GLYOXYLATE CYCLE ENZYME-ACTIVITIES ARE INDUCED IN SENESCENT PUMPKIN FRUITS

The presence of isocitrate lyase (EC 4.1.3.1) and malate synthase (EC 4.1.3.2) was investigated in detached pumpkin fruits (Cucurbita pepo L ., var. Alberello di Sarzana). After incubation of pumpkin fruit slice s in permanent darkness, the enzyme activities appeared after only 1 d ay and then declined to zero at day 6. Catalase (EC 1.11.1.6) specific activity increased during the first 2 days of incubation and decrease d thereafter. Hydroxypyruvate reductase (EC 1.1.1.81) and 3-hydroxyacy l CoA dehydrogenase (E.C. 1.1.1.35) specific activities changed very l ittle, but NADP(+)-dependent isocitrate dehydrogenase (E.C. 1.1.1.42) and Fumarase (E.C. 3.2.1.2) specific activities increased during the f irst day and declined thereafter. Sucrose density gradient fractionati on of cell organelles showed that isocitrate lyase-and malate synthase are localized in peroxisomal fractions. The presence of the two key e nzymes of the glyoxylate cycle was confirmed by immunoblolting, both i n crude extracts and in peroxisomal fractions. It is concluded that fo llowing detachment and dark incubation, a transition from peroxisomes to glyoxysomes occurs in pumpkin fruit.