AMMONIA PRODUCTION AND KINETIC-PROPERTIES OF GLUTAMATE-DEHYDROGENASE IN THE SIPUNCULID PHASCOLOSOMA-ARCUATUM EXPOSED TO ANOXIA

The amounts of total NH4+ detected in the external media in which Phas colosoma arcuatum had been exposed to various periods of anoxia were s ignificantly greater than those in which the worms were exposed to nor moxia for a similar period. The increased NH4+ production by P. arcuat um during anoxic exposure was unlikely to be due to an increased catab olism of adenine nucleotides or urea. In contrast, there were signific ant decreases in the concentrations of several free amino acids in the coelomic plasma and body tissues of individuals during the 48 h of an oxic exposure. The amount of NH4+ produced by the anoxic P. arcuatum c ould be accounted for by the decreases in the concentrations of aspart ate or glycine. Increases in the catabolism of free amino acids (FAA), leading to the increased production of NH4+, in P. arcuatum during an oxia were supported by the detection of significant changes in the kin etic properties of glutamate dehydrogenase (GDH), in the deaminating d irection, from worms exposed to anoxia for 48 h. The apparent increase in the affinity of GDH from the anoxic worm to glutamate would bring about a greater deaminating activity at physiological concentrations o f this substrate. P. arcuatum used in these experiments were collected from the mangrove swamp at Mandai, Singapore between 1990 and 1993.