Interference by pro-apolipoprotein A-I in apolipoprotein E phenotyping using chemical precipitation procedures

Lipoproteins of density, d less than 1.063, isolated by polyanion-cation precipitation methods, gave isoelectric focussing patterns of apolipoprotein E isoforms by rod-gel electrophoresis which differed from the corresponding patterns obtained from ultracentrifugally-derived very low density lipoproteins. The differences were sporadic and variable but the most common effect was an increased frequency of the E3 isoform. Two-dimensional analyses involving sodium dodecyl sulphate-polyacrylamide gel electrophoresis and immunoelectrophoresis against anti-apolipoprotein A-I indicated that contamination of precipitated lipoproteins with pro-apolipoprotein A-I was responsible for this phenomenon. It is suggested that two-dimensional techniques should be applied for definitive phenotyping if precipitated lipoproteins are used as source material.