Effects of dietary organic chromium on gilthead seabream ( Sparus aurata L.) performances and liver microsomal metabolism P P Gatta 1 , A Piva 1 , M Paolini 2 , S Testi 1 , A Bonaldo 1 , A Antelli 2 & A Mordenti 1 1 Dipartimento di Morfo®siologia Veterinaria e Produzioni Animali, Universita Á degli Studi di Bologna, Via Tolara di Sopra, 50-40064 Ozzano Emilia, Bologna, Italy 2 Dipartimento di Farmacologia, Universita Á degli Studi di Bologna, Via Irnerio, 48-40126 Bologna, Italy Correspondence: P P Gatta, Dipartimento di Morfo®siologia Veterinaria e Produzioni Animali, Universita Á degli Studi di Bologna, Via Tolara di Sopra, 50-40064 Ozzano Emilia, Bologna, Italy. Tel: (+39) 051792995, Fax: (+39) 051792869, e-mail: ppgatta@vet.unibo.it Abstract The effect of yeast and chromium yeast on gilthead seabream (Sparus auratus L) performance, carcass indices and body composition was studied. Whether supplementation affected liver microsomal mixed function oxidases using either multibioprobes (test- osterone) or highly speci®c substrates to cytochrome P450 (CYP) isoforms was also investigated. Seabream juveniles (35±37 g initial weight) were allocated into 12 800 L tanks of 50 ®sh each for 87 days and fed pelleted experimental diets, i.e. control, yeast supplemented (1.6%) and chromium yeast supplemented at both low (800 p.p.b.) and high chromium level (53 810 p.p.b.). At the end of the experiment, growth, feed conversion ratio, thermal-unit growth coef®cient, carcass yield, hepatosomatic index, and carcass and ®llet proximate compositions were similar among treatments and only condition factor was statistic- ally different. Organic chromium at both doses affected CYP-catalysed drug reactions slightly, as shown by the modest effect on the regio- and stereoselective hydroxylations of testosterone, as well as the metabolism of the selected probes. Overall, we found that chromium yeast did not change performance substantially, nor carcass indices, carcass and ®llet chemical compositions, or hepatic xenobiotic metabolizing enzymes of gilthead seabream. Keywords: chromium, gilthead seabream, nutrition, toxicology Introduction Over the last few decades, chromium has been recognized as a new and important micronutrient, essential in human (Jeejebhoy et al. 1977) and animal nutrition (Shwartz & Mertz 1959), in¯uenc- ing many aspects of metabolism (Mordenti, Piva & Piva 1997). In particular, it has been shown to have a positive in¯uence on the growth, reproductive ef®ciency and carcass composition of pigs and cattle. Furthermore, there is evidence that dietary chro- mium supplementation can exert some bene®cial effects on both the nonspeci®c and speci®c immune systems of cattle (Mallard & Borgs 1997). So far, several studies dealing with the effect of chromium in ®sh have been related to its role in carbohydrate utilization. Shiau & Lin (1993) found a signi®cant improvement in weight gain, energy deposition and liver glycogen content when tilapia (Oreochromis niloticus L. 3 O. aureus L) juveniles were fed a glucose-based diet supplemented with chromium chloride. In the same experiment, no differences were recorded when diets contained starch instead of glucose as a carbohydrate source. Another study (Shiau & Chen 1993), designed to 60 ã 2001 Blackwell Science Ltd Aquaculture Research, 2001, 32 (Suppl. 1), 60±69