INCREASES IN TISSUE AMINO-ACID LEVELS IN RESPONSE TO AMMONIA STRESS IN THE TERRESTRIAL ISOPOD PORCELLIO SCABER LATR

Terrestrial isopods excrete waste nitrogen by intermittent volatilizat ion of ammonia and must therefore accumulate amino groups in a non-tox ic form between excretory bouts. High-performance Liquid chromatograph y (HPLC) was used to analyze concentrations of 23 amino acids in the o niscidean Porcellio scaber following a 7-day exposure to elevated P-NH 3 (''ammonia stressing''). Analyses were performed on four separate ti ssues: hindgut, hepatopancreas, pleopodal endopods, and the body wall. Concentrations of all free amino acids except histidine and cysteine increased in ammonia-stressed animals. The greatest whole-animal conce ntrations (30-40 mu mol . g fw(-1)) were seen in glutamine and glycine , which also showed the largest increases (fourfold). Most glutamine a nd glycine is accumulated in the body wall and hepatopancreas with con centrations in the hepatopancreas reaching 400-500 mu mol . g fw(-1). Ammonia-stressed animals showed smaller but substantial accumulations of alanine, arginine, proline, and glutamate. Arginine, by virtue of i ts 3N R-group, constitutes the major nitrogen-storage compound, togeth er with glutamine. Calculations based on these and our previous data i ndicate that the amino acids constitute the primary form in which nitr ogen is accumulated under ammonia-stressing conditions. (C) 1996 Wiley -Liss, Inc.