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The enzymatic activity (expressed as milliunits per milligram total proteins) of three intestinal brush-border membrane enzymes, leucine aminopeptidase, alkaline phosphatase and maltase, measured over a range of temperatures between 1.5 and 37 °C, has been found to be much higher in the Antarctic fish Pagothenia bernacchii than in the temperate fish Anguilla anguilla. To explain this experimental observation the apparent Michaelis-Menten constant, the maximal velocity, the activation energy values and the thermal stability of these three enzymes were measured. The apparent Michaelis-Menten constant values of leucine amino peptidase and alkaline phosphatase were different in the intestine mucosal homogenate of the two fish at each measured temperature (from a minimum of 2.5 to a maximum of 37 °C). However, the values found at 2.5 °C for the Antarctic species and 15 °C for the eel where comparable. Furthermore, its value was unchanged in eel intestine apical membranes, both in the presence and without enzyme lipid microenvironment. While the maximal enzymatic activities of the leucine aminopeptidase and maltase did not decrease without their enzyme lipid microenvironment, produced by treatment with Triton X-100, the impairment of alkaline phosphatase maximal activity cannot be significantly differentiated from a non-specific inhibitory effect of the detergent. The activation energy values of leucine amino peptidase, alkaline phosphatase and maltase were lower in the Antarctic fish (11.7, 5.6 and 11.8 kcal·mol-1, respectively) than in the eel (13.6, 7.6 and 13.1 kcal·mol-1, respectively). The thermal stability of alkaline phosphatase and maltase is different in Pagothenia bernacchii and Anguilla anguilla intestinal homogenate.

The monoclonal antibody 6313/G2 raised against the mammalian type I (AT1) angiotensin II (Ang II) receptor (Ang II-R) also recognises a component in teleost (eel) tissue preparations that binds radiolabelled Ang II, and has an isoelectric point (pI) of 6·5 and molecular mass of 75 kDa. Immunohistochemical analysis using this antibody showed specific binding sites in eel intestine, kidney, gill and liver sections.The same antibody was used here to evaluate the presence and distribution of Ang II-R in target tissues of the Antarctic teleost icefish (Chionodraco hamatus).Immunocytochemistry of intestine and gill sections showed that the antibody bound to uniformly distributed intracellular sites and cell surface membranes in absorptive cells in the intestine and chloride and pavement cells in the gills. It also stained endothelium and both the longitudinal and circular layers of smooth muscle cells in the intestine. In the kidney, only the tubules in the trunk stained positively while the head (atubular part of the kidney) was negative. In kidney tubules, in contrast with other tissues, the receptor was most concentrated in the cytoplasm underlying the basolateral membranes, with somewhat weaker staining beneath the apical cell membrane. Immunoblotting identified a single component from trunk kidney preparations that focused at pI 5·9 in isoelectric focusing gels and showed a molecular mass of 75 kDa in SDS–polyacrylamide gels.The data suggest that, as in other teleosts, Ang II has a physiological role in the icefish.Journal of Endocrinology(1997) 154,193–200