Molecular Cloning and Expression of Lipid Transfer Inhibitor Protein Reveals Its Identity with Apolipoprotein F

Published studies demonstrate that lipid transfer inhibitor protein (LTIP) is an important regulator of cholesteryl ester transfer protein (CETP) activity. Although LTIP inhibits CETP activity among different lipoprotein classes, it preferentially suppresses transfer events involving low density lipoprotein (LDL), whereas transfers involving high density lipoprotein as donor are less affected. In this study, we report the purification of LTIP and the expression of its cDNA in cultured cells. Purification of LTIP, in contrast to other published protocols, took advantage of the tight association of this protein with LDL. Ultracentrifugally isolated LDL was further purified on anti-apoE and apoA-I affinity columns. Affinity purified LDL was delipidated by tetramethylurea, and the tetramethylurea-soluble proteins were separated by SDS-polyacrylamide gel electrophoresis. The protein migrating at a molecular mass of ∼33 kDa was excised from the gel and its N-terminal amino acid sequence determined. The 14-amino acid sequence obtained showed complete homology with the sequence deduced for apolipoprotein F (apoF) cDNA isolated from Hep G2 cells. On Western blots, peptide-specific antibodies raised against synthetic fragments of apoF reacted with the same 33-kDa protein in LTIP-containing fractions purified from LDL and from lipoprotein-deficient plasma. In contrast to that previously reported, apoF was shown to be associated almost exclusively with LDL, identical to the distribution of LTIP activity. The cDNA for apoF was cloned from a human liver cDNA library, ligated into a mammalian expression vector, and transiently transfected into COS-7 cells. Conditioned media containing secreted apoF demonstrated CETP inhibitor activity, whereas cells transfected with vector alone did not. This CETP inhibitor activity was efficiently removed from the media by nickel-Sepharose, consistent with the 6-His tag incorporated into recombinant apoF. By Western blot, the 6-His-tagged protein had a molecular weight slightly larger than native apoF. The CETP inhibitor activity of recombinant apoF possessed the same LDL specificity, oleate sensitivity, and dependence on lipoprotein concentration as previously noted for LTIP. We conclude that LTIP and apoF are identical. Published studies demonstrate that lipid transfer inhibitor protein (LTIP) is an important regulator of cholesteryl ester transfer protein (CETP) activity. Although LTIP inhibits CETP activity among different lipoprotein classes, it preferentially suppresses transfer events involving low density lipoprotein (LDL), whereas transfers involving high density lipoprotein as donor are less affected. In this study, we report the purification of LTIP and the expression of its cDNA in cultured cells. Purification of LTIP, in contrast to other published protocols, took advantage of the tight association of this protein with LDL. Ultracentrifugally isolated LDL was further purified on anti-apoE and apoA-I affinity columns. Affinity purified LDL was delipidated by tetramethylurea, and the tetramethylurea-soluble proteins were separated by SDS-polyacrylamide gel electrophoresis. The protein migrating at a molecular mass of ∼33 kDa was excised from the gel and its N-terminal amino acid sequence determined. The 14-amino acid sequence obtained showed complete homology with the sequence deduced for apolipoprotein F (apoF) cDNA isolated from Hep G2 cells. On Western blots, peptide-specific antibodies raised against synthetic fragments of apoF reacted with the same 33-kDa protein in LTIP-containing fractions purified from LDL and from lipoprotein-deficient plasma. In contrast to that previously reported, apoF was shown to be associated almost exclusively with LDL, identical to the distribution of LTIP activity. The cDNA for apoF was cloned from a human liver cDNA library, ligated into a mammalian expression vector, and transiently transfected into COS-7 cells. Conditioned media containing secreted apoF demonstrated CETP inhibitor activity, whereas cells transfected with vector alone did not. This CETP inhibitor activity was efficiently removed from the media by nickel-Sepharose, consistent with the 6-His tag incorporated into recombinant apoF. By Western blot, the 6-His-tagged protein had a molecular weight slightly larger than native apoF. The CETP inhibitor activity of recombinant apoF possessed the same LDL specificity, oleate sensitivity, and dependence on lipoprotein concentration as previously noted for LTIP. We conclude that LTIP and apoF are identical. One of us (R. E. Morton), in collaboration with D. B. Zilversmit, was the first to identify and partially characterize a novel protein in human plasma that inhibits cholesteryl ester transfer protein (CETP) 1The abbreviations used are: CETP, cholesteryl ester transfer protein; LTIP, lipid transfer inhibitor protein; CE, cholesteryl ester; TG, triglyceride; VLDL, very low density lipoprotein; LDL, low density lipoprotein; HDL, high density lipoprotein; apoF, apolipoprotein F; TMU, 1,1,3,3-tetramethylurea; BSA, bovine serum albumin; PCR, polymerase chain reaction; PAGE, polyacrylamide gel electrophoresis; CAPS, 3-(cyclohexylamino)propanesulfonic acid. activityin vitro (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar). Subsequent reports by Son and Zilversmit (2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar) and by Nishide et al. (3Nishide T. Tollefson J.H. Albers J.J. J. Lipid Res. 1989; 30: 149-158Abstract Full Text PDF PubMed Google Scholar) have confirmed and extended the initial characterization of this factor as a unique ∼30,000-35,000 molecular weight acidic glycoprotein. This protein, LTIP, equally inhibits the CETP-mediated transfer of TG and CE (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar, 2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar). The mechanism of inhibition has not been studied in detail; however, limited studies by us and others (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar, 2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar, 4Morton R.E. J. Biol. Chem. 1985; 260: 12593-12599Abstract Full Text PDF PubMed Google Scholar) suggest that LTIP disrupts the association of CETP with lipoproteins. In contrast to the effects of other apoproteins that have been shown to possess CETP inhibitory activity when assayed at high concentrationsin vitro (2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar, 5Lagrost L. Perségol L. Lallemant C. Gambert P. J. Biol. Chem. 1994; 269: 3189-3197Abstract Full Text PDF PubMed Google Scholar, 6Guyard-Dangremont V. Lagrost L. Gambert P. J. Lipid Res. 1994; 35: 982-992Abstract Full Text PDF PubMed Google Scholar, 7Connolly D.T. Krul E.S. Heuvelman D. Glenn K.C. Biochim. Biophys. Acta. 1996; 1304: 145-160Crossref PubMed Scopus (21) Google Scholar, 8Kushwaha R.S. Hasan S.Q. McGill Jr., H.C. Getz G.S. Dunham R.G. Kanda P. J. Lipid Res. 1993; 34: 1285-1297Abstract Full Text PDF PubMed Google Scholar), LTIP shows lipoprotein specificity in its inhibitory capacity. LTIP preferentially suppresses lipid transfers involving LDL (1.019 < d <1.063 g/ml)(9, 10) while having the least effect on transfers between VLDL (d <1.006 g/ml) and HDL (1.063 < d <1.21 g/ml). In mass lipid transfer assays with whole plasma, LTIP supplementation leads to decreased efflux of CE from LDL to VLDL and increased efflux of CE from HDL to VLDL, resulting in HDL that are better substrates for lecithin-cholesterol acyltransferase (10Morton R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). We have that this plasma TG, not CETP, is for lipid transfer in plasma J. PubMed Scopus Google Scholar, T. L. Biol. PubMed Scopus Google Scholar). in the of LTIP, less TG is by transfer to LDL, transfer from VLDL to HDL to be LTIP activity in whole plasma with the of transfer between VLDL and LDL and with HDL R.E. 1993; PubMed Scopus Google Scholar). studies have in a in LTIP activity R.E. Biol. PubMed Scopus Google Scholar). we have shown that CETP has for lipid transfer from LDL HDL the transfer of CE from HDL in plasma is a of LTIP activity R.E. Scholar). of LTIP R.E. Greene D.J. Biol. PubMed Scopus Google Scholar), that LTIP activity be by suggest that LTIP the of lipid transfer events between by as a lipid between while others to the of lipid transfer activity in plasma is by CETP and LTIP in the to proteins that CETP have the that a protein is for not of the inhibitory activity in whole human plasma. Purification of LTIP has been by its to with other plasma (10Morton R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar), with assays by low of used to has limited the that be used to for further In this report we have an to LTIP purification that advantage of that LTIP activity is associated with LDL in plasma (10Morton R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar) and that a of this LTIP activity is on LDL its by We report the of LTIP from human LDL and the and expression of its was from and for and were obtained from and were from and was from was by of and at in the and protein were from human plasma from the of the was the of VLDL, LDL, and were isolated by J.H. J. 34: PubMed Scopus Google Scholar), against and at were on In were with by a (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar). was isolated from human plasma by a of HDL and of human Acta. PubMed Scopus Google Scholar). purified CETP was isolated from lipoprotein-deficient human plasma by and as previously R.E. Zilversmit D.B. J. Lipid Res. Full Text PDF PubMed Google Scholar) and in LTIP was partially purified from lipoprotein-deficient plasma as previously (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar, R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). for CETP and LTIP activity were to previously published Zilversmit D.B. Biochim. Biophys. Acta. PubMed Scopus Google Scholar, R.E. Zilversmit D.B. Biochim. Biophys. Acta. 1981; PubMed Scopus Google Scholar, R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). CETP activity was in assays of lipid transfer between LDL and HDL of the of the LDL and HDL were separated by the Zilversmit D.B. Biochim. Biophys. Acta. PubMed Scopus Google Scholar, R.E. Zilversmit D.B. Biochim. Biophys. Acta. 1981; PubMed Scopus Google Scholar). CETP activity was from the of to the and as Zilversmit D.B. Biochim. Biophys. Acta. PubMed Scopus Google Scholar). CETP inhibitor activity was in the assays in assays lipid transfer was between LDL and LDL R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). were separated by with activity is as the inhibition of a of CETP to the apoA-I and anti-apoE were purified from by affinity on protein as by the fractions were and to of the fragments of apoF were by a antibodies to this were in as previously for apoF isolated from delipidated HDL by E. P. PubMed Scopus Google Scholar), was the of human plasma was obtained from the of the and LDL was isolated by as the < d < J.H. J. 34: PubMed Scopus Google Scholar). LDL was LDL was further purified by on affinity columns. of and were and at with LDL was to the and by at a of fractions containing LDL, as by were and by as the were for by with with by the same at of LDL was by a of the et al. T. J. PubMed Scopus Google Scholar). in a affinity purified LDL of was to with and at for was to and at in a the at the of the a the proteins were against and were for as by the were with and for at were and at for were by Western proteins in were to at as by et al. T. J. PubMed Scopus Google Scholar). were with in reacted with the by with were by with and to apoproteins on in serum in isolated were by gel J. J. Biol. Chem. Full Text PDF PubMed Google Scholar). were to was with by for were with and in for the and in the important to lipid in the lipoproteins. were by with as were and by with J. J. Biol. Chem. Full Text PDF PubMed Google Scholar). on a 14-amino acid N-terminal sequence obtained for the 33-kDa protein from LDL, a was The and used to a human liver cDNA by of cDNA with cDNA The was ligated into a by and into to the by the was with a the Res. PubMed Scopus Google Scholar). on homology of with apoF Albers J.J. Biophys. Res. 1994; PubMed Scopus Google Scholar), were to of the cDNA of apoF for amino to the complete sequence of the secreted was into a mammalian expression vector, to the and of apoF with COS-7 cells were transfected by the in cells were in of the with bovine serum and at in a the cells were and of in a of of with for cells were and in media for The media were on and CETP inhibitory activity in the media determined. was by the of et al. J. Biol. Chem. Full Text PDF PubMed Google Scholar) as by PubMed Scopus Google Scholar), with as of was assayed by a amino acid proteins were to by in transfer CAPS, at for P. to and Purification for Google Scholar). proteins were by with in and of were N-terminal protein sequence was with an protein with a and a P. J. PubMed Scopus Google Scholar, PubMed Scopus Google Scholar). the purification of LTIP by an that the for (10Morton R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar), we a to this protein from LDL, LTIP activity in native plasma (10Morton R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). on the of LTIP activity in the d with that in lipoprotein-deficient plasma by LTIP from R.E. 1993; PubMed Scopus Google we have that of plasma LTIP activity is in this lipoprotein Although LDL a of its LTIP, this a of on PubMed Scopus Google Scholar). with isolated LDL, the purification affinity of and with antibodies to by T. J. PubMed Scopus Google Scholar) of the affinity purified LDL. The proteins from LDL inhibitory activity. This purification in purification of CETP inhibitor activity with The CETP inhibitor activity in the LDL the same for lipid transfers involving LDL and by oleate as we have previously for LTIP in the of LTIP isolated from lipoprotein-deficient plasma In contrast to the inhibitory activity in the and apolipoprotein suppresses CETP activity (2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar, R.E. Greene D.J. Biol. PubMed Scopus Google Scholar), was by oleate not The from LDL was further by SDS-polyacrylamide gel slightly kDa was in the VLDL HDL protein of molecular consistent with the association of LTIP with LDL. This molecular mass with that for LTIP on the of LTIP activity on gel (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar). Although other molecular weight proteins molecular were in the proteins were not in LTIP from lipoprotein-deficient plasma that not to the LTIP activity in the LDL protein the inhibitor activity in the LDL is to the 33-kDa of inhibitor protein was to CETP activity by in the LDL to LDL transfer of This to an of of LTIP activity from activity of purified not was purified from human plasma as in the LTIP activity was from the activity in the lipoprotein-deficient plasma by LTIP activity in isolated LDL was deduced from the of plasma LTIP activity to the of LTIP activity used the are from a purification with of activity of not determined. in a of CETP inhibitory inhibitor of inhibitor activity by of inhibitor in and transfer CETP inhibitor in of lipoprotein-deficient and LDL were activity was for in assays were the in the effect of oleate on the expression of inhibitor activity was determined. for of inhibitor in and transfer in a LTIP was purified from human plasma as in the LTIP activity was from the activity in the lipoprotein-deficient plasma by LTIP activity in isolated LDL was deduced from the of plasma LTIP activity to the of LTIP activity used the are from a purification with of plasma. The CETP inhibitor in of lipoprotein-deficient and LDL were activity was for in assays were the in the effect of oleate on the expression of inhibitor activity was determined. for transfer to the 33-kDa was excised and its N-terminal amino acid sequence determined. N-terminal sequence a sequence of of sequence complete homology of this 14-amino acid sequence with amino of human apoF Albers J.J. Biophys. Res. 1994; PubMed Scopus Google Scholar). acid to the N-terminal amino acid of secreted apoF, is from the of a larger other proteins of sequence were has a of is the acidic we and others (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar, 2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar) previously for partially purified LTIP fragments of apoF, on and of homology with other proteins in published were and into to antibodies against of apolipoprotein to amino of secreted apoF, and amino reacted with the same 33-kDa on of of CETP inhibitor activity and the to fragments of apoF be used to this in serum by and shown for was almost exclusively on LDL in whole serum was noted with VLDL, whereas HDL in whole serum in isolated not did not with the distribution of apoF in serum is the same as the distribution of LTIP activity in plasma by gel (10Morton R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). Although the high used in us to low of apoF were associated with other not the apoF of fractions to the in this on the of serum and isolated LDL we that isolated LDL of the apoF on this lipoprotein in with the to the d not This the of LTIP activity in the d of plasma with the LTIP activity in lipoprotein-deficient plasma by LTIP from R.E. 1993; PubMed Scopus Google Scholar, J. Lipid Res. Full Text PDF PubMed Google Scholar). This lipoprotein distribution for apoF is to that previously by and E. P. PubMed Scopus Google Scholar). a against to us by the for this was in the (d g/ml) of plasma, with in HDL and LDL not This distribution is from that obtained with antibodies against apoF fragments that are against different from et al. Albers J.J. Biophys. Res. 1994; PubMed Scopus Google Scholar) suggest that this with a of plasma We to that the against apoF by et is not for apoF, it apoF on with antibodies against synthetic fragments of apoF, we conclude that apoF is an be in other is important to that antibodies with apoF on LDL when the lipoprotein was did not with apoF on LDL in is antibodies were not to the of apoF when in not with antibodies were not to LTIP activity of inhibitory activity to a This that the for antibodies are not on native apoF. on the 14-amino acid sequence obtained for the 33-kDa shown we a and a human liver cDNA by of cDNA The was ligated into the vector and into E. The sequence of isolated was with the published sequence for the apoF cDNA cloned from Hep G2 cells Albers J.J. Biophys. Res. 1994; PubMed Scopus Google Scholar). The sequence of liver apoF was to Hep G2 the of this was to the of of the in the of the Hep G2 sequence were to of a cDNA of apoF for amino to the complete sequence of secreted apoF This was into a mammalian expression vector, to the and of apoF with into COS-7 cells and media from cells transiently transfected with vector containing apoF cDNA inhibitor activity that CETP in a whereas cells transfected with the vector alone not did not CETP activity. of media with to the 6-His-tagged recombinant apoF removed of the inhibitor activity. The proteins from the demonstrated CETP inhibitor activity The protein was recombinant apoF as by and The molecular weight of recombinant apoF was slightly than that of native apoF consistent with the amino incorporated into the apoF showed the same specificity for lipid transfer assays involving LDL and by oleate as previously noted for LTIP (10Morton R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar, R.E. Greene D.J. Biol. PubMed Scopus Google Scholar) and Although the of inhibitor in the and assays be for recombinant apoF for apoF and this was not on its protein by of recombinant apoF with proteins on we a activity of of protein, resulting in an for recombinant apoF of This is to the for native apoF the inhibitory activity of recombinant apoF was and to the of apoF equally the of CETP to TG and CE transfers of inhibition as was the for that inhibition is by CETP not by the of CETP to with lipid of CETP inhibitory activity in COS-7 the CETP inhibitor activity in assays transfer from LDL to and LDL to HDL was determined. are as LTIP and recombinant apoF media from COS-7 cells transiently activity are as that in of LTIP that in of media for recombinant apoF. LDL specificity is as the of are of and for LTIP and recombinant of inhibitor activity by LTIP and recombinant inhibitor were assayed by the LDL to in the of the of are the for (LTIP) dependence of inhibitor activity on the concentration of lipoproteins. LTIP and recombinant inhibitor were in assays lipid transfer from LDL to The of was as while an LDL to HDL of are the for of LTIP from studies on the of partially purified LTIP isolated from lipoprotein-deficient plasma. on of LTIP activity, we and others (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar, 2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar, T. Tollefson J.H. Albers J.J. J. Lipid Res. 1989; 30: 149-158Abstract Full Text PDF PubMed Google Scholar, R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar) have shown that LTIP is an acidic of molecular weight that is associated almost exclusively with LDL. This association is consistent with the that LTIP has activity CETP activity involving LDL with other lipoprotein substrates R.E. 1993; PubMed Scopus Google Scholar, R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). of LTIP, on its activity, are important for LTIP with CETP inhibitory activity isolated from plasma by other The in this report the of LTIP and apolipoprotein in molecular lipoprotein distribution in plasma, and in for a This is further by that of LTIP activity isolated by apoF. recombinant apoF CETP inhibitory activity that has specificity for transfer involving LDL and is by as has been for LTIP R.E. 1993; PubMed Scopus Google Scholar, R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar, R.E. Greene D.J. Biol. PubMed Scopus Google Scholar, R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). on of we conclude that LTIP is apoF. In this we the of et al. Albers J.J. Biophys. Res. 1994; PubMed Scopus Google Scholar) apoF is as the amino from the of a acid the of the it that this this has not been determined. The of LTIP as apoF a to this We suggest that this of apoF is unique among apoproteins the CETP inhibitory of better studied have not been by further studies in purified was shown in studies to CETP activity vitro assays L. Perségol L. Lallemant C. Gambert P. J. Biol. Chem. 1994; 269: 3189-3197Abstract Full Text PDF PubMed Google Scholar, 6Guyard-Dangremont V. Lagrost L. Gambert P. J. Lipid Res. 1994; 35: 982-992Abstract Full Text PDF PubMed Google Scholar, 7Connolly D.T. Krul E.S. Heuvelman D. Glenn K.C. Biochim. Biophys. Acta. 1996; 1304: 145-160Crossref PubMed Scopus (21) Google Scholar), CETP activity is not decreased in and CETP C. E. J. 1994; PubMed Scopus Google Scholar), and HDL containing apoA-I apoA-I and are in CETP P. C. T. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar). In other and CETP activity with of to (2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar, 7Connolly D.T. Krul E.S. Heuvelman D. Glenn K.C. Biochim. Biophys. Acta. 1996; 1304: 145-160Crossref PubMed Scopus (21) Google Scholar). of in CETP not plasma CETP activity in C. E. J. 1994; PubMed Scopus Google Scholar), and of human apoA-I in CETP leads to not CETP activity T. T. P. J. PubMed Scopus Google Scholar). with this in other studies isolated and transfer protein have been shown to CETP activity J. PubMed Scopus Google Scholar, J.H. Albers J.J. J. Lipid Res. Full Text PDF PubMed Google Scholar, T. Biochim. Biophys. Acta. PubMed Scopus Google Scholar, T. 1993; PubMed Scopus Google Scholar). The plasma CETP inhibitor amino of by et al. R.S. Hasan S.Q. McGill Jr., H.C. Getz G.S. Dunham R.G. Kanda P. J. Lipid Res. 1993; 34: 1285-1297Abstract Full Text PDF PubMed Google Scholar) suppresses CETP activity in with an of D.T. Krul E.S. Heuvelman D. Glenn K.C. Biochim. Biophys. Acta. 1996; 1304: 145-160Crossref PubMed Scopus (21) Google Scholar) it that it has in in human plasma are C. and of Scholar). the of apoproteins to CETP activity has been and not by in effects of isolated apoproteins have not been demonstrated in assays that the of whole plasma. By the inhibitory activity of has been shown in a of assays isolated synthetic and plasma as The for is than that for other In contrast to that previously reported, we demonstrate that apoF is an protein and that is associated with HDL in The that apoF be isolated from HDL that of this to this lipoprotein with the of apoF is the of apoF in HDL The association of with LDL and the that LTIP preferentially suppresses transfer with LDL that lipoprotein association is important to its This is consistent with limited that LTIP to and inhibits CETP by its association with the lipoprotein R.E. J. Biol. Chem. 1985; 260: 12593-12599Abstract Full Text PDF PubMed Google Scholar), and with and with published (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar, 2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar, 4Morton R.E. J. Biol. Chem. 1985; 260: 12593-12599Abstract Full Text PDF PubMed Google Scholar) that inhibitor activity is to lipoprotein native we that a of inhibitor to lipoprotein is to CETP activity by This that are a limited of and CETP for on the lipoprotein The plasma concentration of apoF has been to be P. PubMed Scopus Google Scholar), is to a that be from purification at a plasma LDL concentration of R.E. J. Biol. Chem. 1985; 260: 12593-12599Abstract Full Text PDF PubMed Google Scholar), apoF is to on LDL This that LDL in plasma CETP activity at a to the of apoF and CETP for on the lipoprotein The of lipoprotein association and inhibitory that LTIP activity be its to is by in lipoprotein In we have that a of is to the VLDL when plasma is at in the of lecithin-cholesterol acyltransferase E. The of lipoprotein that LTIP and the and that this are In we that apoF possess the same and as LTIP. The weight of leads to the that apoF is LTIP. This the first report of a for this on of LTIP from in vitro studies of its (1Morton R.E. Zilversmit D.B. J. Biol. Chem. 1981; 256: 11992-11995Abstract Full Text PDF PubMed Google Scholar, 2Son Y.-SC. Zilversmit D.B. Biochim. Biophys. Acta. 1984; 795: 473-480Crossref PubMed Scopus (58) Google Scholar, T. Tollefson J.H. Albers J.J. J. Lipid Res. 1989; 30: 149-158Abstract Full Text PDF PubMed Google Scholar, 4Morton R.E. J. Biol. Chem. 1985; 260: 12593-12599Abstract Full Text PDF PubMed Google Scholar, R.E. 1993; PubMed Scopus Google R.E. Greene D.J. J. Lipid Res. 1994; 35: Full Text PDF PubMed Google Scholar, R.E. Biol. PubMed Scopus Google Scholar, R.E. Scholar, R.E. Greene D.J. Biol. PubMed Scopus Google Scholar), we suggest that apoF is an important regulator of between plasma the of lipoprotein The in this report a of this than previously The of the is for N-terminal amino acid sequence and We P. for in protein sequence