Reaction of phenylglyoxal and {p-hydroxyphenyl}glyoxal with arginines and cysteines in the .alpha. subunit of tryptophan synthase

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Abstract
The .alpha. subunit of tryptophan synthase from Escherichia coli is inactivated by phenylglyoxal and by (p-hydroxyphenyl)glyoxal. The use of these chemical modification reagents to determine the role of arginyl residues in the .alpha. subunit of tryptophan synthase was complicated by the finding that these reagents react with SH groups of the .alpha. subunit, as well as with arginyl residues. Analyses of the data for incorporation of phenyl[2-14C]glyoxal, for inactivation and for sulfhydryl modification in the presence and absence of indole-3-glycerol phosphate, indicate that 2 SH groups and 1 arginine are essential for the activity. The finding that the substrate protects the single essential arginyl residue but not the 2 SH groups is consistent with the observed kinetics of partial protection by substrate or by a substrate analog, indole-3-propanol phosphate. In contrast to phenylglyoxal, (p-hydroxyphenyl)glyoxal modifies 2-3 SH groups that are not protected by indole-3-glycerol phosphate and modifies none of the arginyl residues that are modified by phenylglyoxal.