Molecular characterisation of catalase in Antarctic teleosts: insights into oxidative stress management in an extreme environment.

The permanently sub-zero temperatures and high oxygen solubility of the Southern Ocean promote the formation of reactive oxygen species (ROS) in the tissues of resident organisms, imposing persistent oxidative stress. Antarctic notothenioid fish have evolved efficient antioxidant systems to counter this pressure. This study reports the first molecular characterisation of the catalase (cat) transcript and deduced protein (CAT) sequence, together with tissue-specific CAT activity, in two Antarctic species with contrasting physiological traits: the red-blooded notothenioid Trematomus bernacchii and the haemoglobin-less icefish Chionodraco hamatus. Integrating cDNA sequencing with transcriptomic data, we reconstructed full-length cat coding sequences, performed Bayesian and maximum-likelihood phylogenetic analyses, and compared electrostatic surface properties using structural modelling. The cat transcript levels and CAT activity were quantified in liver, gills, heart, and skeletal muscle. Catalytic, NADPH-binding, and substrate-channel residues were fully conserved across Antarctic and non-Antarctic teleosts, consistent with strong purifying selection. T. bernacchii displayed a markedly more negative electrostatic surface than the icefish and non-Antarctic species, a pattern consistent with the increased surface acidity typical of cold-adapted proteins and with the higher oxidative burden of the red-blooded condition. The icefish showed a near-neutral electrostatic profile, consistent with its haemoglobin-less physiology. The liver was the principal site of both cat transcript accumulation and CAT activity in both species. Non-hepatic tissues showed a translation index below unity, consistent with post-transcriptional and/or post-translational regulation, with maximum translational efficiency shifting from the liver in T. bernacchii to the heart in C. hamatus. These findings suggest that enzyme surface properties and tissue-specific translational regulation may jointly contribute to antioxidant homeostasis under extreme marine conditions.