Mouse embryonic stem (ES) glucose-6-phosphate (G6P) dehydrogenase-deleted cells ( G6pd delta), obtained by transient Cre recombinase expression in a G6pd -loxed cell line, are unable to produce G6P dehydrogenase (G6PD) protein (EC 1.1.1.42). These G6pd delta cells proliferate in vitro without special requirements but are extremely sensitive to oxidative stress. Under normal growth conditions, ES G6pd delta cells show a high ratio of NADPH to NADP(+) and a normal intracellular level of GSH. In the presence of the thiol scavenger oxidant, azodicarboxylic acid bis[dimethylamide], at concentrations lethal for G6pd delta but not for wild-type ES cells, NADPH and GSH in G6pd delta cells dramatically shift to their oxidized forms. In contrast, wild-type ES cells are able to increase rapidly and intensely the activity of the pentose-phosphate pathway in response to the oxidant. This process, mediated by the [NADPH]/[NADP(+)] ratio, does not occur in G6pd delta cells. G6PD has been generally considered essential for providing NADPH-reducing power. We now find that other reactions provide the cell with a large fraction of NADPH under non-stress conditions, whereas G6PD is the only NADPH-producing enzyme activated in response to oxidative stress, which can act as a guardian of the cell redox potential. Moreover, bacterial G6PD can substitute for the human enzyme, strongly suggesting that a relatively simple mechanism of enzyme kinetics underlies this phenomenon.

Glucose-6-phosphate dehydrogenase-deleted embryonic stem (ES) cells (G6pdD) proliferate in vitro without special requirements, but when challenged with oxidants fail to sustain glutathione disulphide reconversion to reduced glutathione (GSH), entering a condition of oxidative stress. Here, we investigate the signalling events downstream of GSH oxidation in G6pdD and wild-type (wt) ES cells. We found that G6pdD ES cells are very sensitive to oxidants, activating an apoptotic pathway at oxidant concentrations otherwise sublethal for wt ES cells. We show that the apoptotic pathway activated by low oxidant concentrations is accompanied by mitochondria dysfunction, and it is therefore blocked by the overexpression of Bcl-X L . Bcl-X L does not inhibit the decrease in cellular GSH and reactive oxygen species formation following oxidant treatment. We also found that oxidant treatment in ES cells is followed by the activation of the MEK/ extracellular signal-regulated kinase (ERK) pathway. Interestingly, ERK activation has opposite outcomes in G6pdD ES cells compared to wt, which has a proapoptotic function in the first and a prosurvival function in the latter. We show that this phenomenon can be regulated by the cellular GSH level.

Haptoglobin (Hpt) was previously found to bind the high density lipoprotein (HDL) apolipoprotein A-I (ApoA-I) and able to inhibit the ApoA-I-dependent activity of the enzyme lecithin:cholesterol acyltransferase (LCAT), which plays a major role in the reverse cholesterol transport. The ApoA-I structure was analyzed to detect the site bound by Hpt. ApoA-I was treated by cyanogen bromide or hydroxylamine; the resulting frag