Human E Apoprotein Eterogeneity

It has been postulated that 3 alleles at a single gene
locus are responsible for the 3 major isoforms of the
human E apoprotein (E-4, E-3, and E-2) and that the
minor isoforms arise by post-translational glycosylation
of the major isoforms (Zannis, V. I., and Breslow,
J. L. (1981) Biochemistry 20,1033-1041). We have found
that the heterogeneity of the 3 major isoforms of the
human E apoprotein is due to differences in primary
structure and involves cysteine-arginine interchanges.
Based on amino acid analyses, the E isofoms differ in
the number of cysteine residues per mol of protein, ie.
E-4 has no cysteine, E-3 has 1 cysteine residue, and E-
2 has 2 cysteine residues. The sites of substitution have
been identified in 2 cyanogen bromide fragments and
occur at positions 4 and 33 in the 17- and 89-residue
fragments, respectively. Based on sequence and amino
acid analysis, the E-4, E-3, and E-2 contain arginine/
arginine, cysteine/arginine, and cysteine/cysteine at
these sites, respectively. These cysteine-arginine interchanges
are sufficient to account for the known charge
differences between the E-4, E-3, and E-2 isoforms.
Furthermore, thec ysteine differences in theE isoforms
have been confirmed in a large number of subjects
using a rapid screening procedure involving charge
modification of the cysteine residues of the E isoforms
with cysteamine. This method provides a sensitive
measurement of the cysteine content of the isoforms
and, as predicted from the cysteine contents, the E-4,
E-3, and E-2 isoforms are shifted 0, 1, and 2 positive
charge units, respectively, on isoelectric focusing gels
after cysteamine modification. These cysteine-arginine
interchanges in the human E isoforms may directly
affect the metabolic activity of the various isoforms
and have profound metabolic consequences.

(The Journal of biological chemistry)