Lipoprotein lipase in rat lung. The effect of fasting.

We measured lipoprotein lipase activity in dried defatted preparations of rat lung using doubly labeled chylomicron triglyceride as substrate. The enzyme activity was linear for the first hour of incubation at 37 degrees C, had a pH optimum of 8.1 and was completely inhibited by 0.5 M NaC1. Lungs from fed rats hydrolyzed chylomicron triglyceride at a rate of 13.00 mumoles/g per h; the activity rate was unchanged by fasting 8-72 h. Heparin infusion into isolated lungs caused immediate release of lipoprotein lipase to the venous effluent. The activity released was equivalent to about 10% of total lung lipoprotein lipase activity in both fed and fasted rats. Since the ability to remove blood triglyceride is directly related to the level of lipoprotein lipase activity, these findings indicate that the lung is one of the few tissues able to remove efficiently blood triglyceride during fasting.     

Phospholipase activity of bovine milk lipoprotein lipase on phospholipid vesicles: influence of apolipoproteins C-II and C-III.

The effect of human plasma apolipoproteins C-II and C-III on the hydrolytic activity of lipoprotein lipase from bovine milk was determined using dimyristoyl phosphatidylcholine (DMPC) vesicles as substrate. In the absence of apoC-II or C-III, lipoprotein lipase has limited phospholipase activity. When the vesicles were preincubated with apoC-II and then phospholipase activity determined, there was a time dependent release of lysolecithin; activity was dependent upon both apoC-II and lipoprotein lipase concentrations. The addition of apoC-III to DMPC did not stimulate phospholipase activity. We conclude that apoC-II has an activator effect on the phospholipase activity of lipoprotein lipase and that the mechanism is beyond that of simply altering the lateral compressibility of the lipid.     

Lipoprotein lipase and acid lipase activity in rabbit brain microvessels.

A preparation of cerebral microvessels was used to demonstrate the presence of lipoprotein lipase and acid lipase activity in the microvasculature of rabbit brain. Microvessels, consisting predominantly of capillaries, small arterioles, and venules, were islated from rabbit brain. Homogenates were assayed for lipolytic activity using a glycerol-stabilized trioleoylglycerol-phospholipid emulsion as substrate. Lipoprotein lipase activity was characterized with this substrate by previously established criteria including an alkaline pH optimum, increased activity in the presence of heparin and heat-inactivated plasma, and reduced activity in the presence of NaCl and protamine sulfate. A different substrate, containing trioleoylglycerol incorporated into phospholipid vesicles, was used to reveal acid lipase activity that was not affected by heparin, plasma, NaCl, or protamine sulfate. Lipoprotein lipase did not show activity with the vesicle preparation as substrate. Intact microvessels, when incubated in the presence of heparin, release lipoprotein lipase into the incubation solution. In contrast, release of acid lipase activity from intact microvessels was not dependent on heparin. The data show the presence of both lipoprotein lipase and acid lipase in brain microvessels and suggest that lipoproteins are metabolized within the cerebral vasculature.     

Lipoprotein lipase in heart and myocytes: characteristics with intralipid as substrate.

1. Intralipid is a suitable substrate for measuring lipoprotein lipase activity in the presence of other triacylglycerol lipases in heart and myocytes. 2. Triacylglycerol lipase activity in heart and myocytes was increased 10-fold in the presence of serum at pH 7.4 and 8.1. The serum-stimulated activity in myocytes was 95% inhibited by saturating concentrations of antiserum to lipoprotein lipase. 3. Both heparin-releasable and non-releasable lipoprotein lipase fractions had similar Km values for Intralipid and a similar pattern of inhibition by high density lipoprotein but different responses to heparin. 4. Isoproterenol did not alter lipoprotein lipase activity in cardiac myocytes.     

Lipoprotein lipase activity in the flight muscle of Locusta migratoria and its specificity for haemolymph lipoproteins

Lipoprotein lipase activity in flight muscle homogenates of Locusta migratoria was measured, using natural radiolabelled lipoproteins as substrates. The flight specific lipoprotein A⁺ (or low density lipophorin) stimulated lipoprotein lipase activity several-fold compared to the resting lipoprotein A_y (or high density lipophorin). However, with the high mol. wt lipoprotein fraction O_AKH as a substrate, lipase activity was even doubled compared to lipoprotein A⁺. Lipase activity was not increased in flight muscle homogenates of insects which had flown. Neither adipokinetic hormone, nor octopamine had any direct effect on lipoprotein lipase activity. Aspects of hormonal regulation and apoprotein activation of the locust flight muscle lipoprotein lipase are discussed and compared with the model for vertebrate lipoprotein lipase.     

Nutritional regulation of lipoprotein lipase in guinea pig tissues

Glucose transport in guinea pig adipocytes has been shown to be markedly resistant to stimulation by insulin. Lipoprotein lipase is another transport catalyst in adipose tissue which is believed to be regulated by insulin. We have therefore studied how feeding-fasting affects lipoprotein lipase activity in guinea pig tissues. There was an even more marked decrease in adipose tissue lipoprotein lipase activity on fasting in guinea pigs (10-20 fold) than in rats or mice (4-5 fold). In adipocytes, the activity decreased only 2.5-4.5 fold; most of the change was in extracellular lipoprotein lipase. On glucose refeeding, the activity was rapidly restored. In the first 4 hours after glucose administration extracellular lipoprotein lipase activity increased to more than 10 times the amount present in adipocytes. After cycloheximide, lipoprotein lipase activity decreased with a half-life of 22 min. It is concluded that lipoprotein lipase is rapidly produced and turned over in guinea pig adipose tissue, and that the system is quite sensitive to feeding-fasting. In contrast to adipose tissue, there was no significant change in lipoprotein lipase activity in any other tissue on fasting. There was a strong correlation between the activities in heart and diaphragm muscle, but this correlation was independent of feeding-fasting.     

Guinea pig very low density lipoproteins are a good substrate for lipoprotein lipase.

In contrast to plasma from most other animals, guinea pig plasma causes little or no stimulation of lipoprotein lipase activity. Very low density lipoproteins (VLDL) isolated by ultracentrifugation of guinea pig serum caused a definite stimulation of lipase activity, whereas the infranatant inhibited the activity. Gel filtration in 5 M guanidinium hydrochloride of delipidated VLDL demonstrated that the activation was caused by a low molecular weight protein. The VLDL themselves were hydrolized at similar rates as human VLDL both by guinea pig and by bovine lipoprotein lipases. Thus, guinea pig VLDL contain an activator for lipoprotein lipase analogous to that in other animals and there is enough of the activator to support rapid hydrolysis of the VLDL lipids by the lipase.     

The mechanism of heparin stimulation of rat adipocyte lipoprotein lipase

Free fat cells and stromal-vascular cells were prepared from rat adipose tissue by incubation with collagenase. NH(4)OH-NH(4)Cl extracts of acetone-ether powders prepared from fat cells contained lipoprotein lipase activity but extracts of stromal-vascular cells did not. Intact fat cells released lipoprotein lipase activity into incubation medium, but intact stromal-vascular cells did not. The lipoprotein lipase activity of the medium was increased when fat cells were incubated with heparin, and this was accompanied by a corresponding decrease in the activity of subsequently prepared fat cell extracts. Heparin did not release lipoprotein lipase activity from stromal-vascular cells. The lipoprotein lipase activity of NH(4)OH-NH(4)Cl extracts of fat cell acetone powders is increased by the presence of heparin during the assay. This increase is not due to preservation of enzyme activity, but to increased binding of lipoprotein lipase to chylomicrons. Protamine sulfate and sodium chloride have little effect on the binding of lipoprotein lipase to chylomicrons, but they inhibit enzyme activity after binding to substrate has occurred. These inhibitors do, however, inhibit the stimulatory effect of heparin on enzyme-substrate binding.     

Immunological studies on bovine milk lipoprotein lipase. Effects of Fab fragments on enzyme activity

Rabbit antiserum was prepared against purified bovine mild lipoprotein lipase. Immunoelectrophoresis of lipoprotein lipase gave a single precipitin line against the antibody which was coincident with enzyme activity. The gamma-globulin fraction inhibited heparin-releasable lipoprotein lipase activity of bovine arterial intima, heart muscle and adipose tissue. The antibody also inhibited the lipoprotein lipase activity from adipose tissue of human and pig, but not that of rat and dog. Fab fragments were prepared by papain digestion of the gamma-globulin fraction. Fab fragments inhibited the lipoprotein lipase-catalyzed hydrolysis of dimyristoylphosphatidylcholine vesicles and trioleoylglycerol emulsions to the same extent. The Fab fragments also inhibited the lipolysis of human plasma very low density lipoproteins. The change of the kinetic parameters for the lipoprotein lipase-catalyzed hydrolysis of trioleoylglycerol by the Fab fragments was accompanied with a 3-fold increase in Km and a 10-fold decrease in Vmax. Preincubation of lipoprotein lipase with apolipoprotein C-II, the activator protein for lipoprotein lipase, did not prevent inhibition of enzyme activity by the Fab fragments. However, preincubation with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol or Triton X-100-emulsified trioleoylglycerol had a protective effect (remaining activity 7.0 or 25.8%, respectively, compared to 1.0 or 0.4% with no preincubation). The addition of both apolipoprotein C-II and substrate prior to the incubation with the Fab fragments was associated with an increased protective effect against inhibition of enzyme activity; remaining activity with dipalmitoylphosphatidylcholine-emulsified trioleoylglycerol was 40.6% and with Triton X-100-emulsified trioleoylglycerol, 45.4%. Human plasma very low density lipoproteins also protected against the inhibition of enzyme activity by the Fab fragments. These immunological studies suggest that the interaction of lipoprotein lipase with apolipoprotein C-II in the presence of lipids is associated with a conformational change in the structure of the enzyme such that the Fab fragments are less inhibitory. The consequence of a conformational change in lipoprotein lipase may be to facilitate the formation of an enzyme-triacylglycerol complex so as to enhance the rate of the lipoprotein lipase-catalyzed turnover of substrate to products.     

Fatty acyl chain specificity of phosphatidylcholine hydrolysis catalyzed by lipoprotein lipase. Effect of apolipoprotein C-II and its (56-79) synthetic fragment

Mixed acyl chain phosphatidylcholine molecules in Triton N-101 micelles were employed as substrates for lipoprotein lipase to test which substrate acyl chain has the greatest effect on activation of the enzyme by apolipoprotein C-II. The phospholipase A1 activity of lipoprotein lipase was measured by pH-stat. The activation factor (lipoprotein lipase activity plus apolipoprotein C-II/activity minus apolipoprotein C-II) increased monotonically with apolipoprotein C-II concentration up to 1 microM apolipoprotein C-II at an enzyme concentration of 0.01 microM. The maximal activation factor for phosphatidylcholine substrate molecules with sn-2 acyl chain lengths of 14 averages 14.8. By contrast, for sn-2 acyl chain lengths of 16 the activation factor was 29.2. Varying the sn-1 acyl chain length had no significant effect on the activation factor. The chain-length dependence of the activation factor is similar with the apolipoprotein C-II peptide fragment comprising residues 56-79, which does not include the lipid-binding region of apolipoprotein C-II. These data are consistent with a model for activation of lipoprotein lipase in which residues 56-79 bind to lipoprotein lipase and alter the interaction of the sn-2 acyl chain of the phosphatidylcholine (PC) substrate or the lysoPC product within the activated state complex.     

Monoclonal antibodies against bovine milk lipoprotein lipase. Characterization of an antibody specific for the apolipoprotein C-II binding site

Ten murine monoclonal antibodies have been produced that are specific for bovine milk lipoprotein lipase. One monoclonal antibody, bLPL-mAb-7, inhibited completely the apolipoprotein C-II (apo-C-II)-dependent enzymic hydrolysis of trioleoylglycerol in a phospholipid-stabilized emulsion, but had no effect on the hydrolysis of the water-soluble substrate p-nitro-phenylacetate. Four times more bLPL-mAb-7 was required to achieve 50% inactivation of lipoprotein lipase activity when the enzyme was preincubated with excess apo-C-II. Disruption of the binding of a dansyl-labeled apo-C-II peptide to lipoprotein lipase by bLPL-mAb-7 was demonstrated by resonance energy transfer, both in the presence and absence of lipid. This antibody thus appears to recognize the apo-C-II binding site of lipoprotein lipase. In addition, bLPL-mAb-7 also inhibited the lipoprotein lipase activity of human post-heparin plasma.     

Beta-adrenergic stimulation enhances translocation, processing and synthesis of lipoprotein lipase in rat heart cells

Cells isolated from newborn rat hearts were cultured for 10-14 days, and lipoprotein lipase activity was present in an intracellular and heparin-releasable pool. Treatment of the cultures with 10(-7) M isoproterenol for 3 min resulted in a 3-fold increase in heparin-releasable lipoprotein lipase and a concomitant decrease in residual cellular enzyme activity. Similar results were obtained by treatment with dibutyryl cAMP. Treatment with isoproterenol or dibutyryl cAMP for 2 h affected glycosylation of immunoadsorbable lipoprotein lipase, so that the ratio of [3H]galactose to [14C]mannose in the heparin-releasable enzyme increased from 3.8 (control) to 13.0 (isoproterenol-treated). The change in the ratio of the sugars in the cellular fraction of the enzyme was from 3.1 to 9.9. 2 h treatment with isoproterenol did not enhance new enzyme synthesis, as determined by incorporation of [3H]leucine into immunoadsorbable lipoprotein lipase. 24 h after addition of either isoproterenol or dibutyryl cAMP to the culture medium, stimulation of enzyme synthesis was demonstrated. The present results permit three effects of isoproterenol on lipoprotein lipase to be distinguished: stimulation of translocation from a cellular to heparin-releasable pool; enhanced processing of mannose residues and terminal glycosylation; stimulation of synthesis of enzyme protein.     

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.     

The site of cartilage matrix degradation.

1. The metabolism of VLD lipoproteins (very-low-density lipoproteins) was studied in intact isolated beating-heart cells and isolated perfused rat heart from starved animals by using [14C]triacylglycerol fatty acid-labelled VLD lipoprotein prepared from rats previously injected with [1-14C]palmitate. 2. 14C-labelled VLD lipoprotein was metabolized by the isolated perfused heart, but was only minimally metabolized by the heart cells unless an exogenous source of lipoprotein lipase was added. 3. Measurements of lipoprotein lipase at pH 7.4 with the natural substrate 14C-labelled VLD lipoprotein indicated that during collagenase perfusion of the heart the enzyme was released into the perfusate, the activity released being proportional to the concentration of collagenase used. Lipoprotein lipase activity in homogenates of hearts that had been perfused with collagenase showed a corresponding loss of activity. 4. At high perfusate concentrations of collagenase, inactivation of the released lipoprotein lipase occurred. 5. Lipoprotein lipase activity was largely undetectable in the homogenate of the isolated heart cells. 6. It is concluded that the lipoprotein lipase responsible for the hydrolysis of VLD lipoprotein triacylglycerol is predominantly located externally to the heart muscle cells and that its release can be facilitated by perfusion of the heart with bacterial collagenase.     

Lipoprotein lipase activity in liver of the rat fetus

Lipoprotein lipase activity was determined in tissue from pregnant and post-partum rats and virgin adult controls and in liver from fetuses and pups. A glycerol-based emulsion of tri-(1-¹⁴C)-oleoyl-glycerol was used as substrate. According to the inhibitory characteristics in the presence of protamine and NaCl, the measured activity corresponded to the extrahepatic lipoprotein lipase of the adults. Compared to control values, the lipoprotein lipase activity was reduced in the mother's adipose tissue in late gestation and during the first days after parturition while it did not change in heart. Liver activity was negligible in mothers and controls while in the fetus it increased until the time of birth. The presence of this enzyme may allow the fetus liver to remove circulating triglycerides and to store them in preparation for early extrauterine life.     

A method for the determination of lipoprotein lipase in postheparin plasma and body tissues utilizing a triolein-coated Celite substrate

A simple and specific method for assaying lipoprotein lipase activity is described. Postheparin plasma, heart homogenates, or extracts of acetone powder of adipose tissue were incubated with a triolein-coated Celite substrate, and enzyme activity was determined from the rate of free fatty acid (FFA) release in the incubation system. FFA release was linear for 30 min, and was proportional to protein concentration in the incubation system. FFA release was decreased by addition of deoxycholate or Triton X-100. Increasing the concentration of heparin in the incubation system caused a gradual decrease in FFA release by postheparin plasma and increases in activity of heart homogenates and adipose tissue lipoprotein lipase. The Celite substrate was found to be satisfactory for assaying pancreatic lipase activity as well.     

Lipoprotein lipase in rat lung. Effect of dexamethasone.

The effect of hormone administration on the activity of lipoprotein lipase in the lung was studied in the rat. The following hormones were administered: dexamethasone, L-thyroxine, estradiol-17beta and progesterone. In addition, lung lipoprotein lipase activity was studied in diabetic and lactating rats. Lipoprotein lipase activity was measured in dried, defatted preparations of rat lung using double labeled ([14C]palmitate, [3H]glycerol) chylomicron triacylglycerol as substrate. Dexamethasone administration caused a rise of 70% in the level of activity of lipoprotein lipase in acetone powders of lung and a 100% increase in the amount of enzyme released during heparin infusion into isolated, perfused lungs. Enzyme activity was higher in lungs of females than of male rats; however; the level of activity was unaffected by estrogen or progesterone administration to either male or ovariectomized rats. Diabetes, hyperthyroidism or lactation did not change lipoprotein lipase activity in the lung. The constant presence of lipoprotein lipase activity in the lung suggests that this organ is able to maintain a steady supply of triacylglycerol-fatty acids under a variety of physiological and pathological conditions. Stimulation of enzyme activity by dexamethasone could lead to increased uptake of triacylglycerol-fatty acids by the lung and may thus be a contributing factor to corticosteroid-induced enhanced surfactant synthesis.     

Mechanism of salt-mediated inhibition of lipoprotein lipase.

The activity of lipoprotein lipase isolated from rat postheparin plasma has been determined with synthetic lipids, in the presence and absence of apoprotein of the natural substrate very low density lipoprotein, as a function of medium ion-pair concentration of a number of different inorganic salts. The several kinetic effects of lipoprotein protein on lipase activity were specifically and quantitatively reversed in the presence of molar sodium chloride or solutions of equivalent effective ion concentrations of other salts. Salt-mediated inhibition was fully reversible by silution and was independent of substrate concentration. Inhibition was a function of the identity of the salt anion within a Hofmeister (lyotropic) series: I- greater than SCN- greater than NO3- greater than Cl- greater than F-, and, in these terms, was not significantly different for a series of inorganic chlorides (Li+, Na+, K+, Cs+). The effects of salts on the natural lipoprotein substrates, chylomicrons, and very low density lipoproteins were similar to those obtained with a synthetic lipid-protein substrate complex. These findings are discussed in the light of recent ideas on the activation of lipoprotein lipase.     

Lipoprotein lipase and hepatic lipase activities are differentially regulated in isolated hepatocytes from neonatal rats.

Lipoprotein lipase and hepatic lipase are members of the lipase gene family sharing a high degree of homology in their amino acid sequences and genomic organization. We have recently shown that isolated hepatocytes from neonatal rats express both enzyme activities. We show here that both enzymes are, however, differentially regulated. Our main findings are: (i) fasting induced an increase of the lipoprotein lipase activity but a decrease of the hepatic lipase activity in whole liver, being in both cases the vascular (heparin-releasable) compartment responsible for these variations. (ii) In isolated hepatocytes, secretion of lipoprotein lipase activity was increased by adrenaline, dexamethasone and glucagon but was not affected by epidermal growth factor, insulin or triiodothyronine. On the contrary, secretion of hepatic lipase activity was decreased by adrenaline but was not affected by other hormones. (iii) The effect of adrenaline on lipoprotein lipase activity appeared to involve beta-adrenergic receptors, but stimulation of both beta- and alpha 1-receptors seemed to be required for the effect of this hormone on hepatic lipase activity. And (iv), increased secretion of lipoprotein lipase activity was only observed after 3 h of incubation with adrenaline and was blocked by cycloheximide. On the contrary, decreased secretion of hepatic lipase activity was already significant after 90 min of incubation and was not blocked by cycloheximide. We suggest that not only synthesis of both enzymes, but also the posttranslational processing, are under separate control in the neonatal rat liver.     

Triglyceridase and phospholipase A1 activities of rat-heart lipoprotein lipase. Influence of apolipoproteins C-II and C-III.

The influence of purified human apolipoprotein C-II on phospholipase A1 and triglyceridase activities of lipoprotein lipase were compared. Lipoprotein lipase was obtained from rat hearts by perfusion with a medium containing heparin and purified on a heparin Sepharose 4-B column. Using phosphatidyl-ethanolamine-coated triglyceride particles as substrate it was found that the phospholipase A1 and triglyceridase activities of lipoprotein lipase similarly depend on the presence of apolipoprotein C-II. Apolipoprotein C-III cannot replace apolipoprotein C-II. However, addition of apolipoprotein C-III in the presence of C-II affects both lipase activities. While strong inhibition of triglyceridase activity was observed under these conditions, phospholipase A1 activity was slightly stimulated. On the basis of these findings a model was constructed for the role of apolipoprotein C-II in lipoprotein lipase action.