The Distribution of the Enzyme Arginase in the Tissues of Selected Cichlidae Species: Tilapia zillii, Sarotherodon galilaeus and Oreochromis niloticus

The paper reports the tissue distribution of the enzyme arginase in three different Cichlids: Tilapia zilli, Sarotherodon galilaeus and Oreochromis niloticus, from the Aiba and Osinmo reservoirs, located in the southwestern Nigeria. The tissues of S. galilaeus showed very high activity of arginase as compared with the other two species. The liver of O. niloticus and the gut of T. zillii showed very high activity of arginase in the Osinmo reservoir. The high arginase activity observed in the tissues of these organisms is attributed to ureotelism and is similar to the result obtained for tilapia, Alcolapia grahami, from lake Magadi, Kenya. component of total nitrogen excretion (Campbell & Anderson, 1991; Wood, 1993). Its waste basically occurs through the gills, + and the ammonium ion (NH ) prevails over 4 the non ionic ammonia (NH ), the practically 3 impermeable form. Environmental conditions have been reported by Campbell & Anderson (1991) to be the common stimulus for urea synthesis in teleost fishes. For instance, Lake Magadi in Kenya Oreochromis alcalicus grabami live in very high alkaline water (pH 10) that is unfavourable for ammonia diffusion across the gills (Mommsen & Walsh, 1992; Wood, 1993). However, the activities around two reservoirs (Aiba & Osinmo reservoirs in south-western Nigeria) have been observed as those that enhance increase in environmental pollution, especially the use of fertilizers and other chemicals by farmers, and poor sewage disposal by the locals (Atobatale, 2008; Komolafe & Arawomo, 2008). These pose difficult environmental conditions such as alkaline waters and seasonal drought. The main objective of the present study, therefore, was to measure the activities of the enzyme arginase in different tissues of some Tilapia species (Tilapia zillii, Sarotherodon galileus and Oreoch-romis niloticus) in these reservoirs. Materials and methods Tris-HCl, tris-base, manganese chloride, and arginine were all purchased from Sigma Aldrich Chemicals, USA. All other reagents used were of analytical grades. Collection of samples Cast-net was used once a month to collect fish samples between April and August 2009 in Aiba and Osinmo reservoirs, both in southwestern region of Nigeria. The fish samples were stored in an ice-chest before transporting to the laboratory where they were stored at temperature below 0 °C until ready for use. The fish species viz. T. zillii, S. galilaeus and O. niloticus were identified using the keys by Holden & Read (1978) and Adesulu & Sydenham (2008) at the Fish Culture Laboratory, Department of Zoology, Obafemi Awolowo University, Ile-Ife, Nigeria. Preparation of tissue extract Prior to extraction, the Cichlid species were slit open and the various tissues of interest (liver, guts and gills) were removed o and stored at 4 C until required. Tissue extracts were prepared by homogenising 10 g (w/v) of each tissue in three volumes of homogenisation buffer (phosphate buffer, pH 7.2). The suspensions were centrifuged for 20 min at 4,000 r.p.m. in a Microfield Centrifuge Model 800 D. The supernatants were used as the source of enzyme. Protein and enzyme assays Arginase activity was determined by the measurement of urea produced by the reaction of Ehrlich’s reagent according to the modified method of Kaysen & Strecker (1973). The reaction mixture contained in final concentration 1.0 mM Tris-HCl buffer, pH 9.5 containing 1.0 mM MnCl 0.1 M 2, arginine solution and 50 mml of the enzyme preparation in a final volume of 1.0 m. The mixture was incubated for 10 min at 37 °C. The reaction was terminated by the addition of 2.5 ml Erhlich reagent (2.0 g of pdimethylaminobenzaldelyde in 20.0 ml of concentrated hydrochloric acid and made up to 100 ml with distilled water). 48 West African Journal of Applied Ecology, vol. 18, 2011 The optical density reading was taken after 20 min at 450 nm. The urea produced was estimated from the urea curve (graph of optical density against urea concen-tration). The unit of activity of arginase is defined as the amount of enzyme that will produce one o mmol of urea per min at 37 C. Bradford method (1976) was used to measure the protein concentration of the enzyme using bovine serum albumin (BSA) as standard. Statistical analysis The results are presented as means ± SD. Data were analyzed by one-way ANOVA by using SAS/PC software to examine whether there was any statistical difference among groups. Duncan multiple range test was used for paired comparisons. A P value less than 0.05 was consideredstatistically significant. Results The activity of arginase in the three Cichlid species is presented in Table 1. In Aiba reservoir, activity of arginase was found to be as high as 0.40 ± 0.00 and 0.49 ± 0.01 in the liver and gut of S. galilaeus, respectively. A slight variation was observed in Osinmo reservoir where arginase activity was highest in the liver (0.53±0.04) of O. niloticus, followed by the gut (0.48±0.01) and gills (0.29±0.02) of T. zillii (Table 1). Discussion Ureogenesis has been considered less important for most Teleosts (Wright, 1995). It was assumed for many years that genes of urea cycle (UC) enzymes had been lost from the Teleost genome (Wright, 1995; Jenkinson et al., 1996; Terjesen et al., 2001). However, the presence of UC enzymes in early life stages is being reported in some species, and proposed as a fact for most (Jenkinson et al., 1996), though it was not very clear whether ammonia excretion prevails over the urea. In spite of a few UC enzymes with different specific activities reported in many species (Nener, 1988; Mommsen & Walsh, 1992), the full set is uncommon. It was reported that in difficult environmental conditions such as alkaline waters and seasonal drought, urea cycle enzymes remain active in a few adult Teleost species (Randall et al., 1989). Arginine conversion to ornithine is catalyzed by arginase. This enzyme is widely distributed and its activity in fishes is supposed to change with the ingested protein level (Berge et al., 1997). Usually, ammoniotelism