Glucose 6-phosphate dehydrogenase (G6PD) plays a key role to maintain the redox state within the red cells. The molecular basis of human G6PD deficiency rely on the presence of point mutations along its 59.2 kDa subunit [l]. The intracellular active enzyme behave mostly as a homodimer. Over 300 different G6PD deficient variants has been already reported and, in spite of their low intracellular activity some of them have normal Km values for glucose-6-phosphate and NADP. We have assumed that the alteration of the conformational stability of the protein could be driven by point mutations in the G6PD molecule and therefore a defective folded state could be incompatible with its normal activity. In order to test our hypothesis in this system we have studied the kinetics of the assisted protein disulfide isomerase (PDI) refolding of the normal G6PD for a subsequent comparison of these data with equivalent data from the variants. In this paper we report the experimental conditions of the PDI catalyzed refolding of the recombinant human G6PD. Human G6PD was expressed and purified to homogeneity as previously described [2] . Purification of bovine PDI was carried out by the method of Hillson et al., [3]. Reduced, denatured G6PD was obtained as previously reported by Creighton [41 with DTT and guanidine hydrochloride (Gdn-HCI). Excess of DTI and Gdn-HC1 was removed from the denatured G6PD by centrifugal gel filtration (Sephadex G25) in 0.1 YO acetic acid. Renaturation of the G6PD activity was monitored spectrophotometrically at 340 nm. As shown in Table 1, the presence of PDI in the folding mixture favoured the oxidative renaturation of G6PD increasing the in vitro rate of this event with respect the absence of PDI. As G6PD monomers do not show any activity, the refolding of the protein must be accompanied by the assembly of the monomers into dimers, allowing the detection of the G6PD activity. NADP, both in the presence and absence of PDI, increased the refolding rate, in agreement with previous studies on the unfolding of G6PD where NADP was shown to drive a conformational drift of the enzyme [5]. As illustrated in Figure 1, this effect