Kleczkowski LA, Villand P, Olsen O-A 
Hysteresis and reversible cold inactivation of ADP-glucose pyrophosphorylase from barley seed endosperm.
Zeitschrift für Naturforschung: 1993  48c:457-460

Abstract:
ADP-glucose pyrophosphorylase (AGP) from barley (Hordeum vulgare L.) seed endosperm showed a lag in activity when assayed after storage at -20-degrees-C. The cold-stored enzyme could regain most, or all, of its activity during 40-60 min following exposure to ambient temperatures. The lags were not observed when 2 mm MgCl2 was added to the storage buffer before freezing. Storage at -20-degrees-C, in the absence of MgCl2, led to the appearance of 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 mm (with and without PGA, respectively). In contrast, storage at - 20-degrees-C in the presence of MgCl2 or incubation at + 20-degrees-C resulted in an active enzyme which was only weakly activated by PGA (up to 30%) and had the respective K(m) values with ATP of 0.1 and 0.3 mm. It is suggested that low temperature may induce a change in the conformation and/or oligomerization state of the AGP protein, resulting in a low activity enzyme form which has distinct regulatory and kinetic properties.