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 (ap o) E concentration in serum, we compared the apo E concentrations measured in fresh human serum samples with those determined after storage at +4 degr ees C, -20 degrees C or -80 degrees C. The serum apo E concentration was me asured by immunoturbidimetry using an anti-human apo E polyclonal antibody from goats. One week storage at +4 degrees C did not significantly affect t he serum apo E concentration. At -20 degrees C or -80 degrees C no signific ant change in apo E concentration occurred during up to three months of sto rage. Moreover, the concentration of apo E was not modified after long-term storage of serum samples kept at -196 degrees C in liquid nitrogen for up to four years. In addition, 15 freeze-thaw cycles, over a 3-week period, di d not affect the apo E concentration in serum. A similar freeze-thaw proced ure 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.