Dietary phenylalanine requirement and tyrosine replacement value for phenylalanine for ﬁngerling Catla catla (Hamilton) Seemab Zehra, Mukhtar A. Khan ⁎ Fish Nutrition Research Laboratory, Department of Zoology, Aligarh Muslim University, Aligarh 202 002, India abstract article info Article history: Received 22 March 2014 Received in revised form 13 June 2014 Accepted 23 June 2014 Available online 1 July 2014 Keywords: Phenylalanine Requirement Growth Fingerling Catla catla Two 12-week experiments were conducted to determine the dietary phenylalanine requirement and tyrosine replacement value for phenylalanine for ﬁngerling Catla catla. In experiment I, phenylalanine requirement was determined by feeding six casein–gelatin based amino acid test diets (330 g kg -1 CP; 16.72 kJ g -1 GE) with grad- ed levels of phenylalanine (3.9, 6.4, 8.7, 11.2, 13.8, 16.2 g kg -1 dry diet) at a constant level (10 g kg -1 ) of dietary tyrosine to triplicate groups of ﬁsh (3.95 ± 0.24 cm; 0.68 ± 0.19 g) near to satiation. Live weight gain (LWG%), speciﬁc growth rate (SGR % day -1 ), feed conversion ratio (FCR), protein retention efﬁciency (PRE%), phenylala- nine retention efﬁciency (PHRE%) and RNA/DNA ratio responded positively with the increasing concentrations of phenylalanine reaching the highest values at 11.2 g kg -1 of dry diet. Quadratic regression analysis of LWG, SGR, PRE, PHRE and RNA/DNA ratio at 95% of maximum response against varying levels of dietary phenylalanine exhib- ited the requirement at 10.3, 10.1, 9.9, 9.7 and 10.6 g kg -1 dry diet, respectively. The above analysis revealed that inclusion of phenylalanine at 10.1 g kg -1 of dry diet, corresponding to 594 g kg -1 of lysine is optimum. In exper- iment II, six diets with different levels of L-tyrosine (1.9, 3.8, 5.9, 8.1, 9.8, 11.8 g kg -1 dry diet) with 10.1 g kg -1 phenylalanine (determined in experiment I) ﬁxed in all the test diets were fed to ﬁsh (3.85 ± 0.25 cm; 0.66 ± 0.16 g) to determine the tyrosine requirement under identical conditions. Quadratic regression anal- ysis of LWG, SGR, PRE, PHRE and RNA/DNA ratio at 95% of maximum response against dietary tyrosine con- centrations indicated the requirement at 6.3, 6.5, 6.5, 7.1 and 7.3 g kg -1 dry diet, respectively. Hence, inclusion of tyrosine at 6.8 g kg -1 of dry diet, corresponding to 378 g kg -1 of lysine is taken as the tyrosine required for optimum utilization of phenylalanine. Based on above data, a total requirement of phenylala- nine and tyrosine for ﬁngerling C. catla was found to be 16.9 g kg -1 (10.1 g kg -1 phenylalanine + 6.8 g kg -1 tyrosine) of dry diet, corresponding to 939 g kg -1 of lysine. Tyrosine replacement value for phenylalanine was computed to be 37% on molar basis. © 2014 Published by Elsevier B.V. 1. Introduction Feed is one of the major inputs in aquaculture and the success of ﬁsh farming depends primarily on the provision of adequate quantity of nutritionally balanced feeds in a form which is acceptable to ﬁsh (Zargar et al., 2012). Hence, the inclusion of optimum quantity of a par- ticular nutrient is necessary for successful aquaculture system. Deter- mining the essential amino acid requirements of cultured ﬁsh is extremely important because of signiﬁcant effects of these nutrients on muscle deposition, feed cost, and nitrogen pollution (Small and Soares, 1999). They are important fuel molecules, signaling factors and major substrates for the synthesis of a wide range of bioactive molecules and proteins (Finn and Fyhn, 2010). Besides being the building blocks of protein synthesis, amino acids in ﬁsh are also used in energy production or for other metabolic purposes (Ronnestad et al., 2001). Quantitative dietary requirements for the ten indispensable amino acids have been determined for several ﬁsh species (NRC, 2011; Wilson, 2002). Phenylalanine, an aromatic indispensable amino acid is re- quired for normal growth and metabolic processes. It is the sole precursor of tyrosine. Phenylalanine can be converted to tyrosine by tetrahydrobiopterin-dependent phenylalanine hydroxylase in liver and kidneys but phenylalanine cannot be synthesized back from tyro- sine (Li et al., 2009). Thus, adding tyrosine to diets for ﬁsh can reduce re- quirement for phenylalanine. Tyrosine is a common precursor for important hormones and neurotransmitters, including thyroxine (T4), triiodothyronine, epinephrine, nor-epinephrine, dopamine, and mela- nin (Li et al., 2009). Pinto et al. (2009) reported that dietary require- ments for phenylalanine and tyrosine of ﬁsh increase substantially during metamorphism. These molecules have important regulatory roles (Chang et al., 2007). Information on the effects of phenylalanine and tyrosine on growth is scarce. Hence, inclusion of sufﬁcient amounts of phenylalanine and tyrosine to optimize the growth, body protein synthesis and also for the other physiological functions in ﬁsh is essential. Aquaculture 433 (2014) 256–265 ⁎ Corresponding author. Tel.: +91 9412272587. E-mail address: khanmukhtar@yahoo.com (M.A. Khan). http://dx.doi.org/10.1016/j.aquaculture.2014.06.023 0044-8486/© 2014 Published by Elsevier B.V. Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online