Hysteresis and Reversible Cold-Inactivation of ADP-Glucose Pyrophosphorylase From Barley Seeds

ADP-glucose pyrophosphorylase (AGP) from barley (Hordeum vulgare L.) seed endosperm showed a lag in activity when assayed after storage at 2 0 °C. The cold-stored enzyme could regain most, or all, o f its activity during 4 0 -6 0 min following exposure to ambient tempera­ tures. The lags were not observed when 2 m M MgCl2 was added to the storage buffer before freezing. Storage at -2 0 °C, in the absence o f MgCl2, led to the appearance o f a low activity AGP form which was activated up to 3-fold by 3-phosphoglycerate (PGA) and had high K m values with ATP of 0.3 and 1.2 m M (with and without PGA, respectively). In contrast, storage at -2 0 °C in the presence o f M gCl2 or incubation at +20 °C resulted in an active enzyme which was only weakly activated by PGÄ (up to 30%) and had the respective K m values with ATP o f 0.1 and 0.3 m M . It is suggested that low temperature may induce a change in the conformation and/or oligomerization state o f the AGP protein, resulting in a low activity enzyme form which has distinct regulatory and kinetic properties.