Effects of dithiothreitol on protein activity unrelated to thiol-disulfide exchange: for consideration in the analysis of protein function with Cleland's reagent.

Dithiothreitol (DTT) is widely used to reduce disulfide bonds in the analysis of protein structure and function. However, thiol-disulfide exchange is not the only mechanism whereby DTT can alter protein function. We observe that DTT diminishes the carbohydrate binding activity of a cysteineless mutant of pigpen as well as it inhibits the intact molecule. Lack of inhibition by threitol, a derivative of the four-carbon sugar threose, indicates that the thiol groups of DTT are required for inhibition, and also that DTT is not acting as a simple carbohydrate competitor. Moreover, inhibition of pigpen-carbohydrate binding is not likely due to metal chelation because pigpen binding to carbohydrate is insensitive to EDTA and 1, 10-phenanthroline, which would otherwise be expected to mimic the DTT effect. Our results suggest that DTT can interact with protein domains in the absence of cysteine residues, and that the biochemical reactivity of DTT is not necessarily one and the same with its assumed biochemical specificity.