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 organs such as germinating cotyledons or senescing leaves. They are the site of beta-oxidation and of the glyoxylate cycle. These consecutive pathways are essential to the maintenance of gluconeogenesis initiated by the degradation of reserve or structural lipids. In contrast to mitochondrial beta-oxidation, which is prevalent in animal cells, glyoxysomal beta-oxidation and the glyoxylate cycle have no direct access to the 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 and/or by transmembrane shuttles. Experimental evidence based on the active metabolic roles of higher plant glyoxysomes and yeast peroxisomes suggests the coexistence of two mechanisms, namely a reductase/peroxidase membrane redox chain and a malate/aspartate shuttle susceptible to transfer electrons to the mitochondrial ATP generating system. Such a model interconnects beta-oxidation, the glyoxylate cycle, the respiratory chain and gluconeogenesis in such a way that glyoxysomal malate dehydrogenase is an essential and exclusive component of beta-oxidation (NAD+ regeneration). Consequently, the classical view of the glyoxylate cycle is superseded by a tentative reactional scheme deprived of cyclic character.
Escher
C L
CL
Institute of Plant Biology and Physiology, University of Lausanne, Switzerland.
Widmer
F
F
D005943:Gluconeogenesis / Q000502:physiology*; D006038:Glyoxylates / Q000378:metabolism*; D050356:Lipid Metabolism; D018521:Plant Physiological Phenomena; D010944:Plants / Q000201:enzymology* / Q000378:metabolism*
D006038:Glyoxylates