passages.id: split_6_test_633_passage
passages.type: progene_text
passages.text: The rate - limiting step in the de novo synthesis of the cellular protectant glutathione is catalyzed by gamma-glutamylcysteine synthetase ( GCS ; also known as glutamine-L-cysteine ligase , GLCL ) , a heterodimer consisting of catalytic ( GCS ( h ) ) and regulatory ( GCS(l) ) subunits .