On the activation of two chlorplastic phosphatases by fructose bisphosphate, sedoheptulose bisphosphate and magnesium

Fructose bisphosphatase and the ‘alternate’ phosphatase, when purified to homogeneity from spinach chloroplasts, are only slightly activated upon pre-incubation with Mg²⁺, fructose bisphosphate or sedoheptulose bisphosphate, as well as by a mixture of the divalent cation and either of these sugar phosphates. This activating effect is much smaller than that produced by thioredoxin f_B and becomes null if thioredoxin f_B is present. It is therefore very unlikely that fructose bisphosphate, sedoheptulose bisphosphate or Mg²⁺ may play a significant role in the light activation of the two phosphatases.

Magnesium slowly deactivates the active fructose bisphosphatase but slightly enhances the activity of the already activated ‘alternate’ phosphatase. Owing to their slowness, these processes are likely not to play any significant biological role.