Lipolytic activity of whole isolated liver cells in aqueous suspension

Purification and properties of a monoacylglycerol lipase in human erythrocytes

A membrane-bound monoacylglycerol lipase (MAGL) activity, previously demonstrated in intact human erythrocytes [Boyer, Somma, Vérine, L'Hôte, Finidori, Merger and Arnaud (1981) J. Clin. Endocrinol. Metab. 53, 143-148], has now been purified to apparent homogeneity by a five-step procedure involving solubilization in CHAPS and sequential chromatographies on Sephacryl S-400, DEAE-Trisacryl, Zn(2+)-chelating Sepharose and Superose 12 columns. The purified protein has a molecular mass of 68 +/- 2 kDa, as determined by SDS/PAGE and gel filtration, suggesting that the enzyme behaves as a monomer. The concentration-dependence of MAGL activity with monooleoylglycerol, the preferred substrate showed kinetics typical of an interfacial lipolytic enzyme displaying optimal activity on emulsified substrate particles; apparent Km values were 0.27 mM and 0.49 mM for the sn-1(3)- and sn-2-isomers respectively. MAGL had no, or negligible, activity towards tri-oleoylglycerol, di-oleoylglycerol, oleoylcholesterol, oleoyl-CoA and phosphatidylcholine; it was inhibited by di-isopropylfluorophosphate, PMSF and diethyl p-nitrophenyl phosphate, suggesting that MAGL is a serine hydrolase. MAGL activity was not modified by bile salt or apolipoprotein C-II, whereas a dose-dependent inhibition was observed with apolipoprotein A-I.

Lipolytic activities of freshly isolated rat liver parenchymal cells

By using a specific collagenase preparation preserving lipolytic enzymes, we could isolate intact rat liver cells with monoester lipase (MEL) and, for the first time, substantial amounts of endogenous neutral triester lipase (TEL) activities assayable as cell-bound enzymes. TEL and MEL activities were found exclusively in parenchymal cells. Virtually all TEL was located on plasma membrane from which it was rapidly released at 37 degrees C in the absence of any additive. MEL was distributed almost equally inside the cell and in the membrane, to which it was firmly attached. Infusion of heparin to the whole animal before liver exposure decreased by 80% the TEL content of parenchymal cells (a property typical of hepatic lipase) whilst MEL was unchanged. These results question the concept that heparin-releasable hepatic lipase acts at the surface of endothelial liver cells and further suggest that TEL and MEL refer to distinct catalytic entities.

Use of specific collagenases for the isolation of rat liver cells with preserved lipase activities

The heparin-releasable neutral lipase (EC 3.1.1.3) from rat liver is inactivated by the common preparations of collagenases (EC 3.4.24.3) used for the isolation of liver cells. We show that two collagenases purified from Clostridium histolyticum allow both the complete preservation of this lipolytic activity and a good viability of liver cells isolated by the usual perfusion protocol.

Effects of apoproteins C on lipoprotein lipase activity bound to rat fat cells

During short-term incubation, a suspension of fat cells isolated from female rats hydrolyzed tri[3H]oleyl[14C]glycerol added as substrate and incorporated an average of 42% of the amounts of [3H]oleic acid released. Lipolysis was catalyzed by cell-bound lipoprotein lipase (LPL), whose activity was influenced by apoproteins (apo) C purified from human plasma. ApoC-II enhanced 12-fold the hydrolysis rates with cells from fed rats versus 3,4-fold with cells from fasted rats. In both cases, the stimulation by apoC-II was greater than that by whole serums from led or fasted rats. Maximal LPL activity at the cell surface depended more on the preexisting nutritional state of the cell than on apoC-II or serums added extracellular. ApoC-I, apoC-III1 and apoC-III2 had no or moderate direct effects on LPL activity, whereas apoC-III1 and apoC-III2 reduced markedly the stimulating effect of apoC-II. At all levels of LPL activity, the amounts of [3H]oleic acid liberated by hydrolysis and incorporated as [3H]acylglycerol in cell lipids were highly correlated with LPL activity, suggesting that the uptake process was directly related to enzyme action.

Lipoprotein lipase. Localization on plasma membrane fragments from lactating rat mammary tissue

Subcellular fractionation studies were done using lactating rat mammary tissue. Lipoprotein lipase (E.C. 3.1.1.34) was found to be associated with plasma membrane-enriched and endoplasmic reticulum-enriched fractions. Considerable amounts of soluble lipoprotein lipase were found after homogenization of this tissue. The membrane-associated lipoprotein lipase activities described here were found to be resistant to release, by heparin, from their membrane sites. Buoyant density gradient fractionation experiments suggest that the lipoprotein lipase activity of lactating mammary tissue is located in part on the secretory epithelial cell plasma membrane. This conclusion is discussed in the context of the known site of action, in vivo, of mammary gland lipoprotein lipase, and of probable routes of its transport to that site.

Triacylglycerol hydrolysis by cells isolated from lactating rat mammary gland

The hydrolysis of exogenous trioleoylglycerol emulsions by suspensions of cells prepared from lactating rat mammary gland has been investigated. Cell integrity remains high throughout short (at least 30 min) incubations, during which extracellular hydrolysis of trioleoylglycerol proceeds at a mean rate (11 preparations) of 1.9 nmol oleate (and 0.6 nmol glycerol) released/min per mg protein. This hydrolysis shows partial dependence upon added serum and partial inhibition by protamine sulphate - both characteristic properties of lipoprotein lipase-catalyzed lipolysis. One of more monoacylglycerol hydrolase enzymes may also contribute to the measured lipolysis. Evidence is presented consistent with the hypothesis that a surface-located lipoprotein lipase is responsible for the observed lipolysis. Very little lipoprotein lipase activity is released from the cell surface by heparin. During trioleoylglycerol hydrolysis, non-esterified oleate does not accumulate in the cells or in the medium in quantities stoicheiometric with glycerol release. Analyses indicate that it passes into the cells without prior equilibration with the extracellular oleate pool(s). Once inside the cells, oleate is rapidly re-esterified into the triacylglycerol fraction. The possible relevance of these findings to the physiological mechanism of fatty acid uptake from triacylglycerol at the capillary endothelium is discussed.

Characterization of triacylglycerol lipase activity in human amniotic fluid

A radiochemical assay was used to measure the triacylglycerol lipase activity found in normal human amniotic fluid at term. Enzyme activity was characterized in a partially purified extract of amniotic fluid and was found to be optimal at pH 8.0 +/- 0.2 in the presence of 5mM sodium taurocholate, with the use of emulsified tri-[3H]oleoyl glycerol as the substrate. The assay described made it possible to determine the lipase activity in as little as 25 microliters of a 12,000 x g supernatent of whole amniotic fluid as the source of enzyme. The lipase appeared to be distinct from another triacylglycerol lipase measurable in fetal membranes. In turn, it was shown that the amniotic fluid enzyme exhibited several catalytic properties which resembled those of pancreatic lipase. i.e., its substrate specificity, the bimodal effect of bile salt, and the influence of authentic pancreatic colipase on the catalytic process. The results suggest that the assay of triacylglycerol lipase activity may be clinically useful in the detection of enzyme abnormalities in human amniotic fluid.

Hydrolysis and uptake of an aliphatic fatty ester by whole isolated fat cells

During 9-min incubations at 37 degrees C, whole isolated fat cells are able to hydrolyze a water-insoluble ester, ethyl [3H]oleate, added as an emulsion to the extracellular medium. The monoester lipase exerts its activity as a membrane-bound enzyme and is not releasable by heparin. Variation of the reaction rate with substrate concentration suggests a diffusion-limited process, consonant with the fact that enzyme and substrate are associated with particles of enormous sizes (the fat cell and the lipid droplet, respectively). Variation of lipase activity with increasing temperature in the incubation medium shows a break at 46 degrees C +/-1 degrees C, most probably related to a phase transition in the membrane structure. During incubation, 30-60% of the amount of [3H]oleic chains formed by hydrolysis are incorporated into the cell, mainly as triacylglycerols. Controls of cell viability indicate that under the experimental conditions the framework of the membrane and the internal organization of the cell appear to be maintained. Such an experimental model might duplicate physiological conditions since aliphatic fatty esters are thought to be hydrolyzed at the cell surface during uptake.