Glutathione reductase is an enzyme found in red blood cells (and most other cells) that keeps the antioxidant glutathione in its active, reduced form. By recycling oxidised glutathione back to its working state using NADPH, the enzyme maintains the cell’s defences against oxidative damage — a vital role in red blood cells, which carry oxygen and are constantly exposed to oxidative stress. Its activity depends on riboflavin (vitamin B2), which supplies the FAD cofactor: measuring glutathione reductase activation in red cells is actually a functional test of riboflavin status. When the enzyme falters, haemoglobin oxidises and red cells become fragile — one reason riboflavin deficiency can contribute to anaemia.