Abstract In the early steps of chlorophyll (Chl) degradation, the Mg-dechelation of chlorophyllide a (Chlide) to pheophorbide is known to be catalyzed by the so-called Mg-dechelatase. We previously demonstrated that the presence of a smaller metal-chelating substance (MCS) is required for the Mg-dechelation reaction using Chlide as the native substrate. This was further substantiated by this study in which Mg-dechelation activity in extracts from mature leaves of Chenopodium album was examined in complete fractions after gel filtration chromatography using an artificial substrate, Mg-chlorophyllin a (Chlin) and the native substrate, Chlide. A small-molecular-weight MCS and Mg-releasing proteins (MRPs) were present when Chlin was used as the substrate. However, only MCS had Mg-dechelation activity for the native substrate. Glutathione S-transferase (GST) was identified as one of the Mg-releasing proteins in addition to the previously known peroxidase (POD). Spontaneous release of Mg from Chlin was observed after the addition of a low concentration of hydrogen peroxide in the absence of MRPs, but not Chlide. These findings demonstrate that MCS plays a role in the catalysis of the Mg-dechelation reaction in the breakdown pathway of Chl. The release of Mg from Chlin by MRP may relate to the function of active oxygen species such as hydrogen peroxide. The relationship between Mg-release and the molecular plasticity of the artificial substrate compared to native substrate is discussed.