Abstract
Variations in the high concentrations of intracellular amino acids in the tissues of osmoconforming, euryhaline animals such as the ribbed mussel (Modiolus demissus) appear to have a major role in adjusting intracellular osmotic pressure in response to extracellular osmotic change. The experiments discussed here on the metabolic origins and fates of these amino acids are part of a continuing study on the physiological mechanisms regulating the concentrations of these amino acids. Tissues isolated from ribbed mussels adapted at 12 o/oo were incubated for 8 hrs. in artificial seawater media at 12 o/oo or 32 o/oo. The total marked increase in total intracellular amino acid levels at 32 o/oo vs. 12 o/oo was not blocked by addition of 1 mM amino oxyacetic acid (AOA). However, with AOA the usual increase in alanine and proline concentrations was offset by increases in a variety of amino acids including ornithine. AOA inhibited transaminases associated with ornithine, alanine, and glutamate metabolism (I~50~s:10^−8^ to 10^−5^ M) but did not inhibit the L‐amino acid oxidase. ^14^C‐tracer experiments with isolated gill tissue indicated no synthesis of proline from glutamate and enhanced synthesis of proline, alanine, and glutamate from ornithine and arginine at 32 o/oo vs. 12 o/oo. L‐amino acid oxidase, proline oxidase, glutamate dehydrogenase, and enzymes in the metabolic pathway between arginine and proline were found in cell‐free extracts of gill tissue. During hyperosmotic stress, proline was apparently derived from the phosphoarginine‐arginine pool, whereas alanine and the other accumlating amino acids have multiple metabolic origins including the amino acids released during protein turnover, the metabolites of the phosphoarginine pool, and possibly endogenous carbohydrate.