± 0.7 v 6.7 ± 0.7 (P < 0.05), 2.0 ± 0.1 v 1.2 ± 0.1 mg/dl (P < 0.001), respectively. HDL-FPLC apoC-III and apoE (but not TG or cholesterol) levels were also higher in frozen samples: 12.0 ± 1.2 v 7.5 ± 0.6 (P < 0.001), 2.7 ± 0.2 v 1.6 ± 0.2 mg/dl (P < 0.001). Frozen storage led to a decrease in apoC-III (-17 ± 9%) and apoE (-19 ± 9%) in TRL. Redistribution of apoC-III and apoE was most evident in samples with high TG levels. HDL apoC-III and apoE levels were also significantly higher when measured in plasma stored at -80oC. Our results demonstrate that lipoprotein distribution of apoC-III and apoE is affected by storage of human plasma, suggesting that analysis of frozen plasma should be avoided in studies relating lipoprotein levels of apoC-III and/or apoE with the incidence of CAD.

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