Development and characterisation of some intestinal enzymes in Atlantic cod (Gadus morhua L.) and Atlantic halibut (Hippoglossus hippoglossus L.) larvae

To determine the status of development of the intestine, the activities of intestinal leucine–alanine peptidase (Leu–ala), brush border membrane (bbm)-bound alkaline phosphatase (AP) and bbm-bound leucine aminopeptidase N (LAP) were analysed from first feeding to passed weaning in Atlantic halibut (Hippoglossus hippoglossus; 1–78 days post first feeding; dpff) and Atlantic cod (Gadus morhua; 0–72 dpff). Before conducting the ontogenetic studies, the enzyme assays used were optimised in relation to temperature and pH (only AP and LAP), and the Michaelis constant (KM; All enzymes) determined, in order to ensure valid data and to maximise the activity potentials. Leu–ala was used as a marker of intracellular digestion which was expected to decrease during the experimental periods. AP and LAP were used as markers of development of the bbm and achievement of an adult mode of digestion. In halibut, Leu–ala increased 11-folds (P = 0.0002) in specific activity (mU (mg protein)− 1) from first feeding to 34 dpff and thereafter a small decrease (0.65-fold; P = 0.006) in activity was observed. In cod, the specific activity of Leu–ala varied randomly during the experiment. Distinct increases in specific activities of AP and LAP were observed from 29 to 52 dpff in halibut (fivefold; P < 0.002) and from 29 to 37 dpff in cod (two–threefold; P < 0.02). The bbm-bound activities of AP and LAP in percent of total intestinal activity of the respective enzyme increased from approximately 2 to 15% between 34 and 43 dpff in halibut (P < 0.0003), confirming the results of specific activities. In cod, a similar result with AP was found between 29 and 51 dpff (P = 0.0003). The increase in percentage of bbm-bound activity of LAP was later, increasing exponentially from constituting 5 to 31% of total individual activity between 37 and 72 dpff (P = 0.0003). The experiments indicated that the digestion in halibut had advanced towards the adult mode at 40 to 50 dpff, while time of reaching the same maturational level in cod was less certain due to the different activity profiles of AP and LAP. As AP is well recognised as a bbm differentiation marker, it can be assumed that the intestine in cod matured when AP activity had reached an elevated level at approximately 40 to 50 dpff, whereas LAP possibly was induced later. Ceasing of the larval mode of digestion, expected to be indicated by reduced activity of Leu–ala, was not detected.