When the icy waters around Antarctica became isolated from the rest of the planet's oceans, the creatures trapped there had no choice but to adapt or die, and notothenioids were no exception. Evolving from bottom-dwelling ancestors that lacked swim bladders, notothenioids couldn't resort to an air-filled buoyancy aid to get them to the surface. Instead, they retain neutral lipids to give them the extra lift. But having evolved an alternative approach to buoyancy control, the fish were faced with a dilemma: how could they retain buoyant adipose tissue, while using fats for fuel? Puzzled by this apparent conundrum Bruce Sidell and Jeffery Hazel headed south to the US Antarctic Survey's Palmer Station to investigate the machinery of lipid liberation and see how notothenioids had resolved the quandary. Focusing on an enzyme that mobilises fats from lipid stores, Sidell and Hazel were surprised to discover that the enzyme is selective, allowing buoyant lipids to accumulate and keep the fish afloat().
Sidell and Hazel headed south to Antarctica to gather Trematomus newnesi from the icy southern ocean. But instead of returning to their US labs with the fish they trapped, the team took up residence at Palmer Station,where they began analysing lipase enzymes in the fishes' adipose tissue. Sidell explains that the laboratory facilities at Palmer are `terrific[because] the normal daily disruptions melt away down there'. Together, Sidell and Hazel successfully purified two enzyme fractions from the adipose tissue with lipase activity: hormone-sensitive lipase and lipoprotein lipase. Knowing that hormone sensitive-lipase was the rate-limiting fat-freeing enzyme, the team decided to focus on it, to find out whether it has any preference for the fatty acids it liberates.
But the wide range of triacylglycerides found in the fish's adipose tissue made it difficult to test the enzyme's specificity with isotopically labelled fatty acids; they simply weren't available. Instead Sidell and Hazel turned to a competition assay, offering the enzyme a choice between radioactive triolein, a triglyceride found in the fish's adipose tissue, and other triglycerides found in the fish's tissue to see whether the enzyme had a triglyceride preference. Testing the enzyme's selectivity over a range of saturated, monounsaturated and polyunsaturated fatty acids, Hazel and Sidell were able to identify the enzyme's preferred triglycerides. Polyunsaturated and monoenoic fatty acids both came out on top, while the enzyme barely processed saturated fats at all. Not only was the enzyme highly selective, but it seemed to have a preference; the lipase liberated the fatty acids that the fish consumes to meet its catabolic demands.
Pleased that the enzyme only seemed to mobilise fuel from adipose stores,Sidell took a closer look at the triglycerides that the enzyme left behind`The enzyme worked particularly poorly on triacylglycerides containing myristic acid' says Sidell. And when he looked at the fatty acids found in the fishes' adipose tissue, he realised that this was one of the major triacylglyceride components in Trematomus' lipid stores. The enzyme appeared to be actively selecting the fish's favourite fuel while accumulating buoyant myristic acid to keep the fish bobbing about.
