Cold labiUlty of pyruvate, orthopbosphate dikinase was investigated using a homogeneous, purified enzyme preparation from maize (Zea mays L. var. Golden Cross Bantam T51) leaves. Its stability was markedly reduced below about 10 C and the rate of cold inactivation folowed first order kinetics at a concentration lower than about 0.1 miligram of enzyme per milliliter. Cold inactivation was little affected by pH in the range which gives good stabilty for the enzyme at warm temperatures and the enzyme activity was protected strongly by inclusion of substrates (pyruvate and phosphoenolpyruvate) and polyols such as sucrose, sorbitol, and glycerol.Loss of catalytic activity was accompanied by an apparent dissociation of a tetrameric form of the enzyme (9S form) into a new, more slowly sedimenting (5.1S) component. Inclusion of pyruvate at 4 mM in the coldtreated enzyme had no effect on the sedimentation value. A sharp change in activation energy of the dkinase-catalyzed reaction was observed near 12 C and its break point appears to be close to the generally accepted critical low temperature Himit for the growth of maize plants.An unusual inactivation by cold temperatures of pyruvate, Pi dikinase (EC 2.7.9.1) from C4 plants has received considerable attention since the initial report by Hatch and Slack (1). The property of cold lability of the dikinase was confirmed and its reversibility was established in our laboratory using the enzyme from maize leaf purified to homogeneity (11). Our previous data indicated a possible correlation between the cold response of this enzyme and thermal response in maize cultivars (12). Such a correlation supports the view that the enzyme may be a ratelimiting step in photosynthesis in C4 plants at cool temperature, as has been mentioned by Taylor et al. (13).In this paper we investigated the property of cold lability of the maize leaf dikinase and discuss its relation to photosynthetic CO2 fixation at low temperature. This is an attempt to describe more fully the cold-labile nature of the enzyme in view of its possible physiological meaning. MATERIALS AND METHODS PLANT MATERIALSMaize plants (Zea mays L. var. Golden Cross Bantam T5 1) were grown for 2 to 5 weeks in a greenhouse under natural light conditions at 15 to 35 C.
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