To prepare each gill homogenate, 4–5 crabs acclimated to each salinity for 10 days, or to 21‰ for each exposure time, were used. The gills were carefully blotted on filter paper to remove excess buffer, weighed, diced and homogenized in homogenization buffer (20 mL buffer/g wet tissue) using a Potter homogenizer. After centrifuging the crude extract at 20,000 × g for 35 min at 4 °C, the centrifuge tubes containing the supernatant were placed in crushed ice and the pellet was resuspended in an equal volume of homogenization buffer. After further centrifuging as above, the two supernatants were pooled and centrifuged at 100,000 × g for 2 h at 4 °C. The resulting pellet was homogenized in 20 mmol L− 1 imidazole buffer, pH 6.8, containing 250 mmol L− 1 sucrose (5 mL buffer/g wet tissue). Finally, 0.5-mL aliquots were rapidly frozen in an acetone/dry ice bath and stored at − 20 °C. No appreciable loss of activity was seen after 2-month's storage. When required, the aliquots were thawed, placed on crushed ice and used immediately.