For example, losses of ascorbate activity from catfish diets containing ascorbic acid or ethylcellulose-coated ascorbic acid (ECAA) was reported to be 23% to 34% (AA) and 10% to 24% (ECAA) after conventional steam pelleting and 55% to 69% (AA) and 40% to 55% (ECAA) after expansion pelleting (Lovell and Lim, 1978). Retention of vitamin C in extruded catfish feeds ranged from 34% to 100% for fat coated ascorbic acid and 37% to 75% for ECAA (Robinson, 1992).
Although ECAA is generally regarded as more stable than ascorbic acid, Sandnes and Utne (1982) reported a 70% to 80% loss of ascorbate activity with ECAA- supplemented diets after steam pelleting and storage at 4ƒC for 24 weeks. Practically no activity could be detected after 16 weeks when the feed was stored at room temperature. In extruded catfish feeds, stability of the fat-coated ascorbic acid ranged variable ranging from 0 retention after 28 days in some samples to over 40% after 90 days in others. Retention of ECAA averaged 32% and 22% after 14 and 28 days, respectively, but was also highly variable ranging from 0 to 46% and 0 to 35% retention (Robinson, 1992). By contrast, 80% to 100% ascorbate recovery has been reported for ascorbyl-2-poly- and monophosphate within expanded or extruded feeds (Lovell and El Naggar, 1989; Shiau and Hsu, 1993).
The role of ascorbyl-2-sulfate in fish feeds has been controversial. Some researcher has shown that this form of vitamin C is available to fishes (Halver et al., 1975; 1993; Tucker and Halver, 1984a, b, 1986; Halver and Hardy, 1994; Shiau and Hsu, 1995a, b; Abdelghany, 1996, 1998; Kittakoop et al., 1996). Other suggest that ascorbyl-2-sulfate has no or reduced vitamin C activity in fishes (Tsujimura et al., 1978; Murai et al., 1978; Soliman et al., 1986a; Dabrowski and K–ck, 1989; Lovell and El Naggar, 1989; Dabrowski et al., 1990a, 1990b, 1994; Sandnes et al., 1990; El Naggar and Lovell, 1991a, b; Shiau and Hsu, 1994; Hsu and Shiau, 1997; Amerio et al., 1998). The difference in the experimental findings is based on the analyses used to determine the presence of ascorbyl-2-sulphate in fish tissues (Dabrowski et al., 1994). Although work has shown arylsulfatases activity is present in fish (Dabrowski et al., 1990b, 1993), its Km values is much higher than normal liver and intestinal concentrations of ascorbyl-2-sulphate if fed in a diet.
The bioavailability and biopotency of the various salts of ascorbyl-2-polyphosphate and ascorbyl-2-monphosphate have been evaluated in different fishes. In the intestine of rainbow trout (Oncorhynchus mykiss), Atlantic salmon (Salmo salar), channel catfish (Ictalurus punctatus), sea bass (Dicentrarchus labrax), gilthead sea bream (Sparus auratus), eel (Anguilla anguilla), hybrid sturgeon (Acipenser ruthenus x A. Baeri) and European catfish (Silurus glanis), ascorbyl-2-mono- or polyphosphate is converted to ascorbate and absorbed (Sandnes and Waagb¯, 1991; Miyasaki et al., 1992, 1993; Buddington et al., 1993; Dabrowski et al., 1994; Gy. Papp et al., 1995; Matusiewicz and Dabrowski, 1995).
The various phosphate forms and salts have been found to be equal to crystalline ascorbic acid in their ability to support normal growth and prevent ascorbic acid deficiency in rainbow trout (Oncorhynchus mykiss) (Grant et al., 1989; Sato et al., 1991; Dabrowski, 1992b, 1996; Gabaudan et al., 1992; Cho and Cowey, 1993; Matusiewicz et al., 1994, 1995; V–lker and Fenster, 1994); Atlantic salmon (Sandnes and Waagb¯, 1991; Waagb¯ et al., 1991a; Roem and Oines, 1993); channel catfish (Ictalurus punctatus) (Lovell and El Naggar, 1989; Wilson et al., 1989; El Naggar and Lovell, 1991a, b; Mustin and Lovell, 1992); shrimp (Penaeus monodon) (Shigueno and Itoh, 1988; Shiau and Hsu, 1994; Kittakoop et al., 1996; Hsu and Shiau, 1997, 1998; Hunter et al., 1998; Moreau et al., 1998); yellowtail (Seriola quinqueradiata) (Kanazawa et al., 1992); tilapia (Oreochromis spp.) (Shiau and Hsu, 1995b; Abdelghany, 1996, 1998); sea bass (Dicentrarchus labrax) (Amerio et al., 1998; Alexis et al., 1999); gilthead sea bream (Sparus auratus) (Alexis et al., 1999); and striped bass (Morone saxatilis) (Ashraf et al., 1996).
