Kinetic characterization of NADP-specific glutamate dehydrogenase from the sea anemone, anthopleura xanthogrammica: Control of amino acid biosynthesis during osmotic stress

Light induces changes in activities of Na(+)/K(+)-ATPase, H(+)/K(+)-ATPase and glutamine synthetase in tissues involved directly or indirectly in light-enhanced calcification in the giant clam, Tridacna squamosa

The objective of this study was to determine the effects of 12 h of exposure to light, as compared with 12 h of exposure to darkness (control), on enzymatic activities of transporters involved in the transport of NH(+) 4 or H(+), and activities of enzymes involved in converting NH(+) 4 to glutamate/glutamine in inner mantle, outer mantle, and ctenidia of the giant clam, Tridacna squamosa. Exposure to light resulted in a significant increase in the effectiveness of NH(+) 4 in substitution for K(+) to activate Na(+)/K(+)-ATPase (NKA), manifested as a significant increase in the Na(+)/NH(+) 4-activated-NKA activity in the inner mantle. However, similar phenomena were not observed in the extensible outer mantle, which contained abundant symbiotic zooxanthellae. Hence, during light-enhanced calcification, H(+) released from CaCO3 deposition could react with NH3 to form NH(+) 4 in the extrapallial fluid, and NH(+) 4 could probably be transported into the shell-facing inner mantle epithelium through NKA. Light also induced an increase in the activity of glutamine synthetase, which converts NH(+) 4 and glutamate to glutamine, in the inner mantle. Taken together, these results explained observations reported elsewhere that light induced a significant increase in pH and a significant decrease in ammonia concentration in the extrapallial fluid, as well as a significant increase in the glutamine concentration in the inner mantle, of T. squamosa. Exposure of T. squamosa to light also led to a significant decrease in the N-ethylmaleimide (NEM)-sensitive-V-H(+)-ATPase (VATPase) in the inner mantle, and significant increases in the Na(+)/K(+)-activated-NKA, H(+)/NH(+) 4-activated-H(+)/K(+)-ATPase, and NEM-sensitive-VATPase activities in ctenidia, indicating that light-enhanced calcification might perturb Na(+) homeostasis and acid/base balance in the hemolymph, and might involve the active uptake of NH(+) 4 from the environment. This is the first report on light having direct enhancing effects on activities of certain transporters/enzymes related to light-enhanced calcification in the inner mantle and ctenidia of T. squamosa.

Inorganic nitrogen uptake by symbiotic marine cnidarians: a critical review

The physiological data support host involvement in net ammonium uptake by intact symbioses. The evidence for nitrate assimilation by intact symbioses is equivocal. The depletion-diffusion model can account for net ammonium uptake by intact symbioses, but is inadequate to account for phosphate or nitrate uptake by symbioses. There is no evidence for nitrogen limitation as the means by which the host regulates algal growth in symbiosis; phosphorus limitation appears to be more likely.

Localization of two L-glutamate dehydrogenases in the coral Acropora latistella

The coral Acropora latistella has been shown to contain two distinct L-glutamate dehydrogenases. An NADPH-specific GDH, closely resembling the enzyme from other coelenterates, is located in the cytosol; a distinct NADH-specific GDH is localized within the mitochondria. This is in contrast to earlier reports, where the NADPH-specific enzyme was thought to be the only GDH present and was assumed to be mitochondrial. Preliminary kinetic and stability data are presented for the two GDHs.