CL-proteins and the regulation of lipoprotein lipase activity in locust flight muscle

Lipoprotein lipases in the flight muscles of Locusta migratoria show a marked substrate specificity: diacylglycerols associated with the adipokinetic hormone (AKH)-induced lipoprotein, A+, are hydrolysed at 4 to 5 times the rate of those associated with the lipoprotein in resting (non-hormone-stimulated) locusts, Ayellow. To determine the basis for this discrimination, the effect on the activity of flight muscle lipoprotein lipase of CL-proteins, a major constituent of lipoprotein A+, but not of Ayellow, has been investigated; they inhibit the flight muscle enzyme in a competitive manner whether activity is measured with a natural lipoprotein substrate, a lipid emulsion or a water soluble substrate. Experiments in vivo suggest that the flight muscle enzyme is normally inhibited in resting (non-AKH-stimulated) locusts but, interestingly, injection of synthetic AKH-I relieves the inhibition and increases the activity by 30 to 40%. This is not a direct effect of the hormone on the enzyme, but appears to be related to the hormone-induced formation of lipoprotein A+, so that the majority of CL-proteins in the haemolymph become bound to this lipoprotein and the concentration of free CL-proteins is markedly reduced. We suggest that CL-proteins play a major role in the regulation of lipoprotein lipase in locust flight muscle.