I was stimulated by the recent discussion of Met-sulfoxide and my
own recent question about the reduction of oxidized cysteine residues in
proteins to report the results of my ongoing literature search:
1. Reduction of disulfides to thiols
While DTT is a good reducing at alkaline pH, Whitesides and
collaborators have shown that at pH 7, other reagents with lower pKa values
than DTT reduce disulfides more quickly (Methods Enzymol. 251?, Techniques
in Protein Chemistry, Volume 6, 259, 1995)
TCEP can reduce disulfides even under acidic conditions
(Biochemistry, 1998, 37, 12172). Sodium borohydride might be effective
also, but its decomposition rate increases with decreasing pH.
2. Reduction of the sulfenic acid of cysteine to cysteine. A variety of
reagents can effect this transformation (Accts. Chem. Res., 1976, 9, p.
293). TCEP might be effective in this regard.
3. Reduction of the sulfinic acid of cysteine to cysteine. I have not yet
been able to find literature that deals with this, but Saul Patai's series
of books on the chemistry of functional groups is a good bet.
4. Reduction of cysteine sulfonate (cysteic acid) to cysteine. This
cannot be done under reasonable conditions.
5. Reduction of methionine sulfoxide. With respect to Deb MacMillen's
comments on reduction, the lack of variation of the rate with pH is
surprising, given point #1 above. Nevertheless, that is basically what is
reported in Methods in Enzymol., Vol. 91, 549. Perhaps the active species
is the thiol, as opposed to the thiolate anion.
6. Reduction of the sulfoxide form of a biotin labelling reagent. This
question was posted recently (a commercial reagent was 50% in the sulfoxide
form!), and the key issue is what is the functionality of the reagent. It
might be possible to reduce the sulfoxide without chemically altering the
functional group which effects coupling if one works at alkaline pH and
does not use too strong a reducing agent
In general, Methods in Enzymology Volumes 91, 143, and 251 are valuable
resources for sulfur biochemistry.
I hope this is of some use to people. Thanks to those whose imput
is reflected in these references.
Chris
Christopher Halkides
Dept. of Chemistry, UNCW
601 S. College Road
Wilmington, NC 28403-3297
(910) 962-7427