6.3 Protein and Amino Acid Metabolism
6.3.3. Pools of Free Amino Acids
The function of free amino acids in aquatic animal behavior, communica- tion, and metabolism begins in their sensory organs, where amino acids serve as chemical signals (Saglioet al.,1990). For instance, proline was the most effective gustatory amino acid in rainbow trout at an estimated threshold of 10−7M (Kohbara and Caprio, 2001). In marine fish, particularly during em- bryonic stages with endogenous nutrition, amino acids provide stability in body fluid osmolality and serve as substrates for protein synthesis and/or aerobic catabolism (Ronnestad and Fyhn, 1993; Ronnestad et al., 1993;
Sivaloganathan et al., 1998). Extraoral utilization of protein and amino
acids in fish embryos through endocytosis and cellular proteases activity is markedly different from that found during exogenous feeding, and there- fore, comparisons must be drawn with caution. Conceicaoet al. (1998) quan- tified differences in amino acid contents between the time of fertilization and the time of complete yolk protein absorption and suggested a retention efficiency of 50–80% for essential amino acids, with the free amino acid pool being quantitatively unimportant (not exceeding 5%).
Body proteins cannot be stored in major quantities and are continu- ously renewed through degradation and synthesis. The free amino acid pool changes in its profile (composition) and concentrations depending on the tissue (Carteret al.,1994), frequency and time after feeding (Tantikitti and March, 1995), temperature and food (Knapp and Wiser, 1981), and salinity (Dabrowskiet al.,1996; Auerswaldet al.,1997).
Free amino acid concentrations have frequently been used to monitor the postprandial response in fish, however, the relative distribution between plasma and red blood cells was not measured. It became evident that essen- tial amino acids tend to be concentrated in the plasma compartment (more than 55%), with some, such as lysine, showing a significant decrease in con- centration during the postabsorptive stage (Fig. 6.14). Nonessential amino acids tended to be concentrated in the red cell compartment of the blood.
However, aspartic acid has shown an increase in blood plasma partitioning from 10 to 85% in the postabsorptive stage. Therefore, these observations indicate the significance of separating plasma and cellular free amino acids in the circulation for transport and metabolic purposes.
Differences in amino acid availability are reflected in the plasma amino acid concentrations (Tantikitti and March, 1995), however, feeding at 3- to 6-hr intervals significantly eliminates fluctuations. Maximum plasma con- centrations for essential amino acids were obtained in, for instance, in salmonids, between 4 and 24 hr, with dietary proteins as varied as fish meals, casein, corn gluten, and soybeans (Walton and Wilson, 1986). Be- sides the effect of diet, the location of blood sampling has a profound effect on amino acid concentrations in blood plasma (Table 6.1). Schuhmacher et al. (1997) emphasized the nutritional history and duration of fasting prior to an experimental meal to follow postfeeding changes in plasma free amino acids. Plasma concentrations attained their peak at 9 hr postfeeding for most essential amino acids in trout fed a synthetic amino acid diet, earlier than the 12–18 hr in fish fed wheat gluten as the intact protein source (Table 6.1).
The best correlation between essential free amino acids in the hepatopan- creas or blood plasma and dietary amino acids occurred 4 hr after feeding (r=0.914 and 0.896, respectively) (Ogata, 1986). There was no correlation between dietary levels and concentrations of free amino acids in the liver
FIG. 6.14
Postprandial changes in the blood plasma free amino acid levels expressed as a percentage of the whole-blood levels (Dabrowski, 1982).
of rainbow trout (Muraiet al.,1987). On the contrary, in rainbow trout the best correlation(r=0.849) between the total free amino acid concentra- tions in the liver and the dietary consumption of amino acids was found 4 hr after a meal (Carteret al.,1994). However, essential free amino acids in the liver did not correlate at all. The authors calculated that over 83%
of amino acids were recycled to the general pool after protein breakdown,
therefore in starved rainbow trout only a portion of the free amino acid pool (53%) is used for protein synthesis. Someone may argue that a single meal after 7 days of fasting in rainbow trout is probably weak evidence of a free amino acid pool in fish which otherwise feed actively. Thus, it seems hazardous to conclude that there is a “consistency of free amino acid con- centrations in tissues following feeding” (Carteret al.,1994). It appears of great interest to find out how “the anabolic drive,” an increase in protein synthesis following an influx of absorbed free amino acids, is regulated in light of a possible “shutting-off effect” which may occur with the “flooding”
of orally (or injected) administered free amino acids.