Both the reduced glutathione (GSH) molecules in the erythrocytes and the sulfhydryl groups in the erythrocyte membrane proteins can react with 5,5'-dithiol-di-nitrobenzoic acid (DTNB), and the resulting compounds have a maximum absorption peak at 412 nm. The contents of GSH and sulfhydryl groups in membrane proteins can be measured via visible spectrophotometry (Aksenov and Markesbery, 2001 (link)).
Free full text: Click here