β-oxidation of fatty acids is linked to the glyoxylate cycle in the aleurone but not in the embryo of germinating barley

Abstract

An assay for the measurement of overall β-oxidation was optimized for use with extracts from germinating barley. The most important adaptation is the addition of commercially available acyl-CoA synthetase to the incubation medium in order to reactivate the palmitate formed by the high thioesterase activity of the extracts. The β-oxidation activity determined by the overall assay originated exclusively from microbodies, since inhibitors of the electron-transport chain and alternative respiration in mitochondria did not affect the total β-oxidation activity. Activity patterns of β-oxidation upon germination were determined both for barley aleurone and embryo. In the aleurone, β-oxidation activity was already present in the quiescent grain and increased during germination, reaching an optimum 4 days after germination. In addition, in the embryo β-oxidation activity could be demonstrated in the quiescent grain, and it increased upon germination. Within the growing embryo, β-oxidation activity was present in leaves, roots and scutellum. The maximal activity levels reached in the various embryo parts were significantly lower than those in the aleurone. Organelle separation by density gradient centrifugation of homogenates from scutellum suggests that, in this tissue, microbodies constitute the only subcellular compartment carrying out β-oxidation. In the aleurone, the developmental activity profiles of glyoxylate cycle enzymes closely resemble those of β-oxidation indicating that, in this tissue, β-oxidation is localized in so-called glyoxysomes. Inability to detect glyoxylate cycle enzyme activities in the embryo indicates different metabolic functions of the β-oxidation route in aleurone and embryo.