Gluconeogenesis in facultative anaerobic invertebrates: Evidence of oxalacetate decarboxylation and anaerobic end product incorporation into glycogen from the tissues ofTubifex sp.

Abstract

1. Phosphoenolpyruvate carboxykinase from four facultative anaerobic invertebrate species is shown to decarboxylate oxalacetate to phosphoenolpyruvate (Table 1). Knowledge of this reaction is of importance in the evaluation of gluconeogenic capacities of these organisms. 2. InTubifex the PEPCK-catalyzed decarboxylation reaction was characterized and its regulatory properties were studied. PEPCK mainly occurs in the cytosolic fraction and its absolute requirements in the decarboxylation direction are oxalacetate, Mn2+, nucleoside triphosphate (ITP) (Table 2). The pH optima for PEPCK-catalyzed decarboxylation and carboxylation reactions are 8.1 and 7.1, respectively (Fig. 1).Km values for oxalacetate and ITP were 0.25 and 0.39 mM, respectively (Fig. 2b and c). In presence of 0.5 mM AMP, the decarboxylation reaction was inhibited up to 50% of the control while even in presence of 5 mM AMP, the carboxylation reaction was unaffected (Fig. 3). Lombricine phosphate, one of the naturally occurring phosphagens inTubifex, activates the PEPCK-catalyzed decarboxylation reaction. 3-Mercaptopicolinic acid, a known specific inhibitor of vertebrate and invertebrate PEPCKs, also inhibitsTubifex PEPCK (Table 3). 3. UnlikeCrassostrea andAscaris, pyruvate carboxylase activity is present inTubifex tissues which will facilitate the conversion of C3 to C4 compounds and thus their eventual incorporation into glycogen. 4. Various gluconeogenic precursors when provided exogenously under aerobic conditions are incorporated into tissue glycogen by the intactTubifex (Table 4). 5. The potential roles of PEPCK regulatory properties under aerobic and anaerobic conditions are discussed with relation to gluconeogenesis and glycolysis inTubifex.