Metabolic budgets for lysine and glutamate in unfed herring ( Clupea harengus) larvae

Abstract

Fish larvae are thought to have little control of their amino acid (AA) metabolism, leading to higher AA requirements than adult fish and mammals. Therefore, it is important to know to what extent fish larvae have the capacity to spare indispensable amino acids (IAA) at the expense of dispensable amino acids (DAA). This study intended to estimate a metabolic budget both for an IAA and a DAA in fasting herring larvae. A mix of crystalline amino acids solubilised in 1/3 seawater and added as either l- 14C-glutamate or l- 14C-lysine, was tube-fed to first feeding and 47-day-old (pre-metamorphosis) herring larvae. Both when glutamate and lysine were given in high and low doses, the following components of the metabolic budget were estimated: total AA assimilation, retention of AA in the free AA and in the protein pools, AA conversion into lipid and AA oxidation. First feeding herring larvae absorbed around 93% of both lysine and glutamate. However, 23% of the tube-fed lysine was oxidised and 70% was retained in the body, while for glutamate oxidation was as high as 76% and retention is only 17%. A similar picture was observed for 47-day-old larvae. Here, oxidation was 22% and 62% and retention 63% and 32%, for lysine and glutamate, respectively. Therefore, herring larvae use glutamate preferentially to lysine as an energy substrate from first feeding onwards. These results suggest that fish larvae have better capacity of regulating AA catabolism than previously believed.