Turid Synnøve Aas

Purine-induced expression of urate oxidase and enzyme activity in Atlantic salmon (Salmo salar). Cloning of urate oxidase liver cDNA from three teleost species and the African lungfish Protopterus annectens.

The peroxisomal enzyme urate oxidase plays a pivotal role in the degradation of purines in both prokaryotes and eukaryotes. However, knowledge about the purine‐induced expression of the encoding gene is lacking in vertebrates. These are the first published sequences of fish urate oxidase, which were predicted from PCR amplified liver cDNAs of Atlantic salmon (Salmo salar), Atlantic cod (Gadus morhua), Atlantic halibut (Hippoglossus hippoglossus) and African lungfish (Protopterus annectens). Sequence alignment of different vertebrate urate oxidases revealed amino acid substitutions of putative functional importance in the enzyme of chicken and lungfish. In the adult salmon, expression of urate oxidase mRNA predominated in liver, but was also identified in several nonhepatic organs including brain, but not in skeletal muscle and kidney. Juvenile salmon fed diets containing bacterial protein meal (BPM) rich in nucleic acids showed a significant increase in liver urate oxidase enzyme activity, and urea concentrations in plasma, muscle and liver were elevated. Whereas salmon fed the 18% BPM diet showed a nonsignificant increase in liver mRNA levels of urate oxidase compared with the 0% BPM‐fed fish, no further increase in mRNA levels was found in fish receiving 36% BPM. The discrepancy between urate oxidase mRNA and enzyme activity was explained by rapid mRNA degradation or alternatively, post‐translational control of the activity. Although variable plasma and liver levels of urate were detected, the substrate increased only slightly in 36% BPM‐fed fish, indicating that the uricolytic pathway of Atlantic salmon is intimately regulated to handle high dietary purine levels.