LIPID MOBILIZATION AND GLUCONEOGENESIS IN PLANTS - DO GLYOXYLATE CYCLE ENZYME-ACTIVITIES CONSTITUTE A REAL CYCLE - A HYPOTHESIS

Glyoxysomes are specialized peroxisomes present in various plant organ s such as germinating cotyledons or senescing leaves. They are the sit e of beta-oxidation and of the glyoxylate cycle. These consecutive pat hways are essential to the maintenance of gluconeogenesis initiated by the degradation of reserve or structural lipids. In contrast to mitoc hondrial beta-oxidation, which is prevalent in animal cells, glyoxysom al beta-oxidation and the glyoxylate cycle have no direct access to th e mitochondrial respiratory chain because of the impermeability of the glyoxysomal membrane to the reduced cofactors. The necessity of NAD() regeneration can conceivably be fulfilled by membrane redox chains a nd/or by transmembrane shuttles. Experimental evidence based on the ac tive metabolic roles of higher plant glyoxysomes and yeast peroxisomes suggests the coexistence of two mechanisms, namely a reductase/peroxi dase membrane redox chain and a malate/aspartate shuttle susceptible t o transfer electrons to the mitochondrial ATP generating system. Such a model interconnects beta-oxidation, the glyoxylate cycle, the respir atory chain and gluconeogenesis in such a way that glyoxysomal malate dehydrogenase is an essential and exclusive component of beta-oxidatio n (NAD(+) regeneration). Consequently, the classical view of the glyox ylate cycle is superseded by a tentative reactional scheme deprived of cyclic character.