This is the inner layer of the skin called Dermis and the outer surface which is visible in our eyes is the Epidermis. The primary determinant of variability in human skin color is the Melanin and Melanocyte is a cell that synthesizes it. L-glutathione,, starts the lightening process in the Dermis working its way out to the surface.
This is the point where the metabolic pathway of Melanin begins. As you see in the diagram GSH(Reduced Glutathione) intervenes the metabolism. There are certain kinds of compounds that inhibits the Tyrosinase Activity and L-glutathione is one of them.
The oral use of Reduced Glutathione(L-glutathione) inhibits the Tyrosinase Activity. Once it has been inhibited, the metabolic pathway then reverses its course going to the synthesis of light pigmented melanin called Phaeomelanin instead of synthesizing dark pigmented melanin called Eumelanin. The cycle continously flow as long as the presence of L-glutathione co-exist in the metabolic pathway of melanin. Finally, light skin will reveal to the surface gradually changing your complexion.