The responses of the digestive proteases trypsin and chymotrypsin and protein metabolism to differences in feed protein quality were investigated in Atlantic salmon (Salmo salar L.). Two sets of experimental feeds were produced. Each set of high and low quality feeds was provided to either 150 g or 2 kg salmon. Protein in the high quality feeds had significantly higher percentages of free (reactive) sulphydryl (SH) groups than the corresponding feeds based on low quality meals. After 90 days feeding, groups given high and low quality feeds did not differ in their specific growth rates (SGR) in either experiment. However, feed conversion efficiency (FCE) was significantly different between the high and low quality feed groups in 2 kg salmon, where the difference between the high and low feed protein qualities was larger, 10% versus 4% SH/[SH + (S-S)] in 150 g salmon. Higher FCE was preceded by significantly higher trypsin and chymotrypsin specific activities on day 60. SGR, in general, changed after the first month and was stable during the last 2 months in both experiments. Concurrently, both trypsin (T) and chymotrypsin (C) decreased with an increased activity ratio of trypsin to chymotrypsin (T/C ratio), and resulting in significantly lower T/C ratio on day 90 in salmon feeding on high quality feeds in both sizes of fish. Differences in FCE were associated with significant differences in levels of total free amino acids (TFAA) in the plasma and the white muscle, as well as in the ratio of essential to non-essential free amino acids (EAA/NEAA ratio), free hydroxyproline, and RNA in the white muscle. Interestingly, after 3 days starvation (day 93), 5-7 h postprandial EAA/NEAA ratio in the plasma was significantly lower in the high quality diet groups in both experiments. Trypsin specific activity inversely correlated with muscle TFAA levels in 2 kg salmon, concurrent with higher muscle levels of RNA, lower free hydroxyproline and higher FCE in fish fed higher quality diets. KEY WORDS

Biochemical structure of protein (reactive SH content, content ratio of SH/SÐS and concentration of D-Asp as % of total (D L)-Asp) indicating digestibility of dietary protein was changed under different processing conditions. Based on ®sh crude enzyme extract, in vitro digestibility of different ®sh materials processed under different conditions correlated positively with reactive SH content and content ratio of SH/SÐS and negatively with D-Asp concentration. In vitro digestion of different experimental feeds, based on Atlantic salmon crude enzyme extracts, was studied in association with growth trials in order to investigate its value as a criterion for industrial strategy in predicting feed quality. Crude enzymes were extracted from the pyloric caeca before feeding. Signi®cant differences in in vitro digestibility between the experimental feeds were observed whereby there would be differences in feed conversion ef®ciency within 3 months of feeding. There were associations between the in vitro digestibility and other parameters for dietary quality, such as mink digestibility and the biochemical structure parameters of the dietary protein due to different processing conditions. Crude enzyme extracts from rainbow trout and European seabass were also used for in vitro digestibility study of different experimental feeds by standardising trypsin activity to that of Atlantic salmon crude enzyme extract. The results indicated that different ®sh species have different digestion ability to the same feed types, and the effective time for feed utilisation and growth is dependent on ®sh sensitivity and the extent of difference in digestibility between the feeds consumed as observed in the Atlantic salmon trials. For the species investigated, sensitivity ranking of the enzymes to feed quality under the condition studied was Atlantic salmon > rainbow trout > European seabass. The results indicated that in vitro digestibility study of experimental feeds using pyloric caecal crude enzyme extract from a speci®c species at an age of interest could be a practical, quick and reliable method for testing feed quality in growth trials. By standardising the crude enzyme extract with regards to trypsin activity, the in vitro digestibility values could be comparable not only within the same species but also between different species.

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