Insect lipid transfer particle can facilitate net vectorial lipid transfer via a carrier-mediated mechanism

The mechanism of facilitated lipid transfer by insect or mammalian plasma lipid transfer proteins has not been elucidated. Transfer catalysts may act as carriers of lipid between donor and acceptor lipoproteins or, alternatively, transfer may require formation of a ternary complex. This study was designed to determine if Manduca sexta hemolymph lipid transfer particle (LTP) can facilitate net vectorial transfer of lipid without concomitant contact between donor and acceptor lipoproteins and LTP. M. sexta [3H]diacylglycerol-high density lipophorin-larval ([3H]DAG-HDLp-L) and human low density lipoprotein (LDL) were covalently bound to Sepharose matrices and packed into separate columns. In incubations lacking LTP, greater than 98% of the recovered DAG remained associated with HDLp-L. An unrelated hemolymph storage protein, arylphorin, was unable to catalyze the transfer of DAG between solid-phase lipoproteins. Facilitated transfer of DAG from HDLp-L to LDL was observed when LTP was circulated between the columns. Under these conditions, facilitated transfer occurred at a rate of 2.24 ng of DAG/h (versus 0.16 microgram of DAG/h in the control), and after 16 h greater than 26% of recovered labeled DAG was transferred to LDL. This corresponds to a 14-fold rate enhancement induced by LTP. The LTP-specific transfer of DAG between physically separated lipoproteins demonstrates the ability of LTP to facilitate net lipid transfer via a carrier-mediated mechanism in the absence of a ternary complex involving donor, acceptor, and catalyst. In experiments aimed at assessing the relative contribution of ternary complex formation to DAG transfer, acceptor LDL was circulated with HDLp-L remaining immobilized. Under these conditions, LTP induced a 13-fold rate enhancement from 1.3 to 16.3 microgram of DAG/h. The similar rate enhancements observed with both lipoproteins bound and only donor bound suggest the overall contribution of ternary complex formation to facilitated lipid transfer is insignificant. The described system should prove useful in mechanistic studies of other transfer proteins as well as studies of transfer of other lipids.