Effect of Short- and Long-Term Storage on Human Serum and Recombinant Apolipoprotein E Concentration

In order to assess the short- and long-term stability of apolipoprotein (apo) E concentration in serum, we compared the apo E concentrations measured in fresh human serum samples with those determined after storage at +4°C, −20°C or −80°C. The serum apo E concentration was measured by immunoturbidimetry using an anti-human apo E polyclonal antibody from goats. One week storage at +4°C did not significantly affect the serum apo E concentration. At −20°C or −80°C no significant change in apo E concentration occurred during up to three months of storage. Moreover, the concentration of apo E was not modified after long-term storage of serum samples kept at −196°C in liquid nitrogen for up to four years. In addition, 15 freeze-thaw cycles, over a 3-week period, did not affect the apo E concentration in serum. A similar freezethaw procedure applied to purified human recombinant apo E showed that apo E2 isoform was the most stable in comparison with the apo E3 and apo E4 isoforms.