REVERSIBLE PHOSPHORYLATION OF FRUCTOSE 1,6-BISPHOSPHATASE MEDIATES ENZYME ROLE IN GLYCEROL METABOLISM IN THE FREEZE-AVOIDING GALL MOTH EPIBLEMA-SCUDDERIANA

Fructose-1,6-bisphosphatse (FBPase) from larvae of the freeze-avoiding gall moth Epiblema scudderiana occurs in two forms, which are interco nverted by reversible phosphorylation and separable by CM-cellulose co lumn chromatography, The phosphoenzyme has properties that would make it the more active form in vivo, Compared with the dephosphorylated fo rm, the phosphoenzyme had three-fold lower values for K-m fructose-1,6 -bisphosphate and K-a Mg2+ and lower sensitivities to allosteric inhib itors (I-50 values for fructose-2,6-bisphosphate and AMP were 50% and 10-fold higher, respectively), The proportions of the two enzyme forms in the larvae changed with the seasons and with acclimation to warm ( 15 degrees C) vs cold (4 degrees C) temperatures, The phosphorylated e nzyme predominated (70% of total activity) in early autumn and during the spring, as well as in warm acclimated larvae, all situations where gluconeogenesis via FBPase would be favoured, During the autumn cold- hardening period when the larvae are actively synthesizing the antifre eze, glycerol, the ratio of the two enzyme forms changed to about 50:5 0, This, plus allosteric inhibition and low temperature effects on enz yme kinetics, would effectively suppress FBPase activity and prevent f utile recycling of glycerol carbon back into glycogen during the winte r months when the 2 M pool of polyol must be sustained for antifreeze protection, Acclimation studies suggested that low temperature itself might be the signal that triggers enzyme dephosphorylation, and this c ould integrate control over FBPase with the well-known phosphorylation -mediated activation of glycogen phosphorylase by low temperature in c old-hardy insects, (C) 1997 Elsevier Science Ltd.