Cyst(e)ine residues of bovine white-matter proteolipid
proteins were characterized in a highly purified preparation. From a total of 10.6
cyst(e)ine residues/molecule of
protein, as determined by
performic acid oxidation, 2.5-3
thiol groups were freely accessible to
iodoacetamide,
iodoacetic acid and 5,5'-dithiobis-(2-nitrobenzoic acid) (
DTNB), when the
proteins were solubilized in
chloroform/
methanol (C/M) (2:1, v/v). The presence of
lipids had no effect on
thiol-group exposure. One
thiol group available to
DTNB in C/M could not be detected when
proteolipids were solubilized in the more polar
solvent n-butanol. In a C/M
solution of purified proteolipid
proteins, SDS did not increase the number of reactive
thiol groups, but the cleavage of one disulphide bridge made it possible to alkylate six more groups. C.d. and fluorescence studies showed that
rupture of this disulphide bond changed the protein conformation, which was reflected in partial loss of helical structure and in a greater exposure to the
solvent of at least one
tryptophan residue.
Cyst(e)ine residues were also characterized in the different components [PLP (principal proteolipid
protein), DM20 and LMW (low-Mr
proteins)] of the proteolipid preparation. Although the numbers of
cyst(e)ine residues in PLP and DM20 were similar, in LMW fewer residues were alkylated under four different experimental conditions. The differences, however, are not simply related to differences in Mr.