Glyoxylate Cycle Enzymes in Peroxisomes Isolated from Petals of Pumpkin (Cucurbita sp.) during Senescence

The activities of the two unique enzymes of the glyoxylate cycle, isocitrate lyase (EC 4.1.3.1) and malate synthase (EC 4.1.3.2), were undetectable in petals of pumpkin (Cucurbita sp. Amakuri Nankin) until the end of blooming, but they appeared during senescence. The activity of catalase (EC 1.11.1.6) increased, glycolate oxidase (EC 1.1.3.1) activity did not change, while hydroxypyruvate reductase (EC 1.1.1.81) activity peaked at full blooming stage and declined thereafter. After fractionation of cellular organelles on a sucrose density gradient, we detected isocitrate lyase and malate synthase activities in peroxisomal fractions only from petals at the senescing stage. Northern blot analysis revealed that malate synthase mRNA increased during petal senescence. Citrate synthase (EC 4.1.3.7) and malate dehydrogenase (EC 1.1.1.37) activities were also present, while aconitase (EC 4.2.1.3) was not detectable in peroxisomal fractions. Moreover the presence of 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) and urate oxidase (EC 1.7.3.3) in the peroxisomal fractions from senescing petals indicates that peroxisomes could be involved both in the β-oxidation pathway and in the purine catabolism during petal senescence.