Studies on the polymorphism of human apolipoprotein A-I

Upon preparative isoelectric focussing of human apo-HDL, four major forms of apolipoprotein A-I have been isolated. As identified by the following nomenclature and pI, they comprise: apolipoprotein A-I1, pI 5.62; apolipoprotein A-I2, pI 5.53; apolipoprotein A-I3, pI 5.45; apolipoprotein A-I4 pI 5.36. These forms of apolipoprotein A-I were shown to have identical migration on polyacrylamide gel electrophoresis, molecular weights of 26 000 on sodium dodecyl sulfate gel electrophoresis and a common antigenicity with antisera against apolipoprotein A-I or A-I1. Each form had very similar amino acid compositions with the exception of form apolipoprotein A-I4 which contained one isoleucine residue per mol. All forms but apolipoprotein A-I4 were activators of lecithin:cholesterol acyltransferase, the latter was inhibitory to the reaction. From these results, it was concluded that apolipoprotein A-I1, A-I2 and A-I3 are equivalent forms of apolipoprotein A-I whereas apolipoprotein A-I4 is different or heterogeneous. Upon refocussing, the polymorphs were shown to be stable at their pI and not affected by changes in concentration and by the presence of urea or ampholytes. Exposure of a form of apolipoprotein A-I to alkaline pH partially regenerated the original heterogeneity; however, apolipoprotein A-I4 regenerated from apolipoprotein A-I1 did not contain isoleucine, which further demonstrates form apolipoprotein A-I4 heterogeneity.