CATABOLISM OF BRANCHED-CHAIN AMINO ACIDS IN TISSUES OF HYBRID STRIPED BASS Morone chrysops × M. saxatilis

Glutamate in diets is extensively degraded by the small intestine of fish. However, it is among the most abundant amino acids in the body and is likely synthesized by various tissues of fish. The present study was conducted with hybrid striped bass (HSB) to test the hypothesis that leucine (LEU), isoleucine (ILE) and valine (VAL) are degraded in these tissues to generate glutamate. Slices of 10 tissues (Table 1) were obtained from juvenile HSB and incubated at 26^oC for 2 h in oxygenated (95% O₂/5% CO₂) Krebs-Henseleit bicarbonate buffer (pH 7.4, 5 mM D-glucose) containing for 2 mM [1-¹⁴C]leucine, [1-¹⁴C]isoleucine, or [1-¹⁴C]valine. Production of ¹⁴CO₂ and each branched-chain ketoacids (BCKAs) was determined with the use of our established methods. Furthermore, activities of branched-chain amino acid (BCAA) transaminase and BCKA dehydrogenase in HSB tissues were determined to provide bases for understanding BCAA catabolism.

All the fish tissues actively transaminate leucine, isoleucine and valine with α-ketoglutarate to form glutamate and BCKAs, with the highest rate (nmol/g tissue tissue) in the kidney. Data on the activities of BCAA transaminase and BCKA dehydrogenase are summarized in Table 1. The highest activity of BCAA transaminase was observed in the heart (P < 0.05), whereas the highest activity of BCKA dehydrogenase in the heart.  In HSB, rates of glutamate synthesis from BCAAs may vary greatly with tissues and further catabolism of BCKAs may involve interorgan cooperation to provide ATP, glucose and lipids.