The role of dehydroascorbate in disulfide bond formation

Area of Science:

• Biochemistry
• Cellular Biology
• Oxidative Stress

Background:

• Ascorbate (Vitamin C) acts as an intracellular antioxidant, forming dehydroascorbate (DHA).
• The recycling of DHA back to ascorbate is crucial for maintaining antioxidant capacity.
• Protein disulfide isomerase (PDI) has been implicated in DHA reduction, linking ascorbate metabolism to protein folding in the endoplasmic reticulum (ER).

Purpose of the Study:

• To investigate the role of PDI as a dehydroascorbate reductase.
• To compare the efficiency of PDI with other potential DHA reductases.
• To explore non-enzymatic reduction of DHA and identify potential inhibitors.

Main Methods:

• Enzymatic assays measuring the rate of DHA reduction by PDI and other thioredoxin-superfamily members.
• Kinetic analysis to determine rate constants for DHA reduction.
• Investigation of DHA reactivity with dithiol systems, including unfolded proteins.
• In vitro inhibition assays using borate.

Main Results:

• Protein disulfide isomerase (PDI) exhibits a low rate of dehydroascorbate (DHA) reduction (12.5 M(-1)s(-1)), suggesting it is not the primary reductase in the ER.
• Glutaredoxin demonstrates more rapid DHA reduction via a monothiol mechanism.
• DHA reacts rapidly with unfolded proteins and other dithiol systems (up to 186 M(-1)s(-1)) independently of PDI.
• Borate was identified as a potent inhibitor of both catalyzed and non-catalyzed DHA reduction.

Conclusions:

• PDI's slow reaction rate indicates it is unlikely to be the major dehydroascorbate reductase in the endoplasmic reticulum.
• Ascorbate metabolism is linked to oxidative protein folding through rapid, non-enzymatic reactions of DHA with cellular dithiols.
• Borate represents a novel inhibitor of DHA reduction, suggesting a potential link between borate toxicity and ascorbate metabolism.