Insect Biochemistry and Molecular Biology Insect Biochemistry and Molecular Biology 35 (2005) 461–470 Activation of triacylglycerol lipase in the fat body of a beetle by adipokinetic hormone Lutz Auerswald à , Karl J. Siegert, Gerd Ga¨de Zoology Department, University of Cape Town, Rondebosch 7701, South Africa Received 21 January 2005; accepted 21 January 2005 Dedicated to Helmut Auerswald on the occasion of his 70th birthday Abstract The activation of triacylglycerol lipase and the stimulation of proline synthesis in the fat body of the fruit beetle Pachnoda sinuata by the endogenous octapeptide hormone Melme-CC (pQLNYSPDWa), which belongs to the family of insect adipokinetic hormones, were studied, and the correlation of both events investigated. At rest, the activity of triacylglycerol lipase in the fat body of the beetle was higher than in the fat body of the American cockroach, Periplaneta americana, but lower than in the migratory locust, Locusta migratoria. Triacylglycerol lipase of the beetle is activated by: (a) injection of synthetic Melme-CC and (b) the stimulus of flight. Activation of lipase by Melme-CC is time- dependent. Injection of cpt-cAMP activates triacylglycerol lipase in the fat body and causes an increase in the concentration of proline in the haemolymph at the expense of alanine. In contrast, injection of F-inositol-1,4,5-phosphate does not affect the activation state of lipase, nor the levels of amino acids in the haemolymph. High doses of octopamine do not activate lipase. Furthermore, activity of fat body lipase and proline concentration in the haemolymph both follow a circadian rhythm: both parameters are high in the morning, whereas they are low in the evening. When transfer of Melme-CC, released from the corpora cardiaca, to the thorax/abdomen is prevented by neck-ligation, the activity of lipase, as well as the circulating proline levels are low. Regression analysis revealed that activity of triacylglycerol lipase is positively correlated to proline concentration in the haemolymph, whereas there is a negative correlation of the enzyme activity and alanine level in the haemolymph. From these results we conclude that the activation of fat body triacylglycerol lipase by Melme-CC in P. sinuata stimulates proline synthesis. Proline is one of the major substrates to power flight activity in the beetle. r 2005 Elsevier Ltd. All rights reserved. Keywords: Triacylglycerol; Lipase; Proline; Alanine; Insect; Insect flight; Flight metabolism; AKH; Fatty acid 1. Introduction Flight activity in insects requires large amounts of energy and relies on an aerobic metabolism. Thus, energy for the contraction of the flight muscles is provided by the oxidation of either carbohydrates or lipids or the amino acid proline, or a mixture of at least two of these substrates (Ga¨de and Auerswald, 2003). Flight muscles of insects, such as locusts and moths typically metabolize lipids during flight (Beenakkers et al., 1981; Ziegler and Schulz, 1986), whereas cock- roaches and flies use mainly carbohydrates (Sacktor, 1975; Keeley et al., 1994). Partial oxidation of proline as a major source of energy was so far discovered in the tsetse fly (Bursell, 1981) and various beetle species (Weeda et al., 1979; Zebe and Ga¨de, 1993; Ga¨ de, 1997a,b; Ga¨de and Auerswald, 2002). Energy substrates in the flight muscles of insects are limited in quantity; the main energy reserves of glycogen and triacylglycerols are stored in the fat body (Ga¨ de and ARTICLE IN PRESS www.elsevier.com/locate/ibmb 0965-1748/$-see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ibmb.2005.01.010 à Corresponding author. Tel.: +27216503624; fax: +27216503301. E-mail address: lauerswa@botzoo.uct.ac.za (L. Auerswald).