This uncertainty in Michaelis-Menten modeling -arising from product inhibition, is systematicapplying to both enzymes. Therefore, the comparative values are robust, but the absolute Vmax values (and subsequent kcat and kcat/Km interpretations) have some uncertainty that we cannot quantify. In addition, the experimental variability measured for the GSSG substrate is significantly larger than for the other substrates. This may be consistent with a pingpong/branching mechanism suggested previously for this enzyme/substrate pair . Notably, reduced experimental variability among other substrates suggests the putative mechanism of reduction for GSSG may change for other enzyme/substrate pairs.