(Received for publication, January 20, 1995; and in revised form, August 8, 1995)
From the
Hepatic lipase (HL) and lipoprotein lipase (LPL) are key enzymes
that mediate the hydrolysis of triglycerides (TG) and phospholipids
(PL) present in circulating plasma lipoproteins. Relative to
triacylglycerol hydrolysis, HL displays higher phospholipase activity
than LPL. The structural basis for this difference in substrate
specificity has not been definitively established. We recently
demonstrated that the 22-amino acid loops (``lids'') covering
the catalytic sites of LPL and HL are critical for the interaction with
lipid substrate (Dugi, K. A., Dichek, H. L., Talley, G. D., Brewer, H.
B., Jr., and Santamarina-Fojo, S.(1992) J. Biol. Chem. 267,
25086-25091). To determine whether the lipase lid plays a role in
conferring the different substrate specificities of HL and LPL, we have
generated four chimeric lipases. Characterization of these chimeric
enzymes using TG (triolein and tributyrin) or PL
(dioleoylphosphatidylcholine (DOPC) vesicles, DOPC proteoliposomes, and
DOPC-mixed liposomes) substrates demonstrated marked differences
between their relative PL/TG hydrolyzing activities. Chimeric LPL
containing the lid of HL had reduced triolein hydrolyzing activity (49%
of the wild type), but increased phospholipase activity in DOPC
vesicle, DOPC proteoliposome, and DOPC-mixed liposome assay systems
(443, 628, and 327% of wild-type LPL, respectively). In contrast,
chimeric HL containing the LPL lid was more active against triolein
(123% of the wild type) and less active against DOPC (23, 0, and 30%,
respectively) than normal HL. Similar results were obtained when the
lipase lids were exchanged in chimeric enzymes containing the
NH-terminal end of LPL and the COOH-terminal domain of HL.
Exchange of the LPL and HL lids resulted in a reversal of the
phospholipase/neutral lipase ratio, establishing the important role of
this region in mediating substrate specificity.
In summary, the lid covering the catalytic domains in LPL and HL plays a crucial role in determining lipase substrate specificity. The lid of LPL confers preferential triglyceride hydrolysis, whereas the lid of HL augments phospholipase activity. This study provides new insight into the structural basis for the observed in vivo differences in LPL and HL function.
Hepatic lipase (HL) ()and lipoprotein lipase (LPL)
are critical enzymes that play a central role in lipoprotein
metabolism. Their main function is to hydrolyze triglycerides and
phospholipids present in circulating plasma lipoproteins, including
chylomicrons, very low and intermediate density lipoproteins, and
HDL(1) . In the process, they generate free fatty acids, which
can be utilized for storage or generation of energy. By influencing HDL
metabolism(2, 3, 4) , these two lipases may
also play a role in reverse cholesterol transport.
Together with pancreatic lipase, HL and LPL are members of the human lipase family and share a high degree of primary sequence homology(5) . Their catalytic domain consists of a Ser-His-Asp triad(6, 7, 8) . Conservation of disulfide bonds suggests a similar folding pattern(9) , and by homology to the reported three-dimensional structure of pancreatic lipase(10) , LPL and HL may be organized into two distinct amino- and carboxyl-terminal domains. Both lipases are anchored to the capillary endothelium via glycosaminoglycans and can be released by intravenous administration of heparin(1, 11) .
There are, however, striking differences between LPL and HL. LPL is primarily synthesized by adipocytes, muscle cells, and macrophages, whereas HL mRNA is primarily detected in hepatocytes(12) . LPL is inhibited by 1 M NaCl (11) and requires its cofactor, apoC-II, for full activation(13, 14) . HL, on the other hand, is fully active even in the presence of high salt concentrations and in the absence of a cofactor, although its activity may be modulated by apoA-II (15, 16) and apoE(17, 18, 19) .
The two enzymes also
differ in their substrate specificity, demonstrating preferences in
their fatty acid positional specificity (20, 21) ,
fatty acid chain length(22, 23) , and degree of fatty
acid saturation(24, 25) . In addition, LPL and HL
differ in their preferred lipoprotein substrates. Thus, patients with
LPL deficiency accumulate primarily chylomicrons as well as very low
density lipoproteins in plasma(11) , whereas HL deficiency
results in elevated plasma concentrations of intermediate density
lipoproteins and HDL(26, 27) . Characterization of
these patients demonstrates that in vivo, the preferred
substrate of LPL are the large triglyceride-rich lipoproteins, whereas
HL is more active in the hydrolysis of smaller lipoproteins such as
HDL(28, 29) , especially HDL(30) ,
and intermediate density lipoproteins (26, 31, 32) . Another physiologically
important difference in the function of HL and LPL is the relative
phospholipase versus triacylglycerol hydrolase activity of the
two enzymes. Several studies have demonstrated that, relative to
triglyceride hydrolysis, HL is a more active phospholipase than
LPL(1, 33, 34, 35) . In fact,
inhibition of HL by infusion of antisera in cynomolgus monkeys led to a
marked accumulation of not only triglycerides, but also of
phospholipids in different lipoprotein fractions, especially
HDL(36) . Similar changes in plasma phospholipid levels have
been observed in some patients with HL deficiency(32) , but not
with LPL deficiency(37, 38) . This difference in
relative phospholipase to triacylglycerol hydrolase activity of the two
enzymes may be important for modulating the function of LPL and HL in vivo.
Recent studies involving the analysis of chimeric LPL-HL mutants (39, 40, 41) have suggested that the COOH-terminal domains of LPL and HL (Fig. 1) may play a role in determining the preferred substrate of the respective lipase(42) . In addition, removal of the COOH-terminal 58 amino acids of LPL by chymotryptic cleavage resulted in the inability of LPL to bind to chylomicrons(43) , suggesting that this region of the enzyme may mediate binding to lipoproteins. However, to date, the structural basis for the differences in substrate specificity between the two lipases remains poorly defined. We have recently demonstrated that the lid covering the catalytic domain of the human lipases (Fig. 1) is critically involved in the interaction of the lipases with their lipid substrate(42) . In this report, we investigate the role of the lipase lid in determining the substrate specificity of HL and LPL for phospholipids and triglycerides by generating four different chimeric lipases in which the LPL and HL lids have been exchanged. Our studies demonstrate that the LPL lid enhances triglyceride hydrolysis, whereas the HL lid augments phospholipase function, thus establishing the important role of the lid in mediating lipase substrate specificity.
Figure 1:
Schematic representation of the
structure of the -carbon backbone of horse pancreatic lipase as
reported by Bourne et al.(66) . The loop covering the
catalytic pocket and the COOH-terminal domain are highlighted.
Residues of the catalytic triad (corresponding to Ser-132, Asp-156, and
His-241 in human LPL) and the disulfide bridges are represented as ball-and-sticks. Solid arrows represent
-sheet
structures. The region of highest homology between pancreatic lipase on
one hand and HL and LPL on the other hand is the
NH
-terminal region, whereas the lid region and the
COOH-terminal domain are less strictly conserved. The drawing was
generated with the program
MOLSCRIPT(67) .
To evaluate a potential role of the lipase lid in modulating the substrate specificities of LPL and HL, we first established the importance of the lid region in the hydrolysis of different lipase substrates. We had previously demonstrated that the integrity of the amphipathic helices in the lipase lid is essential for the hydrolysis of water-insoluble long chain fatty acid triglycerides(42) . In a similar manner, we now investigated the role of the amphipathic helices in the hydrolysis of water-insoluble phospholipid substrates. Table 1summarizes the concentration as well as activity in the medium of cells transfected with wild-type LPL, wild-type HL, or lipase-lid mutant plasmids. Disruption of the amphipathicity of helix 1 or helix 2 of the lid or deletion of the helices destroyed the ability of the lipases to hydrolyze water-insoluble triglycerides, while the esterase activity remained intact. Extensive rearrangement of both helices in HL and LPL markedly reduced esterase activity, possibly by reducing the ability of the lid to change to the open conformation upon interfacial activation(42) . The tributyrin activity, however, was still 5 times above background, whereas the more sensitive triolein assay revealed absent activity. These results indicate that the disruption of both helices in HL and LPL selectively prevented the hydrolysis of liposoluble substrate. Similarly, disruption of the amphipathic properties of the lipase lid abolished the ability of all mutant enzymes to hydrolyze phospholipid substrate presented as a DOPC vesicle (Table 1). Taken together, these studies demonstrate that the lipase lid is essential for the hydrolysis of not only liposoluble triglycerides, but also of liposoluble phospholipid substrates.
To
investigate a potential role of the lid in conferring substrate
specificity, we generated several mutant lipases. As illustrated in Fig. 2, in Mut I, the lid of LPL was replaced by the lid of HL,
and in the reciprocal mutant (Mut II), the lid of HL was replaced by
the lid of LPL. Mut III is a chimeric lipase in which the
carboxyl-terminal 134 amino acids of human LPL were replaced by the
carboxyl-terminal 146 amino acids of human HL. Mut IV is a chimeric
lipase that contains the NH-terminal domain of LPL and the
COOH-terminal domain as well as the lid of HL. In addition to these
four chimeric constructs, plasmids containing wild-type LPL and HL
cDNAs and a negative control consisting of the parent vector were
transfected into 293 cells. Greater than 95% of the total triolein
hydrolyzing activity was found in the culture medium of all transfected
cells, indicating that the mutant lipases were secreted to a similar
extent as wild-type LPL and HL (data not shown).
Figure 2:
Schematic representation of wild-type and
mutant LPL and HL constructs used for the analysis of substrate
specificity. The name as well as a general description of the
constructs utilized in this study are listed. LPL sequences are shown
in white, and HL sequences are shown in black. The
numbers indicate the amino acid residues of the particular lipase
comprising the NH-terminal domain of the respective
construct.
Table 2summarizes the tributyrin, triolein, and phospholipid hydrolyzing activities in the medium of 293 cells transfected with the different plasmids. Tributyrin is water-soluble at the concentrations used in the assay and thus measures the esterase function of the lipases. Table 2shows that substitution of the COOH-terminal domain of LPL with that of HL (Mut III) leads to a parallel reduction of tributyrin and triolein activities compared with LPL WT possibly due to a destabilization of the active dimer(40) . Comparison of Mut I with LPL WT, Mut II with HL WT, and Mut IV with Mut III indicates that despite the exchange of the lids, the catalytic domain and esterase function of the mutant lipases are preserved. Further analysis reveals that the presence of the HL lid (Mut I versus LPL WT and Mut IV versus Mut III) increased phospholipid and lowered triolein hydrolysis, whereas the presence of the LPL lid (Mut II versus HL WT and Mut III versus Mut IV) had the opposite effect.
Similar results were obtained when the triacylglycerol hydrolase and phospholipase activities of the different lipases were directly compared after normalization for tributyrin (esterase) activity (Fig. 3). The ability of wild-type LPL and HL as well as Mut I-IV to hydrolyze triglyceride (triolein) versus phospholipid (DOPC vesicles, DOPC proteoliposomes, and DOPC-mixed liposomes) substrates is summarized in Fig. 3(A-E). Replacement of the LPL lid with the lid of HL (Mut I) resulted in a 51% reduction in the ability of mutant LPL to hydrolyze triolein as well as a 317-618% increase in the phospholipid hydrolysis relative to that of wild-type LPL (Fig. 3A). Conversely, replacement of the HL lid with that of LPL (Mut II) led to a 23% increase in triolein hydrolysis and a reduction of phospholipase activity to <30% of wild-type HL (Fig. 3B).
Figure 3: Triolein and DOPC hydrolyzing activities of wild-type and mutant lipases. Activities are normalized for esterase (tributyrin hydrolyzing) activity and presented as percent of wild-type lipase activity. Wild-type lipase is shown as open bars, and mutant lipase as closed bars. Standard deviations are calculated from three transfection experiments. A, LPL WT and Mut I; B, HL WT and Mut II; C, LPL WT and Mut III; D, LPL WT and Mut IV; E, Mut III and Mut IV. In A, D, and E, hatch marks on the x axis indicate the presence of separate scales for triolein and DOPC activities.
Previous studies have proposed a role of the COOH-terminal domain of the human lipases in mediating the interaction with lipid substrates (40, 41) . We thus analyzed a mutant LPL in which the COOH-terminal domain was replaced by the COOH-terminal domain of HL (Mut III); the lipase lid in this chimeric enzyme is from LPL. Fig. 3C shows that the presence of the COOH-terminal domain of HL in LPL leads to only a slight decrease in the ability to hydrolyze long chain fatty acid triglycerides. The phospholipase activity of Mut III, however, was significantly reduced using all three different phospholipid substrates. To examine further the role of the lipase lid in modulating lipase substrate specificity, we replaced the lid of LPL in Mut III with the lid of HL to create a double mutant of LPL. In this case, both the lid and the COOH-terminal domain of LPL are replaced by the analogous structures of HL. Fig. 3D demonstrates that compared with wild-type LPL, the double mutation leads to a 50% reduction in the ability of the lipase to hydrolyze water-insoluble triglyceride substrate and up to a >5-fold increase in phospholipase activity, depending on the DOPC substrate used. When the ability of the double mutant LPL Mut IV to hydrolyze different substrates was directly compared with that of the chimera Mut III (Fig. 3E), a 42% reduction of triolein activity but up to a 13-fold increase in phospholipase activity were evident. Thus, the additional presence of the HL lid again resulted in marked enhancement of phospholipase function, stressing the importance of the lipase lid in conferring substrate specificity.
Table 3summarizes the DOPC/triolein ratios for wild-type LPL and HL as well as the mutant lipases using either the DOPC vesicle or DOPC-mixed liposome substrate as a measure of phospholipase activity. Compared with native LPL, the ratio for native HL is 5-7-fold greater. To facilitate the evaluation of the role the lipase lid plays in determining substrate specificity, the data were separated into two groups. In the top half of Table 3, all lipases containing the LPL lid are listed: wild-type LPL, HL with the LPL lid (Mut II), and LPL with the COOH-terminal domain of HL (Mut III). The constructs containing the lid of hepatic lipase are listed in the bottom half of Table 3: wild-type HL, LPL with the HL lid (Mut I), and LPL with the COOH-terminal domain and lid of HL (Mut IV). When normalized for wild-type LPL, all constructs containing the lid of LPL have a DOPC/triolein ratio of 1.4 or less, whereas the constructs with the HL lid have a ratio of 3.5-11.5. Analysis of Mut I and Mut II demonstrates that the exchange of the lipase lid leads to a reversal of DOPC/triolein ratios of LPL and HL, indicating that the lid may be the single most important region in conferring lipase substrate specificity.
In this study, we have investigated a structural motif in LPL and HL that mediates the different ability of the two enzymes to hydrolyze triglyceride versus phospholipid substrates. Relative to triolein hydrolyzing activity, HL has been shown to be a more potent phospholipase than LPL in vitro(1, 33, 34, 35) . This difference in substrate specificity may play an important role in the biological function of LPL and HL (35) and becomes apparent in the markedly different phenotype observed in LPL (37, 38) and HL (36) deficiency states. We have recently demonstrated that the 22-amino acid lid that covers the catalytic site (Fig. 1) plays a crucial role in mediating the interaction of the lipases with emulsified triglycerides. Since there is little sequence homology between the LPL and HL lids(5) , we postulated that the different primary structures of the lids of LPL and HL may, in part, modulate lipase substrate specificity(42) .
The data presented in Table 1demonstrate that the lids of LPL and HL are important for phospholipid as well as triglyceride hydrolysis. This is in contrast to guinea pig pancreatic lipase, which contains only a ``minilid'' of 5 amino acids, but is still able to hydrolyze phospholipid substrate(61) . Our mutant in which the 22-amino acid lid was replaced by a 4-amino acid peptide had absent phospholipase activity even though its esterase activity was >2-fold higher than that of wild-type LPL. This, together with the data from the other mutants listed in Table 1, indicates that in human LPL and HL, the lid plays a crucial role in triglyceride as well as phospholipid hydrolysis.
Analysis of Mut I and Mut II (Fig. 3, A and B) demonstrates that the simple
exchange of the lids between LPL and HL markedly altered the ability of
the lipases to hydrolyze certain substrates. The lid of HL increased
the phospholipase and reduced the triglyceride hydrolase activity,
whereas the lid of LPL had the opposite effect. Similar findings were
obtained in lipase mutants in which the COOH termini alone or both the
COOH termini and the lids of LPL and HL were exchanged. Analysis of the
chimeric enzyme containing the NH-terminal domain of human
LPL and the COOH-terminal domain of human HL (Mut III) demonstrated a
slight reduction of triolein as well as a significant decrease in DOPC
hydrolyzing activity compared with normal LPL (Fig. 3C). This finding is consistent with previous
studies (39, 40, 43) that suggest that the
COOH-terminal domain of LPL may play a role in mediating the initial
interaction of the lipase with lipoproteins and perhaps modulate the
major type of particle with which either lipase will
interact(42) . Davis et al.(41) conclude from
their analysis of similar chimeric enzymes in which the COOH-terminal
domains of LPL and HL are exchanged that the COOH-terminal domain of HL
augments phospholipase function. Based on the results presented here,
the lid appears to be the more important determinant of phospholipase
hydrolyzing activity. Analysis of the data reported by Davis et al.(41) indicates that replacement of the
NH
-terminal domain of HL with the NH
-terminal
domain and thus the lid of LPL resulted in a 2.5-fold reduction in the
dipalmitoylphosphatidylcholine/triolein ratio. Replacement of the
NH
-terminal domain of LPL with that of HL, on the other
hand, led to a 20-fold increase in this ratio. In addition, direct
comparison of the activity of our Mut III with that of Mut IV, the
latter containing the COOH-terminal domain as well as the lid of HL in
the LPL backbone (Fig. 3E), demonstrates that the
presence of the HL lid in this chimera leads to a 40% reduction of
triolein activity and up to a 13-fold increase in DOPC hydrolyzing
activity. Thus, the presence of the HL lid markedly enhanced
phospholipase function in Mut IV, indicating that the lid may be more
important in determining phospholipase function than the COOH terminus.
Analysis of the DOPC/triolein ratios as an indicator of the relative
phospholipase/triglyceride hydrolase function of normal and mutant
lipases demonstrated that this ratio is higher for HL than for LPL (Table 3). Our studies clearly demonstrate that whenever the LPL
lid was present (Mut II and Mut III) (Table 3), the DOPC/triolein
ratio was similar to that of LPL (1.4). Conversely, whenever the HL
lid was present (Mut I and Mut IV) (Table 3), the ratio increased
to >3.5, similar to that of normal HL. Thus, regardless of the type
of COOH terminus or lipase backbone present, exchange of the lids
between the lipases resulted in a reversal of the
phospholipase/triglyceride hydrolase ratios of LPL and HL.
The mechanism by which the 22-amino acid lid mediates the different substrate specificities of LPL and HL may be complex. Previous studies have demonstrated that the lid must be repositioned to allow access of substrate to the catalytic pocket(10) . We have suggested that, upon opening of the lid, the hydrophobic sides of its amphipathic helices are exposed(42) , thus creating a large hydrophobic area, which may then function as a binding site for lipid substrates. A similar mechanism has been previously demonstrated for an unrelated fungal lipase(62, 63) . More recently, studies in which the three-dimensional structure of pancreatic lipase was cocrystallized with mixed micelles of phosphatidylcholine and bile salt (64) have demonstrated that, indeed, one of the two fatty acids of phospholipid binds to the hydrophobic side chains of the amino acid residues in the lipase lid. A molecular model of LPL based on the pancreatic lipase x-ray structure (65) indicates that the lid of LPL is indeed likely to be organized into amphipathic helices and that it may play a role similar to that of the pancreatic lipase lid. Thus, like in pancreatic lipase, the lids of LPL and HL may come into intimate contact with lipase substrates and thus play a major role in mediating lipase substrate specificity.
In this context, the primary, secondary, and tertiary structures of the lid may be important. Although the secondary structural organization of the lid into highly amphipathic helices appears to be very similar in LPL and HL(42) , there is only a small degree of primary sequence homology. In fact, 15 out of 22 amino acids of the lid are different between LPL and HL. Further analysis of the lipase lids reveals that the LPL lid contains four acidic and three basic amino acids, whereas the lid of HL contains only one acidic but five basic residues. In the investigation of substrate specificity, we controlled for different fatty acid preferences between LPL and HL (22, 23, 24, 25) by using oleic acid as the fatty acid in triglyceride (triolein) and phospholipid (DOPC) substrates. Thus, the observed differences in substrate specificity may, in part, be due to different abilities of the LPL and HL lids to accommodate the polar head group of phospholipids. The differences in the distribution of charged residues in the lid may thus be crucial since a negatively charged lid could interfere with the entrance of a phospholipid with its negatively charged polar head group into the catalytic pocket. Therefore, the lid of HL may be more suited for interaction with a phospholipid substrate than the LPL lid.
A molecular model of LPL based on the x-ray structure of pancreatic lipase reveals another, smaller surface loop between residues 54 and 64 as well as three loops (residues 91-95, 157-160, and 187-196) that make up the hydrophobic groove surrounding the active site(65) . These loops are more highly conserved between LPL and HL, and the region contains two additional negative charges in HL, making it less likely that this region confers preferential phospholipid hydrolysis. While a contribution of these loops to the determination of substrate specificity cannot be ruled out, the reversal of phospholipid/triglyceride hydrolyzing ratios by the mere exchange of the lids suggests that the lids of LPL and HL are the single most important regions determining substrate specificity.
In conclusion, characterization of chimeric enzymes in which the LPL and HL lids have been exchanged has demonstrated that the lipase lid plays a crucial role in mediating the different substrate specificities of LPL and HL. The lid of HL augments phospholipase activity, whereas the lid of LPL increases triglyceride hydrolysis. This study provides new insights into the structural basis for the observed in vivo differences in LPL and HL function.