Lipid transfer particle catalyzes transfer of carotenoids between lipophorins of Bombyx mori

The yellow color of Bombyx mori hemolymph is due to the presence of caroten oids, which are primarily associated with lipophorin particles. Carotenoids were extracted from high density lipophorin (HDLp) of B. mori and analyzed by HPLC. HDLp contained 33 mu g of carotenoids per mg protein. Over 90% of carotenoids were lutein while alpha-carotene and beta-carotene were minor components. When larval hemolymph was subjected to density gradient ultrace ntrifugation, a second minor yellow band was present, which was identified as B, mori lipid transfer particle (LTP). During other life stages examined however, this second band was not visible. To determine if coloration of L TP may fluctuate during development, we determined its concentration in hem olymph and compared it to that of lipophorin. Both proteins were present du ring all life stages and their concentrations gradually increased. The rati o of lipophorin: LTP was 10 similar to 15:1 during the fourth and fifth ins tar larval stages, and 20 similar to 30:1 during the pupal and adult stages . Thus, there was no correlation between the yellow color attributed to LTP and its hemolymph concentration. It is possible that yellow coloration of the LTP fraction corresponds to developmental stages when the particle is a ctive in carotene transport. To determine if LTP is capable of facilitating carotene transfer, we took advantage of a white hemolymph B. mori strain w hich, when fed artificial diet containing a low carotene content, gives ris e to a lipophorin that is nearly colorless. A spectrophotometric, carotene specific, transfer assay was developed which employed wild type, carotene-r ich HDLp as donor particle and colorless low density lipophorin, derived fr om the white hemolymph strain animals, as acceptor particle. In incubations lacking LTP carotenes remained associated with HDLp while inclusion of LTP induced a redistribution of carotenes between the donor and acceptor in a time and concentration dependent manner. Time course studies suggested the rate of LTP-mediated carotene transfer was relatively slow, requiring up to 4 h to reach equilibrium By contrast, studies employing H-3-diacylglycerol labeled HDLp as donor particle in lipid transfer assays revealed a rapid e quilibration of label between the particles. Thus, it is plausible that the slower rate of LTP-mediated carotene transfer is due to its probable seque stration in the core of HDLp. (C) 1998 Elsevier Science Ltd. All rights res erved.