Stephen H. Bishop

NITROGEN METABOLISM AND EXCRETION: REGULATION OF INTRACELLULAR AMINO ACID CONCENTRATIONS

ABSTRACT Amino acids and taurine are used by osmoconforming and osmo-regulating estuarine invertebrate animals to control the intracellular osmotic pressure. These amino acids are readily derived from intermediates of glycolysis and the tricarboxylic acid cycle. Isolated molluscan hearts adapted to low and high salinities in vitro show adaptive changes in amino acid levels which correlate with changes in mechanical properties. Transaminases are at high levels in all tissues of all organisms studied. High levels of the primary ammonia forming-fixing enzyme (GDH) have been found only in crustaceans. Ammonia and small amounts of free amino acids are the major nitrogen excretory products. These are released through the gills, body wall, and gut. Animals adapted to low salinities show increased ammonia production. In animals with low GDH activities (flatworms, molluscs, annelids), the serine cycle and purine nucleotide cycle have been suggested as primary ammonia forming mechanisms. The enzyme responsible for ammonia fixation as these animals adapt to high salinity has not been described.

Ammonia forming mechanisms: Deamination of 5′-adenylic acid (AMP) by some polychaete annelids

Abstract 1. 1. AMP∗ and adenosine are deaminated by the polychaete annelids Abarenicola pacifica, Abarenicola vagabunda, Amphitrite robusta, Nereis branti and Schizobranchia insignis. 2. 2. [14C]-AMP is converted to [14C]-IMP by gut homogenates of all these species. 3. 3. The AMP deaminase (E.C. 3.5.4.6) in these species is active at high AMP concentrations (5 mM), is strongly activated by ADP or ATP (2· mM) and is more active at pH 8 than at pH 6–6·5. 4. 4. Under the conditions of assay, the ATP activated AMP deaminase is two to five times greater than the adenosine deaminase (E.C. 3.5.4.4) activity. 5. 5. A physiological role for this AMP deaminase is postulated.

Ornithine transcarbamylase: steady-state kinetic properties.

Abstract A steady-state kinetic study of Streptococcus faecalis ornithine transcarbamylase has been carried out. A ping-pong kinetic pattern was observed. Phosphate acted as a competitive inhibitor of carbamyl phosphate, δ-hydroxy-α-aminovaleric acid and norvaline were competitive inhibitors of ornithine. Citrulline did not inhibit the reaction even at a 0.1 M concentration level. The formation of a carbamyl-enzyme intermediate is suggested as a likely possibility.

Phosphonoglycoprotein from Metridium senile—heterogeneity of glycoproteins containing aminoethylphosphonic acid☆

1. After separation by SDS gel-chromatography, analysis of AEP-containing glycoproteins from M. senile, indicated 66% amino acids with 220 AEP res./1000 res. and 30% carbohydrate for high mol. wt (greater than 10(7) forms and 80% amino acids with 25-50 AEP res./1000 res. and 10% carbohydrate for low mol. wt (2-4 x 10(4) forms. 2. Uronic acids, sulfate, lipid, and sialic acids were absent. 3. Mild base digestion released AEP-hexosamine containing oligosaccharides and destroyed ser-thr residues in the high mol. wt components. 4. Phosphonoglycoproteins appear to be acidic connective tissue components with AEP linked to hexosamine containing oligosaccharide side chains.

Adenosine deaminase from Metridium senile (L.), a sea anemone

Abstract 1. 1. The enzyme adenosine deaminase (E.C. 3.5.4.4) was purified from a sea anemone, Metridium senile . 2. 2. The substrate specificity and kinetic properties were similar to the calf intestine enzyme. 3. 3. The apparent molecular weight by Sephadex gel filtration was 70,000 daltons. 4. 4. No aminophosphonic acids were detected as constituent amino acids in this protein.