Ad M.Th. Beenakkers

Role of Lipids in Energy Metabolism

Most reviews of lipid metabolism in insects have covered all classes of lipid compounds. Since lipids are generally defined as substances poorly soluble in water but soluble in organic solvents, the authors had to deal with compounds with divergent physiological functions, e.g., phospholipids and pheromones. The subject of this chapter limits the lipoidal substances to be discussed to those that provide a direct source of metabolic energy, though we recognize that other important lipid classes, for example those contributing to (sub)cellular components, are also essential for metabolism.

Formation of partially phosphorylated glycogen phosphorylase in the fat body of the migratory locust

Abstract Glycogen phosphorylase b partially purified from fat body of migratory locusts was incubated with phosphorylase kinase and [γ- 32 P]ATP. DEAE-Sephacel chromatography revealed the conversion of phosphorylase b (absolute AMP dependence for activity) to almost equal amounts of phosphorylases ab (high AMP dependence) and a (largely AMP independent). Electrophoresis and autoradiography showed the incorporation of 32 P into the ab and a forms, and its absence from phosphorylase b . The labeling of phosphorylases ab and a , both being dimers of identical subunits, corresponded to respectively 1 and 2 mol phosphate incorporated/mol of enzyme. This suggests that in phosphorylase a both subunits are phosphorylated with one single phosphate group/subunit, while in phosphorylase ab only one subunit is phosphorylated.

Binding of lipophorin to the fat body of the migratory locust

The interaction of locust high density lipophorin (HDLp) with pieces of fat body tissue was studied at 33°C using a radiolabelled ligand binding assay. Under the assay conditions, binding of tritium-labelled HDLp ((3H)HDLp) was demonstrated to correlate linearly with tissue concentration up to ~ 7 mg of fat body protein per ml of incubation medium. The (3H)HDLp binding that was displace.able by a 20-fold excess of unlabelled HDLp (which is an approximation of the specific binding) reached equilibrium after ~ 2 h, whereas low levels of non-displaceable binding increased linearly during this time interval. Analysis of the concentration dependent total binding of (3H)HDLp revealed the presence of a specific binding site with an equilibrium dissociation constant of Ko = 3.1 ( + 0.5) x 10 -7 M and a maximal binding capacity of 9.8 (+0.5)ng/zg-~ tissue protein. Competition experiments demonstrated that the affinity of unlabelled HDLp for the binding site is similar to the affinity of (3H)HDLp. Unlabelled low density lipophorin (LDLp), however, was shown to have an approx. 20-fold lower affinity for the binding site. Key Word Index: Locusta migratoria, lipophorin0 lipophorin receptor, lipid transfer, fat body

Lipophorin conversions during flight of the death's-head hawkmoth Acherontia atropos

Abstract Flight activity or injection of the deaths-head hawkmoth Acherontia atropos with locust synthetic adipokinetic hormone (AKH I) results in a dramatic increase in the concentration of hemolymph diacylglycerol which is carried by specific lipophorins. In resting hawkmoths diacylglycerols are associated with a high-density lipophorin (HDLp, density ∼1.13 g/ml) consisting of two major apolipophorins (apoLp-I and -II, mol. wt ∼240,000 and 70,000, respectively). During flight or after AKH injection the formation of a new low-density lipophorin is induced (LDLp, density ∼1.03 g/ml), exhibiting a much higher lipid loading and consisting of HDLp subunits and an additional subunit (apoLp-III, mol. wt approx. 20,000). This subunit is a regular constitutent of hemolymph proteins in resting hawkmoths and consists of two protein components with slightly different molecular weights. The component with the lowest molecular weight seems to be preferentially incorporated into the newly generated LDLp. In the resting situation the HDLp already contains some apoLp-III. In spite of some minor differences, the overall mechanism of lipophorin rearrangements upon flight activity in the hawkmoth appears to be very similar to the known systems established for both Locusta migratoria and Manduca sexta .

Occurrence of three forms of glycogen phosphorylase and flight-induced enzyme activation in fat body of the american cockroach, Periplaneta americana

1. 1. Ion exchange chromatography of fat body glycogen phosphorylase of Periplaneta americana yielded three activity peaks, identified as phosphorylases b, ab and a, with - AMP/+AMP activity ratios of 0, 10 and 49%, respectively. 2. 2. In cockroaches at rest, phosphorylase occurred largely in the inactive b form, while both flight and corpus cardiacum extract caused a strong increase in the relative proportions of the active a and intermediate ab forms. 3. 3. After a post-flight resting period of 1 hr, phosphorylase activity in Periplaneta was still high, while in Locusta migratoria the enzyme activity had returned to the pre-flight resting level. 4. 4. Results are discussed in relation to the regulation of substrate mobilization for flight.