HeLp, a heme lipoprotein from the hemolymph of the cattle tick, Boophilus microplus

The main protein of the hemolymph of the cattle tick Boophilus microplus ha s been isolated and shown to be a heme lipoprotein (HeLp). HeLp has an appa rent 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 phospholipi ds, and 3% of sugars. A remarkable feature of Help is the abundance of chol esterol ester (35% of total lipids), a lipid not previously reported in inv ertebrate lipoproteins. Western blot analysis showed Help in hemolymph from adult females and males, but not in eggs. Although Help contains 2 heme mo lecules, it is capable of binding 6 additional molecules of heme. Boophilus feeds large amount of blood, and we recently showed that this tick is unab le to perform de novo synthesis of heme (Braz, G, R. C., Coelho, H. S. L., Masuda, H., and Oliveira, P. L. (1999) Curr. Biol. 9, 703-706). Injection o f tick females with Fe-55-labeled heme-HeLp indicated that this protein tra nsports heme from hemolymph to tissues. Help is suggested to be an essentia l adaptation to the loss of the heme synthesis pathway.