Evonik Degussa Africa (Pty) Ltd. | IBG Business Park | Cnr. New & 6th Road Midridge Offi ce Estate (South) | Cnr. Pioneer and Atlantis Road | Midrand | South Africa 24 Would you be happy with less? Expect the best Methionine source with 100 % quality and reliability, backed by decades of experience. And more profi t in your pocket. You know what really counts. www.evonik.com/feed-additives | feed-additives@evonik.com Porcus Januarie/Februarie 2012
nutrition The roles of tryptophan in pig nutrition By Dr J Htoo and F Crots – Evonik Industries E-mail: franscois.crots@evonik.com The key important points of Tryptophan are: • Required for protein synthesis • Precursor for serotonin • Maximise feed intake and growth performance • Required in the immune response system Tryptophan (Trp) is one of the most complex essential amino acids (AA). This complexity is due to the many different metabolic roles that Trp has in the body, despite the fact that the concentration of Trp is the lowest of all AA in the body of pig. In addition to its role as a building block of body protein, Trp is needed for synthesis of serotonin, a neurotransmitter, which is known to be involved in regulation of feed intake, aggression and stress response behaviors. A metabolite of serotonin degradation, melatonin, may act as free radical scavengers and have antioxidant properties. Another pathway of Trp metabolism, quantitatively a more important one, is the kynurenine pathway which is associated with immune response mechanism. In addition to its involvement in different roles in the body, Trp also is complex due to its low concentrations in several of the feedstuffs used in swine feed, as well as the difficulty associated with analysing for Trp content. Unlike other AA that are isolated by acid hydrolysis, Trp content has to be measured separately following alkaline hydrolysis since it is destroyed during acidic hydrolysis with hydrochloric acid. The content and digestibility of Trp also greatly vary among common feedstuffs. Ingredients that contain relatively high Trp content include blood meal, casein, fish meal, whole egg, potato protein and soybean meal. Tryptophan content is extremely low in corn, tapioca and sorghum. Tryptophan is usually considered the fourth limiting AA in typical cereal-based swine diets. There is considerable variation in Trp requirements of pig as well as ideal dietary Trp:Lys ratios among published data. Undoubtedly, the reasons for these inconsistencies may be attributed to many factors highlighted above. This aim of this review is not to give Trp requirement or ideal ratios recommendations, but rather focuses on the following objectives: 1) to describe metabolic pathways and roles of Trp besides for body protein synthesis, 2) to review the effects of dietary Trp on brain serotonin concentration and its mechanisms in regulating in feed intake and stress response, and 3) to review the effect of dietary Trp on immune response and mechanisms involved. The role of tryptophan for serotonin synthesis As one of the dietary essential AA, Trp plays an important role in body protein synthesis. A stimulatory effect of Trp on protein synthesis in the liver, muscle and skin of pigs has been demonstrated (Ponter et al., 1994). In addition to protein synthesis, Trp is also involved in many complex metabolic pathways. Once absorbed in the small intestine, Trp enters the portal vein and pass into the liver; a portion is used for protein synthesis and the remaining Trp not utilised for protein synthesis can follow two major metabolic pathways. First, a small proportion of it is used to synthesise a neurotransmitter serotonin mainly in the gut, platelets and brain, while the second pathway, known as the kynure- Figure 1: Chemical structure of tryptophan nine pathway, leads to the formation of various products including 3-hydroxykynurenine, 3-hydroxyanthranilic acid, quinolinic acid, kynurenic acid and niacin (Brown, 1995). In the brain, synthesis of serotonin mainly occurs in serotonergic nerves, enterochromaffinic cells, thrombocytes and mast cells. It is also widely distributed in the hypothalamus (Saavedra et al., 1974). In mammals about 90% of total plasma Trp is bound to albumin. The remaining Trp, which is in a free form, can enter the brain through the blood-brain barrier (BBB; Madras, et al., 1974) to be converted by the enzyme tryptophan hydroxylase in the pinealocytes into 5-hydroxytryptophan, which is then converted by decarboxylation to serotonin (Figure 2). As a neurotransmitter, serotonin is involved in regulating a variety of behavioral processes such as appetite, feeding, impulsivity, aggression, sexual behavior, temperature regulation, pain perception and mood control. As a neurohormone, melatonin plays a role in the control of the day- and night-rhythm and serves as an intracellular scavenger of hydroxyl- and peroxide-radicals (Reiter et al., 1994). Figure 2: Metabolic pathway of serotonin and melatonin synthesis Continued on p 26 Porcus January/February 2012 25
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