ALTERATION OF KINETIC PROPERTIES OF PYRUVATE KINASE IN PHASCOLOSOMA ARCUATUM (SIPUNCULID) UPON ANOXIC EXPOSURE

Abstract

The local sipunculid worm Phascolosoma arcuatum could survive for 48 h of environmental anoxia. During this period of anoxic exposure, the worm used glycogen, aspartate and possibly other amino acids in anaerobic energy metabolism, producing succinate and alanine as by products. Glycogen and aspartate were utilized during early anaerobiosis. When the anoxic exposure period was prolonged, the kinetic properties of one of the enzymes at the PEP-branch point P. arcuatum, pyruvate kinase (PK) was modified. The modified PK had a lower affinity for the substrate PEP and was inhibited to a greater extend by alanine compared to the unmodified aerobic form of PK, The other enzyme at the PEP-branch point of P. arcuatum, phosphoenolpyruvate carboxykinase (PEPCK) was however unchanged by anoxic exposure. Hence, the carbon flux at the PEP-branch point was directed to PEPCK upon extended periods of anoxic exposure, There were three distinct PK isozymes in P. arcuatum. Only the kinetic properties of the PK isozymes in the body wall and the proboscis cum attached retractor muscles were modified during prolonged anoxia, The known mechanism of PK alterations during anoxia is that via anoxia induced phosphorylation. In vitro dephosphorylation of the modified PK using alkaline phosphatase provided conclusive evidence that anoxia induced phosphorylation of PK indeed occurs in P. arcuaturn during prolonged anoxic exposure. The kinetic properties of glutamate dehydrogenase (GDH, deamination) was also altered during extended anaerobiosis in P. arcuatum. The enzyme's affinity for the substrate glutamate was higher compared to the GOH (deamination) in the aerobic worm. This would promote greater catabolism of glutamate during anoxia.