Serine Dehydratase and Tyrosine Aminotransferase Activities Increased by Long-term Starvation and Recovery by Refeeding in Rainbow Trout (Oncorhynchus mykiss) JUAN PERAGO ´ N 1Ã , JUAN B. BARROSO 1 , M. DE LA HIGUERA 2 , AND JOSE ´ A. LUPIA ´ N ˜ EZ 3 1 Biochemistry and Molecular Biology Section, Department of Experimental Biology, University of Jae´n, Jae´n, Spain 2 Department of Animal Biology and Ecology, University of Granada, Granada, Spain 3 Department of Biochemistry and Molecular Biology, Centre for Biological Sciences, University of Granada, Granada, Spain ABSTRACT Here, we study a cycle of long-term starvation followed by refeeding in relation to the kinetics of serine dehydratase (SerDH) and tyrosine aminotransferase (TyrAT) in rainbow trout (Oncorhynchus mykiss). We determine SerDH- and TyrAT- specific activity at different substrate concentrations in liver and white muscle of juvenile trout starved for 70 days and then refed for 6 hr, 32 hr, 4 days, and 9 days. SerDH showed a hyperbolic kinetic with a K m for L-serine of 77.0778.78 mM in the liver of control trout. After 70 days of starvation, the SerDH activity at saturate substrate concentration rose 100% over control. No significant changes were found in the K m values of the enzyme. After refeeding, the SerDH activity declined to control values. TyrAT also showed a hyperbolic kinetic with a K m for L-tyrosine of 1.8670.12 and 2.5570.57 mM in liver and white muscle, respectively. In starved trout, TyrAT activity in liver and white muscle was about 64 and 267%, respectively, higher than control. After 9 days of refeeding, the control values recovered, although, at 6 hr of refeeding, hepatic TyrAT activity was higher than that for starvation. This work shows that SerDH and TyrAT are present in rainbow trout and that the two enzymes have regulatory functions in the catabolism of their respective amino acids in this species. J. Exp. Zool. 309A:25–34, 2008. r 2007 Wiley-Liss, Inc. How to cite this article: Peragon J, Barroso JB, De la Higuera M, Lupianez JA. 2008. Serine dehydratase and tyrosine aminotransferase activities increased by long-term starvation and recovery by refeeding in rainbow trout (Oncorhynchus mykiss). J. Exp. Zool. 309A:25–34. Rainbow trout, a carnivorous fish, uses dietary protein as a main energy source. Amino acids produced by the breakdown of exogenous or endogenous proteins are used as energy fuels or gluconeogenic precursors in different metabolic states (De la Higuera and Ca ´rdenas, ’85; Walton, ’86). During certain life-cycle stages, many salmo- nids survive extended periods without food by breaking down endogenous nutrients to obtain energy and glucose. In a previous study with long periods of starvation (70 days) followed by refeed- ing, we found that the strong regulation of liver protein-turnover rates mobilized protein during the starvation period and replenished the tissue Published online 20 November 2007 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jez.429 Received 7 June 2007; Revised 1 October 2007; Accepted 23 October 2007 Grant sponsor: Plan Andaluz de Investigacio´n, Junta de Andalucı ´a, Spain; Grant number: CVI-157. Abbreviations: cAMP, 3 0 ,5 0 -cyclic adenosine monophosphate; FCE, food-conversion efficiency; h, Hill coefficient; a-KG, alpha- ketoglutarate; K m , Michaelis constant; mU, milliUnits; PER, protein- efficiency ratio; PLP, pyridoxal 5 0 -phosphate; mRNA, messenger ribonucleic acid; SEM, standard error of the mean; SerDH, serine dehydratase; TyrAT, tyrosine aminotransferase; V max , maximum velocity. Ã Correspondence to: Juan Perago´n, Biochemistry and Molecular Biology Section, Department of Experimental Biology, University of Jae´n, Campus Las Lagunillas, 23071 Jae´n, Spain. E-mail: jperagon@ujaen.es r 2007 WILEY-LISS, INC. JOURNAL OF EXPERIMENTAL ZOOLOGY 309A:25–34 (2008)