Glutathione reductase, together with ubiquinol 2, catalyzes the reduction of glutathione disulfide (GSSG) to give glutathione (GSH) product. The co-factor of this enzyme is flavin adenine dinucleotide. Thus lack of FAD caused by riboflavin shortage may lower the activity of glutathione reductase. The GRAC may be used to evaluate the nutrition level of riboflavin in vivo.