TABLE 2.

Methionine sulfoxide levels in α/β-type SASP oxidized in vitro^a

Protein	Treatment	% Methionine sulfoxide at:
		Conserved residueb		Less well conserved residue M67
		M27	M28
SspC	None	3 (<1)		21
	Gel purification	13 (<1)		24
	tBHP	57 (57)		61
	tBHP plus DNA	10 (10)		31
	H2O2	89 (89)		>97
	H2O2 plus DNA	23 (23)		61
SASP-C	None		8 (<1)
	Gel purification		15 (5)
	tBHP		55 (15)
	tBHP plus DNA		17 (4)
	H2O2		88 (23)
	H2O2 plus DNA		22 (6)

^aSASP were oxidized, separated from the oxidizing agent, and purified by Tris-Tricine-SDS-PAGE as described in Materials and Methods, except for untreated samples, which were not electrophoretically purified. Note that in these experiments, oxidation was stopped by rapid removal of the oxidizing agent by HPLC, in contrast to the experiment shown in Fig. 1, in which the oxidizing agent was removed by dialysis. 

^bSignificant oxidation also occurred at lysines 28 and 29 of SspC and SASP-C, respectively. This resulted in larger tryptic peptides, which were fully oxidized at M27 and M28. The amounts of these larger peptides have been included in calculations of methionine oxidation, and the percentages of oxidation at lysines 28 and 29 are given in parentheses.