HeLp, a Heme Lipoprotein from the Hemolymph of the Cattle Tick,Boophilus microplus

Abstract

The main protein of the hemolymph of the cattle tick <i>Boophilus microplus</i> has been isolated and shown to be a heme lipoprotein (HeLp). HeLp has an apparent molecular mass of 354,000 and contains two apoproteins (103 and 92 kDa) found in equal amounts. HeLp presents a pI of 5.8 and a density of 1.28 g/ml and contains 33% lipids, containing both neutral lipids and phospholipids, and 3% of sugars. A remarkable feature of HeLp is the abundance of cholesterol ester (35% of total lipids), a lipid not previously reported in invertebrate lipoproteins. Western blot analysis showed HeLp in hemolymph from adult females and males, but not in eggs. Although HeLp contains 2 heme molecules, it is capable of binding 6 additional molecules of heme. <i>Boophilus</i> feeds large amount of blood, and we recently showed that this tick is unable to perform<i>de novo</i> synthesis of heme (Braz, G. R. C., Coelho, H. S. L., Masuda, H., and Oliveira, P. L. (1999)<i>Curr. Biol.</i> 9, 703–706). Injection of tick females with⁵⁵Fe-labeled heme-HeLp indicated that this protein transports heme from hemolymph to tissues. HeLp is suggested to be an essential adaptation to the loss of the heme synthesis pathway.