INDUCTION OF THE INACTIVATION AND DEGRADATION OF PHOSPHOENOLPYRUVATE CARBOXYLASE AND RIBULOSE-1,5-BISPHOSPHATE CARBOXYLASE OXYGENASE IN MAIZE LEAVES BY FREEZING AND THAWING/

When frozen leaves of 24-day-old maize (Zea mays L.) plant were thawed on moist filter paper at 26 degrees C (freeze-thaw treatment) several enzymes, including phosphoenolpyruvate carboxylase (PEPC) and ribulos e-1,5-bisphosphate carboxylase (RuBPC), were rapidly inactivated and d egraded. The kinetics of the inactivation and degradation were pseudo first-order, and the half-times for inactivation of PEPC and RuBPC wer e 3.2 and 2.4 min, respectively. The effect of the freeze-thaw treatme nt on the inactivation and degradation differed among various enzymes: the residual activities of RuBPC, PEPC, hydroxypyruvate reductase, Cy t c oxidase, NADP-malic enzyme and a-mannosidase 10 min after the star t of the thawing treatment were 7, 16, 54, 64, 97 and 98% of the initi al respective levels. Thirty min after the starting of thawing treatme nt, the amounts of total soluble protein, the large subunit of RuBPC, the small subunit of RuBPC, the PEPC subunit and the NADP-malic enzyme subunit had fallen to 61, 2, 16, 8, and 66% of the initial respective amounts. The effect of freeze-thaw treatment on PEPC was greater in o ld leaves than in young leaves. There was a steady increase of the rat e of degradation of PEPC by freeze-thaw treatment as plants aged from 6 to 24 days. These results are discussed in the context of protein de gradation in plant cells.