Hysteresis and Reversible Cold Inactivation of Maize Phosphoenolpyruvate Carboxylase

Maize (Z ea mays L.) leaf phosphoenolpyruvate (PEP) carboxylase (PEPCase) (EC 4.1.1.31) showed a lag in activity when assayed after storage at 0 4 °C. The lag was prom oted by high pH on storage (7.8 8 .5) and was observed over a range o f assay pH (7.1 -8 .5 ). Therm al reacti­ vation o f the cold-stored enzyme by assay tem perature (18 °C) accounted for most o f the hysteretic effect, but presence o f PEP in the reaction m ixture was required to completely eliminate the lag. Based on steady-state rates after the lag, stability o f PEPCase in the cold was inde­ pendent o f protein concentration. It is suggested that low tem perature and high pH induce a change in the oligom erization state o f PEPCase, resulting in a less active but relatively stable form o f the enzyme. The lag probably reflects a reversal o f this process, prom oted by assay tem perature and presence o f PEP.