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 slices in permanent darkness, the enzyme activities appeared after only 1 day 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 decreased thereafter. Hydroxypyruvate reductase (EC 1.1.1.81) and 3-hydroxyacyl CoA dehydrogenase (E.C. 1.1.1.35) specific activities changed very little, but NADP(+)-dependent isocitrate dehydrogenase (E.C. 1.1.1.42) and Fumarase (E.C. 3.2.1.2) specific activities increased during the first day and declined thereafter. Sucrose density gradient fractionation of cell organelles showed that isocitrate lyase-and malate synthase are localized in peroxisomal fractions. The presence of the two key enzymes of the glyoxylate cycle was confirmed by immunoblolting, both in crude extracts and in peroxisomal fractions. It is concluded that following detachment and dark incubation, a transition from peroxisomes to glyoxysomes occurs in pumpkin fruit.