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

Abstract

A DP-glucose pyrophosphorylase (AGP) from barley (Hordeum vulgare L.) seed endosperm showed a lag in activity when assayed after storage at - 20 °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 m M MgCl2 was added to the storage buffer before freezing. Storage at - 20°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 Km values with ATP of 0.3 and 1.2 m M (with and without PGA, respectively). In contrast, storage at - 20°C in the presence of 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 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. © 1993, Walter de Gruyter. All rights reserved.