Post-heparin triacylglycerol lipases in ovine plasma

Lipoprotein lipase and hepatic lipase have been shown to be present in the post-heparin plasma of sheep. Intravenous injection of heparin into sheep produced a rapid increase in the free fatty acid concentration and lipolytic enzyme activity of the plasma, both peaking within 5-15 min and then falling to pre-heparin levels within 30-60 min. Lipolytic activity was not detected in plasma before heparin treatment. Two distinct lipolytic activities were separated from the plasma by chromatography on heparin-Sepharose 6B. Lipoprotein lipase was identified on the basis that the lipolytic activity was dependent upon the addition of plasma, inhibited by 1M NaCl, and inhibited by a specific antiserum against lipoprotein lipase. The second lipolytic activity of plasma was identified as hepatic lipase, as it was not dependent upon plasma for activity, nor was it inhibited by 1M NaCl or antiserum against lipoprotein lipase. Its properties were identical to the lipase extracted from the liver of sheep. Lipoprotein-lipase activity, but not hepatic-lipase activity, was dependent upon the nutritional state of the sheep at the time of heparin injection. However, hepatic lipase comprised a significant proportion of the total lipolytic activity.     

Lipoprotein lipase in cultured heart cells: characteristics and cellular location.

Lipase activity extracted from cultured neonatal rat heart cells was characterized and identified as lipoprotein lipase. Enzyme activity was stimulated by human apoC-II and rat serum; serum stimulation was prevented by human apoC-I and by apoC-II. Lipolysis was maximal at pH 8.0 and was inhibited by protamine sulfate, NaCl, and high concentrations of heparin. About 50% of heart cell lipase activity applied to heparin-Sepharose bound to the gel and was eluted with a NaCl gradient. A peak of lipase activity was observed at 0.84 M NaCl. Neonatal rat heart cells in culture are a mixture of muscle and non-muscle cells. To determine the cellular location of the lipoprotein lipase, enzyme activity and muscle cell content of the cultures were determined. Myosin ATPase was used as an index of muscle cell content since ATPase specific activity correlated (r = +0.97) with muscle cell content determined immunofluorescently. When muscle cell content of cultures was decreased or increased by differential plating, lipase specific activity was constant. Moreover, lipase specific activity was constant during culture growth despite a decrease in muscle cell content. It was concluded that lipoprotein lipase activity of cultured heart cells is not associated solely with either muscle or non-muslce cells.     

Characterization of the lipolytic activity of endothelial lipase

Endothelial lipase (EL) is a new member of the triglyceride lipase gene family previously reported to have phospholipase activity. Using radiolabeled lipid substrates, we characterized the lipolytic activity of this enzyme in comparison to lipoprotein lipase (LPL) and hepatic lipase (HL) using conditioned medium from cells infected with recombinant adenoviruses encoding each of the enzymes. In the absence of serum, EL had clearly detectable triglyceride lipase activity. Both the triglyceride lipase and phospholipase activities of EL were inhibited in a dose-dependent fashion by the addition of serum. The ratio of triglyceride lipase to phospholipase activity of EL was 0.65, compared with ratios of 24.1 for HL and 139.9 for LPL, placing EL at the opposite end of the lipolytic spectrum from LPL. Neither lipase activity of EL was influenced by the addition of apolipoprotein C-II (apoC-II), indicating that EL, like HL, does not require apoC-II for activation. Like LPL but not HL, both lipase activities of EL were inhibited by 1 M NaCl. The relative ability of EL, versus HL and LPL, to hydrolyze lipids in isolated lipoprotein fractions was also examined using generation of FFAs as an end point. As expected, based on the relative triglyceride lipase activities of the three enzymes, the triglyceride-rich lipoproteins, chylomicrons, VLDL, and IDL, were efficiently hydrolyzed by LPL and HL. EL hydrolyzed HDL more efficiently than the other lipoprotein fractions, and LDL was a poor substrate for all of the enzymes.     

Lipoprotein lipase activity in bovine aorta.

A lipoprotein lipase in the bovine arterial wall has been identified and partially characterized. The enzyme has a Km apparent of 1 mM for triolein in a phosphatidylcholine stabilized emulsion. The lipase was stimulated 20- to 30-fold by the addition of heated rat plasma to the assay medium. The activity exhibited a pH optimum at 8.6. Protamine sulfate (1.0 mg/ml) inhibited the activity by 50%, whereas 1.4 M sodium chloride inhibited by 85%. Sodium fluoride, an inhibitor of the hormone-sensitive lipase, had no effect on the activity. Additions of low concentrations of heparin or Ca-2+ to the enzyme caused a slight stimulation of the lipolytic activity. A crude sectioning of the aorta revealed specific activity of lipoprotein lipase to be highest at the endothelial side of the artery.     

Enhanced release and synthesis of lipoprotein lipase in rat heart cell cultures exposed to high concentrations of Hepes

While attempting to optimize conditions for synthesis of lipoprotein lipase by cultured heart cells, we encountered an unexpected rise in enzyme activity when media were supplemented inadvertently with 100 mM Hepes buffer (4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid). This finding was further investigated and optimal results were obtained at pH 7.0-7.2. The increase in lipoprotein lipase activity was time dependent; after 3-6 h there was a rise in medium activity but cellular activity increased only after 24 h. The increased enzyme activity was defined as lipoprotein lipase by inhibition with antiserum to rat adipose tissue lipoprotein lipase. A 72-h exposure to Hepes resulted in a 30% increase in the incorporation of [35S]methionine into cellular proteins and a 2-fold increase into heparin-releasable proteins. Using heparin Sepharose chromatography and stepwise elution, a lipoprotein lipase enriched fraction was recovered with 2 M NaCl. The amount of [35S]methionine and [3H]galactose incorporated into protein of this fraction derived from Hepes-treated cells was 2-6-fold that of controls. A 4-fold increase in cellular lipoprotein lipase mass in Hepes-treated cells was shown by immunoblotting. Results obtained with Hepes-conditioned medium suggest the presence of cell-derived compounds that enhance release and subsequent synthesis of lipoprotein lipase. The effect of Hepes-conditioned medium on lipoprotein lipase resembled to some extent that of the addition of heparin. Therefore, it appears that when Hepes is first added to the culture medium, it might promote a release of heparan sulfate or related compounds, possibly by virtue of its negatively charged sulfonic acid residue. The accumulated heparan sulfate could then promote a sustained release of lipoprotein lipase into the culture medium which in turn leads to increased enzyme synthesis.     

Lipolytic activity in the flight muscles of Locusta migratoria measured with haemolymph lipoproteins as substrates

A radiochemical assay is described in which neutral lipids presented as part of authentic haemolymph lipoproteins have been used as substrates to measure the lipolytic activity in the flight muscles of Locusta migratoria. The radiolabel in the substrate was located almost exclusively in the glycerol backbone of long chain acylglycerols (predominantly diacylglycerols) and the release of radiolabelled glycerol during incubation with muscle homogenates must therefore be due to “true” lipase activity rather than non-specific esterases. As the diacylglycerols were lipoprotein-bound, such enzymes may be regarded as lipoprotein lipases and this work therefore provides the first direct evidence for the existence of this group of enzymes in an insect tissue. Lipolytic activity in flight muscles has a single pH optimum with a peak at 7.5–8.0 and is only partially inhibited by 1 M NaCl which suggests dissimilarity with mammalian lipoprotein lipase. The activity and characteristics of the locust enzyme are discussed in relation to the energy requirements of flying locusts and in relation to what is known of the vertebrate enzyme.     

Dibutyryl cAMP-induced increases in triacylglycerol lipase activity in developing L8 myotube cultures

Triacylglycerol (TG) lipase activity, with an alkaline pH optimum, has been identified in the cellular fraction of L8 myotube cultures. This TG lipase activity was stimulated by serum and inhibited by NaCl and protamine sulfate. These characteristics have been classically described for lipoprotein lipase. It was possible to increase the activity of this TG lipase three- to five-fold by incubating the cells with dibutyryl cAMP. Maximal enzyme activity was observed 16 h following the addition of 10-100 microM dibutyryl cAMP to the cultured cells. Enzyme activity returned to control levels 24 h after removal of the nucleotide from the culture medium. Serum-sensitive alkaline TG lipase activity was also identified in five other myotube preparations of cultured muscle cells. The highest levels of activity were found in rat skeletal muscle primary, H9, and L6 cell types. The finding that dibutyryl cAMP is an effective inducer of alkaline TG lipase activity provides us with a valuable model to investigate mechanisms regulating synthesis, compartmentalization, and transport of lipoprotein lipase in muscle.     

Importance of the different steps of glycosylation for the activity and secretion of lipoprotein lipase in rat preadipocytes studied with monensin and tunicamycin

Lipoprotein lipase synthesized by cultured rat preadipocytes is present in three compartments: an intracellular, a surface-related 3-min heparin-releasable, and that secreted into the culture medium. 30 min after addition of 6 microM monensin, the lipoprotein lipase activity in the heparin-releasable compartment starts to decrease; by 4 h of monensin treatment the lipoprotein lipase activity in the heparin-releasable pool and in the culture medium is about 10% of that found in control dishes. The intracellular activity, which had been identified as lipoprotein lipase by an antiserum to lipoprotein lipase, increases slowly and doubles by 24 h. However, since the cellular compartment accounts for 10-25% of total activity, this increase does not account for the missing enzyme activity. To determine whether this enzyme molecule is synthesized but is not active, incorporation of labeled leucine, mannose and galactose into immunoadsorbable lipoprotein lipase was studied in control, monensin- or tunicamycin-treated cells. Addition of tunicamycin (5 micrograms/ml) for 24 h caused a 30-50% reduction in immunoadsorbable lipoprotein lipase, but the enzyme activity was reduced by 90%. On the other hand, 4 h monensin treatment reduced both incorporation of [3H]leucine into immunoadsorbable lipoprotein lipase and heparin-releasable and medium lipoprotein lipase activity by 57 to 77%. The immunoadsorbable lipoprotein lipase in the intracellular compartment has a [14C]mannose to [3H]galactose ratio of 0.15 and this ratio increased 6-fold in monensin-treated cells. The intracellular lipoprotein lipase in monensin-treated cells had the same affinity for both the native and synthetic substrate as the lipoprotein lipase in control cells, yet its spontaneous secretion into the culture medium and its release by 3 min heparin treatment was markedly decreased. The present results indicate that: the presence of asparagine-linked oligosaccharide (formation of which is inhibited by tunicamycin) is mandatory for the expression of lipoprotein lipase activity; lipoprotein lipase is active also in a high mannose form; and terminal glycosylation and oligosaccharide processing, which is inhibited by monensin, may be important for the appearance of heparin-releasable lipoprotein lipase and secretion of lipoprotein lipase into the medium.     

Effects of heparin-induced lipolytic activity on the structure of rat high-density lipoprotein

Following its secretion into the plasma compartment, the high-density lipoprotein (HDL) is presumed to be acted upon by both soluble enzymes, such as lecithin:cholesterol acyltransferase (LCAT), and membrane-associated enzymes, such as lipoprotein lipase and hepatic lipase. Rats were injected intravenously with heparin to release membrane-associated lipolytic activities into the circulation and the collected plasma was incubated overnight at 37 degrees C in the presence or absence of an LCAT inhibitor or an inhibitor of lipoprotein lipase (1 M NaCl). It was observed that lipoprotein lipase accounted for most of the triglyceride hydrolase activity in the heparin-treated plasma, and that the heparin-releasable activities caused an increase in HDL density but no measurable change in particle size when LCAT was inhibited. Heparin treatment caused about a 60% decrease in plasma triacylglycerol during the interval between injection of heparin and blood collection. Although this caused marked compositional changes in the d less than 1.063 g/ml lipoproteins, no changes were observed in the lipid composition or apoprotein distribution in the HDL. Subsequent incubation for 18 h at 37 degrees C produced marked increases in the apoE content of HDL from heparin-treated plasma even when LCAT was inhibited. Time-course studies showed that in the presence of an LCAT inhibitor there was considerable conversion of phosphatidylcholine to lysophosphatidylcholine in heparin-treated plasma, and that this activity was diminished by 1 M NaCl, but that no phospholipolysis was observed in control plasma. By contrast, both heparin-treated and control plasma possessed substantial triglyceride hydrolase activity. The concurrent action of lipases and LCAT was observed to reduce the maximum level of cholesterol esterification which could be achieved in the absence of lipase activity. It is concluded that changes in HDL particle size are mainly attributable to LCAT, but that lipase activities, which are either free in rat plasma or releasable by heparin, play a role in restructuring the phospholipid moiety and altering the protein composition of the HDL, especially with respect to apoE, a potential ligand to cellular receptors.     

Rapid preparation of a relatively stable, partially purified lipoprotein lipase from perfused rat heart

Rat hearts, extensively washed with cold 0.15 M NaCl solution, were perfused with 5 ml of 0.15 M NaCl containing 16 U of heparin and 10% glycerol to release endothelium-bound lipoprotein lipase. Approximately 100 mU of enzyme activity could be released from each heart (weighing about 1.7 g). Several hearts could be sequentially perfused with the same heparin solution to enrich it in lipase activity. When compared with other equally rapid and frequently used sources of rat lipoprotein lipase (such as heart acetone powder or postheparin plasma), our enzyme preparation had a much higher specific activity suggesting that a greater purification level had been already achieved in a single step. In addition, this lipoprotein lipase preparation contained only trace amounts of lipids, was stable for an hour at 37 degrees C and retained 75% of its activity after 10 days at 4 degrees C. The described procedure is a quick way to prepare a soluble, partially purified and relatively stable lipoprotein lipase that may be useful especially for the in vitro preparation of triacylglycerol-rich lipoprotein remnants.     

Effect of ingestion of unsaturated fat on lipolytic activity of rat tissues

Homogenates of some rat tissues, incubated in Tris-maleate buffer containing bovine serum albumin, olive oil emulsion, heparin, and serum, liberated free fatty acids. The total lipolytic activity in tissues of rats fed a low fat, 20% lard, or 20% corn oil diet for 6 wk was measured. Similar activities were found in all the livers, but there was a significant increase in the total lipolytic activity of the mucosa, epididymal fat, and mesenteric tissues after ingestion of an unsaturated fat diet as compared with that containing a more saturated fat. From measurements of the lipolytic activity in the presence of 1 M NaCl or 0.2 M NaF and in the absence and presence of heparin and serum, the conclusion is drawn that more lipoprotein lipase was present in adipose tissue of rats on unsaturated fat diets. An increase in available lipoprotein lipase after unsaturated fat diets may aid in clearing lipids from the blood of these rats and thus in producing the lower blood lipid levels obtained.     

Human monocytes in culture synthesize and secrete lipoprotein lipase

Within the first day in culture, human monocytes begin to synthesize and secrete a triglyceride lipase. The designation of this activity as lipoprotein lipase is based upon: 1) a requirement of serum or apolipoprotein C-II for full activity; 2) inhibition by 1M NaCl or apolipoprotein C-III₂; 3) a pH optimum of 8; and 4) binding to endothelial cells that is releasable by heparin. The enzyme also exhibits immunological cross reactivity with antibody to purified bovine milk lipoprotein lipase as does human postheparin plasma lipoprotein lipase. Lymphocytes and polymorphonuclear leukocytes do not appear to contain this enzyme.     

An in vitro model to study adipose differentiation in serum-free medium

Adipose differentiation was studied in a teratoma-derived fibroadipogenic cell line (1246) cultured in serum-free medium. The addition of dexamethasone and 1-methyl-3-isobutylxanthine to the serum-free medium induced confluent 1246 cells to differentiate into adipocyte-like cells as evidenced by triglyceride accumulation and increased levels of lipolytic enzyme activities. Hormone-sensitive lipase activity measured 5 days after the addition of dexamethasone and 1-methyl-3-isobutylxanthine increased 17-fold and was activated by cAMP-dependent protein kinase. Neutral diglyceride lipase, monoglyceride lipase, and cholesterol ester hydrolase specific activities increased 23-, 75-, and 73-fold, respectively. Among these three activities, only cholesterol ester hydrolase was activated by cAMP-dependent protein kinase. Differentiated 1246 cells expressed receptors to lipolytic hormones as shown by the stimulation of glycerol release by epinephrine (8.6-fold), glucagon (2.2-fold), and adrenocorticotrophic hormone (5.5-fold). Heparin treatment of 1246 cells in serum-free medium resulted in the release of lipoprotein lipase activity into the culture medium. Thus, 1246 cells can serve as a model for the study of adipose differentiation under defined culture conditions since they are capable of growth and survival in the absence of serum while retaining their ability to differentiate into adipocytes.     

Heparin-released triglyceride lipase from chang liver cells

Heparin-released triglyceride lipase (TGL) from Chang liver cells (ATCC CCL 13) was investigated using very low density lipoproteins (VLDL) as a substrate. The TGL activity was released into the culture medium when the cells were incubated with heparin. The enzyme showed maximum activity at pH 8.5, and 80% inhibition by 0.6 M NaCl. These results indicated that Chang liver cells, a cell line derived from liver, synthesize lipoprotein lipase.     

Characterization of serum-stimulated lipoprotein lipase from bovine heart

1. A triglyceride (TG) lipase is present in whole homogenate and tissue extracts of beef myocardium with characteristics of lipoprotein lipase (LPL); i.e., activity is stimulated by serum, inhibited by NaCl and protamine sulfate, the protein binds to heparin-Sepharose, and the enzyme has an alkaline pH optimum. 2. This TG lipase, eluted from heparin-Sepharose at 0.9-1.0 M NaCl, has an apparent mol. wt of 64 K daltons. Its primary mRNA is 3.7 kb. 3. Expression of LPL mRNA and enzyme activities are in the ratio of approximately 20:8:1 for hearts of mouse, rat and beef, respectively and correlate with r = +0.99.     

Effects of threonine-poor apolipoproteins on post-heparin plasma hepatic triacylglycerol lipase and lipoprotein lipase

Whole-irradiated rabbit pre-heparin plasma had an important inhibitory effect on hepatic triacylglycerol lipase and lipoprotein lipase activities, whereas control rabbit pre-heparin plasma slightly inhibited hepatic triacylglycerol lipase activity at a high concentration and enhanced lipoprotein lipase activity. As some apolipoproteins were known to modulate these two lipolytic enzymes, the inhibitory effects of irradiated rabbit plasma were investigated in apolipoproteins. Three apolipoproteins, with isoelectric points of about 6.58, 6.44 and 6.12, characterized by their low content in threonine (threonine-poor apolipoproteins) were produced in high concentrations in rabbit VLDL and HDL after irradiation. The effects of these apolipoproteins on control rabbit post-heparin plasma hepatic triacylglycerol lipase and extrahepatic lipoprotein lipase were studied. Threonine-poor apolipoproteins substantially inhibited the hepatic triacylglycerol lipase activity and enhanced the apolipoprotein C-II-stimulated activity of lipoprotein lipase. The amounts of these apolipoproteins in triacylglycerol-rich lipoprotein particles may determine the lipolytic activity of lipoprotein lipase and hepatic triacylglycerol lipase in triacylglycerol hydrolysis. The existence of another inhibitor of lipoprotein lipase remains to be determined.     

Lipoprotein lipase of cultured mesenchymal rat heart cells. I. Synthesis, secretion and releasability by heparin

Cell suspensions prepared from rat hearts were separated by replating into F1, F2 and M cultures, and cultured for 3--11 days. Lipoprotein lipase activity was highest in the F1 cultures which consisted mainly of non-beating, mesenchymal cells. The enzyme activity was released into the medium only after addition of heparin. The release occurred by an initial rapid phase and a continuous slow phase. Both the rapid and the slow release of enzyme activity by heparin were inhibited by about 70% after a 4 h pretreatment with colchicine. Thus, it seems that the vesicular transport is responsible for the translocation of lipoprotein lipase to the cell surface also during the slow process of release. The residual activity in the colchicine treated cultures was higher than in the controls indicating that no inhibition of enzyme synthesis occurred. The slow phase of enzyme release continued also after removal of heparin from the medium but was reduced markedly when protein synthesis was inhibited by cycloheximide. Thus the increase in total enzyme activity encountered after exposure to heparin resulted from stimulation of new enzyme synthesis. The half-time of lipoprotein lipase in the F1 cultures was 35 min and full restoration of enzyme activity was found 60 min after complete removal of cycloheximide from the system. These data indicate that the culture system can be used to study regulation of new enzyme synthesis and its turnover.     

Synthesis and secretion of lipoprotein lipase in 3T3-L1 adipocytes. Demonstration of inactive forms of lipase in cells

3T3-L1 adipocytes in culture incorporated [35S]methionine into a protein which could be immunoprecipitated with chicken antiserum to bovine lipoprotein lipase. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed this protein had an Mr of 55,000, similar to that of bovine lipoprotein lipase, and accounted for 0.1-0.5% of total protein synthesis in the adipocytes. Lipoprotein lipase protein was present in small amounts in confluent 3T3-L1 fibroblasts, and the amount increased many-fold as the cells differentiated into adipocytes. This increase was accompanied by parallel increases in cellular lipase activity and secretion. When cells were grown with [35S]methionine, the amount of label incorporated into lipoprotein lipase increased for 2 h and then leveled off. Pulse-chase experiments showed that half-life of newly synthesized lipase was about 1 h. Turnover of lipoprotein lipase in control cells involved both release to the medium and intracellular degradation. When N-linked glycosylation was blocked by tunicamycin, the cells synthesized a form of lipase that had a smaller Mr (48,000), was catalytically inactive, and was not released to the medium. Radioimmunoassay demonstrated that 3T3-L1 adipocytes contained an unexpectedly large amount of lipoprotein lipase protein. 55% of the enzyme protein in acetone/ether powder of the cells was insoluble in 50 mM NH3/NH4Cl at pH 8.1, a solution commonly used to extract lipoprotein lipase; 27% of the lipase protein was soluble but did not bind to heparin-Sepharose and had very low lipase activity; and the remaining 13% was soluble, bound to heparin-Sepharose, and had high lipolytic activity. About one-half of the lipase released spontaneously to the medium was inactive, and lipase inactivation proceeded in the medium with little loss of enzyme protein. Lipoprotein lipase released heparin, in contrast, was fully active and more stable. When protein synthesis was blocked by cycloheximide, the level of lipoprotein lipase activity in adipocytes decreased more rapidly than the amount of lipase protein in the cells. Most of the inactive lipoprotein lipase in adipocytes probably results from dissociation of active dimeric lipase, but some could be a precursor of active enzyme.     

Monoclonal antibodies to avian lipoprotein lipase. Purification of the enzyme by immunoaffinity chromatography

Three monoclonal antibodies to avian lipoprotein lipase have been isolated by fusing spleen cells from immunized BALB/c mice with myeloma P3X-63 Ag 8. The antibodies were detected by their ability to bind immobilized lipoprotein lipase in enzyme-linked immunosorbent assay (ELISA) and by immunoprecipitation of purified enzyme in the presence of second (rabbit anti-mouse) antibodies. Two of these antibodies, CAL1-7 and CAL1-11, inhibited catalytic activity, whereas with CAL1-2 interaction with lipoprotein lipase could be demonstrated only in ELISA and in Western blot assays following denaturation of the enzyme with sodium dodecyl sulfate. An immunoadsorbent column was prepared by coupling immunopurified CAL1-11 to Sepharose-4B. When acetone powder extracts of adipose tissue were applied on the column, 70% of the catalytic activity bound to the matrix. Effective elution was achieved with 1.8 M NaCl, 40% glycerol, 5% acetone, 20 mM Chaps (3[(3-cholamidopropyl)dimethylammonio]propanesulfonate), 0.5 mM EDTA, 1 mM phosphate (pH 6.5). After concentration of the active fractions on a heparin-Sepharose 4B column, the purified enzyme was obtained with an overall recovery of 25%. Sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrates that the preparation is homogeneous with a major band at Mr 60900. Thus, avian adipose lipoprotein lipase has been purified by a one-step immunoaffinity followed by a concentrating step on heparin-Sepharose 4B.     

Heparin decreases the degradation rate of lipoprotein lipase in adipocytes

The mechanism responsible for the stimulation of secretion of lipoprotein lipase by heparin in cultured cells was studied with avian adipocytes in culture. Immunoprecipitation followed by electrophoresis and fluorography were used to isolate and quantitate the radiolabeled enzyme, whereas total lipoprotein lipase was quantitated by radioimmunoassay. Rates of synthesis of lipoprotein lipase were not different for control or heparin treatments as judged by incorporation of L-[35S]methionine counts into lipoprotein lipase during a 20-min pulse. This observation was corroborated in pulse-chase experiments where the calculation of total lipoprotein lipase synthesis, based on the rate of change in enzyme-specific activity during the chase, showed no difference between control (8.13 +/- 3.1) and heparin treatments (9.1 +/- 5.3 ng/h/60-mm dish). Secretion rates of enzyme were calculated from measurements of the radioactivity of the secreted enzyme and the cellular enzyme-specific activity. Degradation rates were calculated by difference between synthesis and secretion rates of enzyme. In control cells 76% of the synthesized enzyme was degraded. Addition of heparin to the culture medium reduced the degradation rate to 21% of the synthetic rate. The presence of heparin in cell media resulted in a decrease in apparent intracellular retention half-time for secreted enzyme from 160 +/- 44 min to 25 +/- 1 min. The above data demonstrate that the increase in lipoprotein lipase protein secretion, observed upon addition of heparin to cultured adipocytes, is due to a decreased degradation rate with no change in synthetic rate. Finally, newly synthesized lipoprotein lipase in cultured adipocytes is secreted constitutively and there is no evidence that it is stored in an intracellular pool.