GSH is synthesized from its 3 amino acids with a biosynthetic pathway shown in Figure 4 . A study in humans, using radioisotopes, 3 has demonstrated that the availability of cysteine,and its precursor methionine, is the rate-limiting factor in GSH synthesis. In general, it is assumed that the two main limiting factors are the levels of cysteine and of the enzyme gamma-glutamyl-cysteine synthetase (also known as glutamate-cysteine ligase). 4