Mass spectral analysis of domestic and wild equine apoA-I and A-II: detection of unique dimeric forms of apoA-II

In pigs, humans, chimpanzees and probably other great apes, a cysteine at residue 6 enables apolipoprotein A-II to form a homodimer. However, the apoA-IIs of other primates, lacking a cysteine residue, are monomeric. We have already reported that horse apoA-IIs form homodimers due also to a cysteine at residue 6. In this study, we wanted to determine whether other equine apoA-IIs might be monomeric. The high density lipoproteins were ultracentrifugally isolated from the plasmas of a horse (Equus caballus), a donkey (Equus asinus) and five wild equines: two types of zebras (Equus zebra hartmannae and Equus zebra quagga boehmi), a Przewalski's horse (Equus przewalskii), a Somali ass (Equus africanus somalicus) and a kiang (Equus kiang holdereri). Using liquid chromatography with electrospray-ionization mass spectrometry, we were able to obtain accurate values for the molecular masses of apoA-I and apoA-II. Homodimeric apoA-IIs were observed in each of the animals studied. The donkey had unique dimers, consisting of the proapolipoprotein A-II linked by a disulfide bond either to a mature apoA-II monomer or another proapoA-II. In addition, our data indicate that small amounts of apoA-I and apoA-II apparently are acylated.