Evidences of glyoxylate cycle in peroxisomes of senescent cotyledons

Abstract

The metabolic pathway of the glyoxylate cycle has been investigated in peroxisomes isolated from senescent pumpkin ( Cucurbita sp.) cotyledons. β-oxidation activity, as well as activities of glyoxylate cycle enzymes isocitrate lyase (EC 4.1.3.1), malate synthase (EC 4.1.3.2), malate dehydrogenase (EC 1.1.1.37) and citrate synthase (EC 4.1.3.7) were detected. In order to establish if there is a channelling of acetyl CoA into the glyoxylate cycle, peroxisomes have been incubated with various substrates. The incubations show acetyl CoA utilization by the glyoxylate cycle. When the incubation medium for citrate formation is used, all the label from [1- 14C]acetyl CoA is recovered in citrate, whereas only 18% of the added radioactivity is recovered in malic acid (by isocitrate lyase and malate synthase) after a long incubation time (3 h). Only by feeding peroxisomes with [1,5- 14C]citric acid and exogenous aconitase (EC 4.2.1.3) can a weak formation of other organic acids (glyoxylate and succinate) be note. The requirement for exogenous aconitase to carry out the peroxisomal glyoxylate cycle points towards the isocitric acid step as a crucial factor for the operation of the global cycle.