Hazel J R, Sidell B D
Department of Biology, Arizona State University, Tempe, AZ 85287-1501, USA.
J Exp Biol. 2004 Feb;207(Pt 6):897-903. doi: 10.1242/jeb.00823.
Antarctic fishes of the suborder Notothenioidei characteristically possess large stores of neutral lipids that have been shown to be important both in conferring buoyant lift and as a caloric resource for energy metabolism. Previous work has established that the aerobic energy metabolism of Antarctic fish is fueled predominantly by the catabolism of fatty acids, with the catabolic machinery displaying a preference for the oxidation of unsaturated fatty acids. The composition of the fatty acids released from adipose tissue of Antarctic fish during lipolysis, however, has not previously been demonstrated. Employing a substrate competition assay, we have characterized the substrate specificity of hormone-sensitive lipase (HSL) from adipose tissue of the Antarctic fish Trematomus newnesi. Rates of oleic acid release from radiolabeled triolein were quantified in the presence and absence of a nonradiolabeled cosubstrate. Polyunsaturated species of triacylglycerols (TAGs) containing 18:2 or 20:4 depressed rates of oleate release by 70-75% below control values. Most of the molecular species of TAG containing monoenoic fatty acids (i.e. those containing 14:1, 16:1 or 20:1) had no significant effect on rates of oleate release. By contrast, oleate release from triolein was actually stimulated (by 2-4-fold) by both saturated species of TAG (i.e. those containing 14:0, 16:0 and 18:0) and those possessing long-chain (22:1 and 24:1) monoenes (by 1.2-1.5-fold). Thus, the rank order of substrate preference for adipose tissue HSL was: polyunsaturates > monoenes > saturates. Degree of fatty acid unsaturation had a more marked effect on rates of hydrolysis than did fatty acid chain length. In addition, the enzyme displayed a preference for the hydrolysis of sn-1,2 rather than sn-1,3 diacylglycerols. These data indicate that the substrate specificity of adipose tissue HSL may be an important factor in determining which fatty acids are mobilized during stimulated lipolysis and which are made available for catabolism by other tissues of Antarctic fishes. Our data further suggest that TAGs containing some saturated fatty acids may be sufficiently poor substrates for catabolism by HSL to explain their disproportionate accumulation in adipose tissue. Such a mechanism could also contribute to the ontogenetic accumulation of fats that has been reported as an underlying basis for the positive correlation of buoyancy with increasing body mass in this group.
南极亚目南极鱼科鱼类的特征是拥有大量中性脂质储备，这些储备已被证明在提供浮力以及作为能量代谢的热量来源方面都很重要。先前的研究已经确定，南极鱼的有氧能量代谢主要由脂肪酸的分解代谢提供燃料，分解代谢机制对不饱和脂肪酸的氧化表现出偏好。然而，此前尚未证实南极鱼脂肪组织在脂肪分解过程中释放的脂肪酸组成。通过底物竞争试验，我们对南极鱼新氏南极鱼脂肪组织中激素敏感性脂肪酶（HSL）的底物特异性进行了表征。在存在和不存在非放射性共底物的情况下，对放射性标记的三油精中油酸的释放速率进行了定量。含有18:2或20:4的多不饱和三酰甘油（TAGs）物种使油酸释放速率比对照值降低了70 - 75%。大多数含有单烯脂肪酸的TAG分子物种（即含有14:1、16:1或20:1的那些）对油酸释放速率没有显著影响。相比之下，饱和TAG物种（即含有14:0、16:0和18:0的那些）以及具有长链（22:1和24:1）单烯的TAG物种实际上刺激了三油精中油酸的释放（提高了2 - 4倍）（提高了1.2 - 1.5倍）。因此，脂肪组织HSL对底物偏好的排序为：多不饱和脂肪酸 > 单烯脂肪酸 > 饱和脂肪酸。脂肪酸不饱和度对水解速率的影响比脂肪酸链长度更为显著。此外，该酶对sn - 1,2二酰甘油的水解表现出偏好，而不是sn - 1,3二酰甘油。这些数据表明，脂肪组织HSL的底物特异性可能是决定在刺激脂肪分解过程中哪些脂肪酸被动员以及哪些脂肪酸可供南极鱼其他组织进行分解代谢的一个重要因素。我们的数据进一步表明，含有一些饱和脂肪酸的TAGs可能是HSL分解代谢的劣质底物，这可以解释它们在脂肪组织中不成比例的积累。这样一种机制也可能有助于脂肪的个体发育积累，这已被报道为该群体中浮力与体重增加呈正相关的潜在基础。
