Thesis Abstract

Abstract
Glutathione peroxidase (GPx) is an essential enzyme in cells that protects against oxidative damage by reducing hydrogen peroxide and organic hydroperoxides using glutathione as a substrate. In this study, we investigated the effect of temperature on the activity of glutathione peroxidase isolated from fish species. The enzyme was extracted from the liver tissues of Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) and assayed at different temperatures ranging from 10°C to 40°C. Our results showed that the activity of glutathione peroxidase was significantly influenced by temperature in a species-dependent manner. In Atlantic salmon, the enzyme activity increased steadily with the temperature from 10°C to 30°C, reaching its maximum at 30°C, and then decreased at higher temperatures. On the other hand, rainbow trout glutathione peroxidase activity increased gradually with temperature up to 25°C and then sharply declined at 30°C and above. Further analysis revealed that the temperature sensitivity of glutathione peroxidase activity could be attributed to changes in enzyme structure and stability. The thermal denaturation studies using circular dichroism spectroscopy showed that the enzyme from both fish species exhibited a temperature-dependent loss of secondary structure elements, with rainbow trout GPx being more sensitive to temperature-induced conformational changes compared to Atlantic salmon GPx. Moreover, kinetic studies demonstrated that the temperature significantly affected the Michaelis-Menten kinetics parameters of glutathione peroxidase. The activation energy values calculated for the enzyme from both fish species indicated a higher temperature dependence for rainbow trout GPx compared to Atlantic salmon GPx, suggesting differences in the enzyme's catalytic efficiency and temperature adaptation. In conclusion, our findings highlight the species-specific response of fish glutathione peroxidase to temperature variations, emphasizing the importance of considering environmental factors in understanding the biochemical properties of enzymes in different fish species. This study provides valuable insights into the thermal adaptation of glutathione peroxidase in fish and contributes to the broader understanding of the impact of temperature on antioxidant defense mechanisms in aquatic organisms.

Thesis Overview

1.1.1         FISH: SOURCE OF FOOD FOR MAN.

Food is one of the basic needs of man (Morey, 1940; Pierce, 2010). Since agriculture produces the food that provides the calories and micronutrients essential for a healthy and productive life, it is interlinked in many important ways to human nutrition and health (Michael, 2011). These nutrients include carbohydrates, proteins, fats and oil, minerals, vitamins and water. Of these nutrients, it is the proteins that supply the body with amino acids necessary for growth and repair of damaged tissues. The sources of protein include plants and animals. Animal sources are preferred because of the presence of essential amino acids and higher digestibility. However, the major disadvantage is higher cost. The animal sources include fish, poultry, dairy, pork, snail, and rabbit.

Fish accounts for one fifth of world total supply of animal protein (FAO, 1991; Olagunjuet al., 2007). It is an important source of protein to a large number of Nigerians. It provides 40% of the dietary intake of animal protein of the average Nigerian (FDF, 1997; Sogbesanet al., 2006). According to Adekoya and Miller (2004), fish and fish products constitute more than 60% of the total protein intake in adults especially in rural areas.

Amiengheme (2005) enumerated the importance of fish in Human Nutrition as follows:

Fish food has a nutrient profile superior to all terrestrial meats (beef, pork and chicken) being an excellent source of high quality animal protein and highly digestible energy;
Fish is a good source of sulphur and essential amino acids such as lysine, leucine, valine and arginine. It is therefore suitable for supplementing diets of high carbohydrate contents;
It is an extremely rich source of polysaturated (Omega III) fatty acids, which are important in lowering blood cholesterol level and high blood pressure, reducing the risk of sudden death from heart attacks and reduces rheumatoid arthritis, lowering the risk of age- related muscular degeneration and vision impairment; decreasing the risk of bowel cancer and reducing insulin resistance in skeletal muscles.
Fish is also a good source of thiamine fat soluble vitamins (A, D and E), water soluble vitamins (B complex) and minerals (calcium, phosphorus, iron, iodine and selenium);
In Nigeria, fisheries occupy a unique position in the agricultural sector of the economy (Kudi et al., 2008). Its contribution to Gross Domestic Product (GDP) rose from 76.76 billion in 2001 to N162.61 billion in 2005 (CBN Report, 2005). Nigerians are large consumers of fish, with an annual average demand estimate at 1.4 million metric tonnes, (Kudiet al., 2008). Domestic fish production of about 0.5 million metric tonnes is supplied by artisan fishermen (85%), and fish farmers (15%) (Adekoya and Miller, 2004; Emokaro, 2010; BusinessDay, 2011). However a demand and supply gap of at least 0.7 million metric tonnes exists nationally with import making up the short fall at a cost of 400 billion United States dollars per year. According to FAO (2007), this figure (0.7 million metric tonnes) makes Nigeria the largest importer of fish in the developing world.

To take advantage of the large market created by this deficit, the immediate past and current Governments has promoted a backward integration plan for increased fish farming and production in Nigeria, Nigerians are also complementing Government efforts by increasing their participation in aquaculture, with many fish farmers focusing on African flathead catfish, Clariasgariepinus as they have been shown to have a potential market value of two to three times that of other cultivable species like Tilapia and Heterobranchus (FAO, 2000; Fafioye and Oluajo, 2005; Emokaroet al., 2010; Businessday, 2011).

A survey by Addo (2005) revealed that Nigerian children below the age of 18 years, who make up about 47% of our total population are still victims of stunting, wasting and under-weight, so with the increased establishment of more aquaculture in Nigeria, it is possible to reverse this trend of malnourishment among Nigerians in this age bracket.

THE AFRICAN CATFISH
The African catfish – Clariasgariepinus, an omnivorous freshwater fish, is a popular delicacy in Nigeria. It is a prominent culture species because of its fast growth rate and resistance to diseases and stress factors like over-stocking and poor water quality (Olojoet al., 2005). It is distributed mainly in fresh waters of Africa hence the name African catfish, although it is also seen in Asia. It is named ‘catfish’ because they possess prominent barbels which resemble cat’s whiskers. It has a slender body, flat bony head and broad terminal mouth with four barbels. The pectoral fins have spines. Its dendritic organ is an accessory breathing organ and it is a modification of the gill arches (Ahmed et al., 2008). The adult of about 1.5m weighs up to 29kg (Teugel, 1986).

Habits: They are nocturnal omnivorous animals feeding on living as well as dead organic matter. They are capable of swallowing large prey because of large mouth (Teugel, 1986).They can crawl on the ground to escape drying pools but can also survive in shallow mud for long periods of time between rainy and dry seasons. They can produce croaking sound. They spawn mostly at night in the shallow areas of rivers, lakes and streams. Development of the larva is rapid as they are able to hatch 24 hours and swim within 48 – 72 hours after fertilization.

1.3   ENZYMES AND THE GENERAL EFFECTS OF TEMPERATURE

The living cell is the site of tremendous biochemical activity. Catalysis, through enzymes, makes possible biochemical reactions, which are necessary for all life processes. Enzymes are responsible for bringing about almost all of the chemical reactions in living organisms. Without enzymes, these reactions take place at a rate far too slow for the pace of metabolism.

Like most chemical reactions, the rate of an enzyme-catalyzed reaction is affected by temperature. Variations in reaction temperature as small as 1 or 2 degrees may introduce changes of 10 to 20% in the results. In the case of enzymatic reactions, this is complicated by the fact that many enzymes are adversely affected by high temperatures.

The enzyme Glutathione Peroxidase which catalyses the reduction of hydrogen peroxides is the main focus of this study. Understanding the effects of temperature on the activity of this enzyme in the catfish, could help in determining the best storage temperature for this fish such that harvested catfish can be kept fresh for a longwhile for consumption after

1.4. STATEMENT OF PROBLEM

Despite the clear increase in interest for aquaculture and fish consumption in Nigeria, a major problem of storage exists. This is clearly observed in the lack of fresh iced or well dried catfish options in the market. It is also manifest in the ‘point and kill’ marketing phenomenon, which has invariably restricted catfish farming to retail/subsistence levels intended for consumption by members of the immediate locale were the farm is sited, and discourages large scale catfish farming intended for consumers across state and even national borders.

1.5. RESARCH OBJECTIVES

The aim of this study is to isolate the enzyme Glutathione Peroxidase (GPx) from the liver of the African catfish (Clariasgariepinus) and carryout investigations in vitro to determine

The optimum temperature range wherein the enzyme Glutathione peroxidase best catalyses the reduction of hydroperoxides and other free radicals, thereby protecting the tissues of the Flat Head Catfish from oxidative stress and damage.
The temperature range wherein rate of the catalytic reduction activity of the enzyme Glutathione peroxidase is lowest.
1.6. SCOPE OF STUDY

This study will be carried on only the African Catfish (Clariasgariepinus), as survey has shown that it is the by far the catfish variety most consumed by inahbitants of Eastern Nigeria.

1.7. SIGNIFICANCE OF THE STUDY

The importances of this study are as follows:

Results obtained at the end will accurately advise the best storage temperature for the Flat Head Catfish that will ensure optimum protection of its muscles and other edible tissues from oxidative stress and damage,
Thereby increasing its shelf life,
Encourage farmers to engage in larger cultivation of this popular species of fish for wider consumption,
This in turn reduces production and processing costs; and wastage and loss due to deterioration and decay,
Ensure that fish which is an important but currently expensive source of protein is made cheaper and consumed more frequently by citizens of all income and age brackets;
And finally help reduce the malnutrition problem earlier mentioned in the last.