Methionine

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Methionine and cystine are classified as sulphur-containing amino acids. Adequate amounts of both methionine and cystine are needed for proper protein synthesis and other physiological functions of the fish. Cystine is considered dispensable because it can be synthesized by the fish from the indispensable amino acid methionine. When methionine is fed without cystine, a portion of the methionine is used for protein synthesis, and a portion is converted into the cystine monomer for incorporation into protein. If cystine is included in the diet, it reduces the amount of dietary methionine needed. Thus, to determine the total sulphur amino acid requirement (methionine plus cystine), the dietary requirement for methionine is determined either in the absence of cystine or with test diets containing very low levels of cystine.

The methionine or total sulphur amino acid requirement values are presented in Table 23.8. It appears that most fish have a requirement value of about 20-30 g kg-1 of dietary protein, whereas catla, chinook salmon and gilthead sea bream appear to require higher levels of methionine.

Rainbow trout appear to be unique in that methionine deficiency results in bilateral cataracts (Poston et al., 1977). These workers observed cataracts in rainbow trout fed diets containing isolated soybean protein. The cataracts were prevented by supplementing the diet with methionine. Cataracts have also been observed in methionine-deficient rainbow trout by Walton et al. (1982), Rumsey et al. (1983) and Cowey et al. (1992). This deficiency sign has also been reported in Arctic charr and actually used as the basis for establishing the methionine requirement for this species (Simmons et al., 1999). Cataracts were also observed in hybrid striped bass fed a diet containing their total sulphur amino acid requirement in a methionine to cystine ratio of 40:60. These fish developed bilateral cataracts within 3 weeks and experienced mass mortality after 4 weeks (Keembiyehetty and Gatlin, 1993).

As indicated above, dietary cystine can reduce the amount of dietary methionine required for maximum growth. The cystine replacement value for methionine on a sulphur basis has been determined to be about 60% for channel catfish (Harding et al., 1977), 44% for blue tilapia (Liou, 1989), 42% for rainbow trout (Kim et al., 1992a) and 40% for red drum (Moon and Gatlin, 1991) and hybrid striped bass (Griffin et al., 1994b).

Robinson et al. (1978) evaluated the utilization of several sulphur compounds for their potential replacement value for methionine in channel catfish. Growth and feed efficiency data indicated that dl-methionine was utilized as effectively as l-methionine. Methionine hydroxy analogue was only about 26% as effective as l-methionine in promoting growth. No significant growth response was observed when taurine or inorganic sulphate was added to the basal diet. Page et al. (1978) were also unable to detect the utilization of taurine and inorganic sulphate as sulphur sources in rainbow trout. d-Methionine has been shown to replace l-methionine on an equal basis in rainbow trout (Kim et al., 1992a). l-Methionine, dl-methionine and N-acetyl-dl-methionine were equally utilized by hybrid striped bass, whereas glutathione and

Table 23.8. Methionine or total sulphur amino acid requirements (g kg-1 of protein) of various fish

Fish

Requirement

Type of diet

Reference

African catfish

32

Purified

Fagbenro et al. (1998a)

Atlantic salmon

24

Purified

Rollin etal. (1994)

Arctic charr

27

Practical

Simmons e/a/.(1999)

Asian sea bass

29

Semipurified

Coloso etal. (1999)

Blue tilapia

28

Purified

Liou (1989)

Catia

36

Purified

Ravi and Devaraj (1991)

Channel catfish

23

Purified

Harding etal. (1977)

Chinook salmon

40

Purified

Halver et al. (1959)

Chum salmon

30

Purified

Akiyama and Arai (1993)

Clarias hybrid

24

Purified

Unprasert (1994)

Coho salmon

27

Purified

Arai and Ogata (1993)

Common carp

31

Purified

Nose (1979)

21

Practical

Schwarz etal. (1998)

European sea bass

20

Practical

Thebault etal. (1985)

Gilthead sea bream

40

Semipurified

Luquet and Sabaut (1974)

Hybrid striped bass

29

Semipurified

Keembiyehetty and Gatlin (1993)

21

Purified

Griffin etal. (1994b)

Japanese eel

32

Purified

Arai (in Nose, 1979)

Japanese flounder

30

Purified

Alam et al. (2000)

Milkfish

25

Semipurified

Borlongan and Coloso (1993)

Mozambique tilapia

32

Practical

Jackson and Capper (1982)

Nile tilapia

32

Purified

Santiago and Lovell (1988)

Rainbow trout

22

Purified

Walton etal. (1982)

23

Purified

Kim etal. (1992a)

19-24

Semipurified

Cowey etal. (1992)

30

Purified

Rumsey etal. (1983)

Red drum

30

Semipurified

Moon and Gatlin (1991)

Rohu

26

Purified

Khan and Jafri (1993)

Yellowtail

26

Practical

Ruchimat etal. (1997a)

dl-methionine hydroxy analogue were only 75% as effective as l-methionine, and taurine was totally ineffective (Keembiyehetty and Gatlin, 1995).

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