lipoproteins in normal and lipoprotein lipase-deficient humans

Transcription

1 Proc. Natl. Acad. Sci. USA Vol. 81, pp , March 1984 Medical Sciences Metabolism of apolipoproteins B-48 and B-100 of triglyceride-rich lipoproteins in normal and lipoprotein lipase-deficient humans ANTON F. H. STALENHOEF*t, MARY J. MALLOYtt, JOHN P. KANEt, AND RICHARD J. HAVELt tcardiovascular Research Institute and Departments of Medicine and IPediatrics, University of California, San Francisco, CA Contributed by Richard J. Havel, December 12, 1983 ABSTRACT The metabolism of apolipoproteins B-48 and B-100 (apo B-48 and B-100) in large triglyceride-rich lipoproteins (300 to 1500 i in diameter) has been compared in three normal subjects and two subjects with genetically determined deficiency of lipoprotein lipase. The triglyceride-rich lipoproteins were obtained from a lipoprotein lipase-deficient donor 4 hr after a fat-rich meal in order to obtain chylomicrons (containing apo B-48) and very low density lipoproteins (VLDL) (containing apo B-100), whose properties had not been modified by the action of this enzyme. The triglyceride-rich lipoproteins were labeled with 1251 and injected intravenously into recipients who had fasted overnight. In normal recipients, most of the apo B-48 was removed from the blood within 15 min, and most of the apo B-100 was removed within 30 min. In the lipoprotein lipase-deficient recipients, most of the injected apo B-100 remained in the blood for more than 8 hr; removal of apo B-48 was only slightly more rapid. In all subjects, only trace amounts of either protein were found in lipoproteins more dense than g/ml. The results indicate that (') the removal of the apo B of both chylomicrons and large VLDL from the blood is dependent upon the hydrolysis of their component triglycerides by lipoprotein lipase, and (it) little or no apo B-48 of chylomicrons or apo B-100 of large VLDL is converted appreciably to low density lipoproteins (LDL). Our results suggest that the reported variability of the conversion of VLDL to LDL may be related to the size and composition of the particles secreted from the liver. The rapid production of remnant particles that are removed efficiently by the liver may minimize the opportunity for further reactions leading to the formation of LDL. Apolipoprotein B (apo B) is thought to be essential for the secretion of triglyceride (TG)-rich lipoproteins from the intestine and liver (1). Recent research has shown that the apo B secreted from the intestine differs from that secreted by the liver of most mammalian species (2). Although the two proteins are closely related immunologically, the intestinal protein (B-48) has a lower apparent molecular weight and a distinct primary structure from the hepatogenous protein (B- 100). In the rat, apo B-48 in intestinal chylomicrons is rapidly removed from the blood by the liver, and essentially none is converted to lipoproteins of higher density (3). By contrast, a variable fraction of apo B-100, secreted from the liver in very low density lipoproteins (VLDL), is converted to lipoproteins of higher density designated low density lipoproteins (LDL), which are removed much more slowly from the blood (1). In humans, most of the apo B-100 of VLDL is thought to be converted normally to LDL (1). However, the fraction so converted appears to be reduced in persons with elevated VLDL levels (4). The fate of apo B-48 of chylomicrons in humans has not been determined. In the present research, we have compared the metabolism of apo B-48 and apo B-100, contained The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C solely to indicate this fact. in TG-rich lipoproteins from a patient with familial lipoprotein lipase (LPLase) deficiency, a condition associated with severely impaired catabolism of plasma TGs (1). In this condition, these lipoproteins are not subject to the action of LPLase, which normally degrades them to remnant particles. The metabolism of these remnants differs from those of their undegraded precursors (5). Here, we report on the rate of removal of apo B-48 and apo B-100 from the TG-rich lipoprotein fraction of plasma of normolipidemic persons and persons with LPLase deficiency and the extent of conversion of these proteins to lipoproteins of higher density. MATERIAL AND METHODS Subjects. Two patients with LPLase deficiency and three normolipidemic subjects were injected intravenously with TG-rich lipoproteins from one of the LPLase-deficient subjects (J.C.). Their clinical characteristics are summarized in Table 1. The subjects were given 1 g of potassium iodide daily for 5 days to block the thyroidal uptake of The protocol was approved by the Committee on Human Research of the University of California, San Francisco. Written informed consent was obtained from each subject. Preparation and Characterization of Labeled TG-Rich Lipoproteins. All procedures were performed under sterile conditions. The donor, J.C., was given a diet containing less than 5 g of fat daily for 3 days to reduce the concentration of large chylomicrons. On two occasions, she received a meal containing 100 g of olive oil. Four hours later, approximately 300 ml of blood was obtained by venipuncture, mixed with 1 mg of EDTA per ml, and centrifuged at 1000 x g for 20 min at room temperature. Plasma (3 ml) was overlayed with 0.15 M NaCl/1 mm EDTA, ph 7.4, in each of 54 polyallomer centrifuge tubes and centrifuged in 40.3 rotors of Beckman ultracentrifuges at 270C for 30 min at 30,000 rpm. The TGrich particles were recovered in the top 1.0 ml. To facilitate layering, the density of the isolated material was increased by addition of 0.5 volume of 2H20 containing 1 mm EDTA. Centrifugation was then repeated as before. The albumin content of the recentrifuged material, estimated immunochemically by rate nephelometry, was less than 20% of total protein. The TG-rich particles were labeled with 125I by the iodine monochloride method (6) and then incubated with 4 volumes of the recipient's plasma for 1 hr at 23 C to reduce the amount of labeled proteins other than apo B by exchange with high density lipoproteins. The particles were then reisolated by a single ultracentrifugation as described above. Tests were performed to ensure the absence of endotoxins (Pyrogent R; Mallinckrodt) and bacteriological sterility. The labeled TG-rich lipoproteins were injected within 60 hr of Abbreviations: apo B, apolipoprotein(s) B; TG, triglyceride; VLDL, very low density lipoproteins; LDL, low density lipoproteins; LPLase, lipoprotein lipase; IDL, intermediate density lipoproteins. *Present address: Department of Medicine, Division of General Internal Medicine, University of Nijmegen, Nijmegen, The Netherlands. 1839

2 1840 Medical Sciences: Stalenhoef et alp Table 1. Characteristics of recipients of TG-rich lipoproteins Serum total Subject* Height, cm Weight, kg cholesterol, mg/dl Serum TG, mg/di Normolipidemic M.B. (30, M) S.M. (31, M) W.V. (30, M) LPLase deficiency J.C. (45, F) F.L. (36, F) *Age in years and sex are shown in parentheses. isolation from the donor. The amounts of TG and protein injected into each recipient were mg and mg, respectively. To measure lipoprotein composition, TGs (7), free and esterified cholesterol (8), phospholipids (9), and protein (10) were estimated. Molecular species of apo B were separated in polyacrylamide gels containing NaDodSO4 as described (11) with the modifications described below. The percentage of total 1251 in apo B-100 and apo B-48 varied from 3.3 to 8.3 and from 2.7 to 5.6, respectively. Lipid labeling was 39-55%, and 5-7% was soluble in 1o trichloracetic acid. Fractions of the TG-rich lipoproteins were separated by gel permeation chromatography (12). Electron microscopy was performed on samples negatively stained with potassium phosphotungstate (13), and particle diameters were measured on the photographic prints (14). Separation of Lipoprotein Fractions. After injection of the TG-rich lipoproteins, the recipients were given fat-free meals for the next 24 hr. Blood was collected at intervals into EDTA. Lipoproteins were immediately separated by sequential ultracentrifugation of 12 ml of plasma in 40.3 rotors of Beckman ultracentrifuges at nonprotein solvent densities of 1.006, 1.019, and g/ml (15). Densities were adjusted to g/ml with 2H20 in 0.15 M NaCl and with KBr to g/ml. Top fractions were collected in a volume of 3.0 ml. KBr was removed by dialysis against 0.15 M NaCl. A Proc. Nad Acad Sci. USA 81 (1984) Measurement of Radioactivity. Apo B-100 and B-48 were isolated by electrophoresis in NaDodSO4/polyacrylamide gels as described (11), with the fbllowing modifications. Cylindrical acrylamide gradient gels (2.5-27%, Isophore electrophoresis gels; Isolab, Akron, OH) were used instead of 3% gels. These gels provided narrower bands, and up to 400,g of VLDL protein or 200,ug of LDL protein could be applied. Gels were prerun with 0.1 M phosphate tank buffer containing 0.1% NaDodSO4 for 30 min. Portions of the lipoprotein fractions ( ,ul) were applied directly to the gel after additions of NaDodSO4 and sucrose (final concentrations each, 3%). Electrophoresis was performed in the presence of reducing agents (1% mercaptoacetic acid and 1% mercaptopropanediol) for 16 hr at 2 ma. The gels were removed, fixed, and stained in methanol/acetic acid/water, 3:1:6 (vol/vol), containing % Coomassie blue for 2 hr at room temperature and 2 hr at 60 C. The gels were destained overnight at room temperature in the same solution without dye. The mobilities of molecular species of apo B were identified with human LDL for B-100 and with rat mesenteric lymph for B-48 (11). Gels were sliced into 7-9 segments, and radioactivity in each segment was measured in a gamma spectrometer. Recovery of radioactivity in VLDL added to the gels was 75-82% in the normal subjects and 60-67% in the subjects with LPLase deficiency. Recoveries did not vary with time after injection. Correction was made for these systematic losses with the assumption that all components were lost equally. All analyses were performed in triplicate. The labeled lipoprotein fractions were assayed for unbound iodine by precipitation with 10%o trichloracetic acid in the presence of carrier albumin. The amount of 1251 in total B-100- B-48 - C 15 B W Alb-m l, Diameter, A FIG. 1. Distribution of particle diameters in photographic prints of the two preparations of negatively stained TG-rich lipoproteins. (A) First preparation. (B) Second preparation. FIG. 2. NaDodSO4 gradient gel electrophoretograms (2.5-27% polyacrylamide) of proteins of the first preparation of TG-rich lipoproteins before (Left) and after (Right) incubation with plasma of recipient M.B. apo B-48 and apo B-100 are well resolved.

3 Medical Science.: Stalenhoef et al ap6 B was assessed in intermediate density lipoproteins (IDL; < p < g/ml) and LDL (1.019 < p < g/ml) from the difference between 1251 in the total sample and the isopropanol-soluble fraction (16). RESULTS Characterization of Labeled TG-Rich Liporoteins. The labeled lipoproteins contained approximately 8% cholesteryl esters, 75% TGs, 4% cholesterol, 10% phospholipids, and 4% protein. The distribution of particle diameters in the two preparations ranged from 250 to 2600 A, with 97% between 300 and 1500 A; median diameters were 560 and 575 A (Fig. 1). NaDodSO4 electrophoretograms of one labeled preparationi before and after incubation with one recipient's plasma are shown in Fig. 2. A separate sample was fractionated by chromatography on 4% agarose gel. Fractions containing particles with a median diameter of 470 A contained 47% of the 1251 in apo B-100 and 18% of that in apo B-48, whereas fractions with a median diameter of 700 A contained 59% of the 1251 in apo B-48 and 26% of that in apo B-100. Removal of apo B-100 and B-48 in TG-Rich Lipoproteins from Blood Plasma. The removal of I251-labeled B-100 and B- 48 (I251-B-100 and 12$I-B-48) from the lipoprotein fraction of density < g/ml is shown in Fig. 3. In normolipidemic subjects, both labeled proteins were removed rapidly. In normolipidemic subjects M.B. and S.M., who received the first preparation of labeled TG-rich lipoproteins, the initial half-times of removal were min for '251-B-100 and 5-15 min for I251-B-48. The second preparation was injected into one normolipidemic subject and the two LPLase-deficient subjects. apo B-100 and B48 were removed very rapidly in the former: within 5 min, one-half of the injected 12 I-B-100 and 125I-B-48 had disappeared. This subject had the lowest plasma TG concentration (Table 1). In LPLase-deficient subject F.L., there was essentially no removal of 125I-B-48 A Proc. NatL Acad Sci. USA 81 (1984) 1841 for several hours; removal was slow in subject J.C. Removal of 125I-B-100 was slow in both subjects. In normolipidemic subjects M.B. and S.M., a small amount of radioactivity was found in IDL, but this amount barely exceeded that observed when a portion of the labeled TG-rich lipoproteins was mixed with the recipient's plasma in vitro to give a concentration similar to that occurring immediately after injection in vivo. Only trace amounts (<1%) of 125I-B-100 (and no i25i-b-48) were detected in LDL. In subject W.V., in whom disappearance of 125I-B proteins from the fraction of density < g/ml was rapid, there was a transient increase in IDL of both 125I-B-100 and 1251I-B- 48, equivalent to about 5% of each injected labeled protein, 5-15 min after injection. No 125I-B-100 or 125I-B-48 was detected in LDL. In the patients with LPLase-deficiency, virtually all of the 1251 in IDL was isopropanol-soluble and essentially no radioactivity was found in LDL. DISCUSSION These studies show remarkable similarities in the catabolism of TG-rich lipoproteins that contain apo B-48 and apo B-100. apo B is not thought to exchange between lipoprotein particles; hence, apo B-48 and apo B-100 uniquely reflect the metabolism of particles derived from intestine and liver, respectively. The slow rate of removal of both populations of particles when injected into LPLase-deficient subjects suggests that they were "nascent" particles-i.e., they had not been acted upon by lipolytic enzymes. The metabolism of apo B-48 conformed to that predicted from studies of this protein in rat chylomicrons (3) and studies of the metabolism of chylomicron TGs in normal and LPLase-deficient humans (17). In normal subjects, most of the protein was removed from the blood in a few minutes, and little or none was retained in lipoproteins of density > g/ml. Similar results have been obtained in a patient 0 * U 10.Q.Ė_, el co a C _ D 4) 4 : 40 m._, ci FIG Hours after injection Removal from blood plasma of apo B-100 and apo B-48 in TG-rich lipoproteins from LPLase-deficient donor J.C. The first preparation was injected into normal subjects M.B. (o) and S.M. (o). The second preparation was injected into normal subject W.V. (A) and LPLasedeficient subjects J.C. (i) and F.L. (e). Values are expressed as the percentage of the calculated dilution of the injected labeled proteins in the plasma volume of each recipient, taken as 40 ml/kg of body weight. In normal subjects (control), removal of apo B-100 (A) was only slightly slower than that of apo B-48 (C). Removal of both proteins was much slower in LPLase-deficient subjects (B and D).

4 1842 Medical Sciences: Stalenhoef et al. with a selective deficiency of apo B-100, into whom autologous chylomicrons were injected, except that removal of apo B-48 was somewhat slower (18). In LPLase-deficient subjects, removal of the protein was markedly delayed, consistent with dependence of the initial step of chylomicron metabolism upon LPLase. A slower removal might result from the expanded size of the chylomicron pool. However, 20 g or more of chylomicron TG normally can be removed from the blood hourly during alimentary lipemia. In the LPLase-deficient subjects, approximately this amount of total TG (chylomicrons plus VLDL) was present in the plasma, but no more than a fraction of this was removed in many hours. Therefore, as in the rat, catabolism of chylomicron TG and apo B- 48 in humans evidently proceeds efficiently only when LPLase is active. Also as in the rat, it seems probable that the apo B-48 in chylomicron remnants is rapidly removed and catabolized in the liver. Schaefer et al. (19) have reported that about 15% of total apo B in human thoracic duct lymph chylomicrons is converted to LDL. Presumably, this represented conversion of apo B-100 derived mainly from contaminating hepatic lymph. By contrast, the metabolism of apo B-100 did not conform to that predicted from published studies in which the entire VLDL fraction of density < g/ml has been labeled. Such VLDL are considerably smaller than those that we injected. Furthermore, they contain not only nascent particles but also a variable amount of remnant VLDL that have been partially metabolized by LPL (20). In normolipidemic humans, such VLDL have residence times of 1-3 hr. By contrast, we observed much shorter residence times for apo B- 100 derived from larger VLDL from a subject with LPLasedeficiency, only slightly longer than that observed for apo B- 48. The rate of removal of chylomicrons (21) and VLDL (22) is a function of particle size in experimental animals. It has been proposed that the more rapid removal of larger particles reflects association with a larger number of LPLase molecules at the surface of the capillary endothelium (23). Our results suggest that the small differences that we observed in the rate of removal of chylomicrons and VLDL from the blood of normal humans reflect the differing size of the particles injected, rather than an intrinsic difference in susceptibility to the enzyme's action. The slower removal of apo B-100 observed when the entire VLDL fraction is injected then may reflect the smaller size of the average particle. If so, the distribution of circulating VLDL particles does not reflect that of secreted particles. Rather, the smaller secreted particles should predominate in plasma VLDL, together with remnants derived from particles of all sizes. Recently, Nestel et al. (24) reported on the rates of removal of radioiodinated apo 1B-100 and apo B-48 in TG-rich lipoproteins obtained from patients with severe hypertriglyceridemia not associated with genetically determined LPLasedeficiency. In most cases, the total VLDL fraction was injected, and the size distribution of particles containing the labeled apo B species was not specified. apo B-100 was removed from the VLDL fraction of hypertriglyceridemic recipients more rapidly than was apo B-48. In two normotriglyceridemic subjects, the rates were comparable, but results were expressed as a fraction of the value found 10 min after injection, a time at which a large fraction of the apo B- 48 should have left the blood. In view of this difference in experimental design and analysis, comparison with our results is not warranted. They did not measure the conversion of either form of apo B to particles more dense than g/ml. Our results also provide strong evidence that the removal of large nascent VLDL is dependent upon the initial action of LPLase. Removal of apo B-100 in the two enzyme-deficient subjects did not differ appreciably from that of apo B- 48. Nicoll and Lewis (25) have reported that the removal of Proc. Natl. Acad Sci. USA 81 (1984) apo B of autologous TG-rich lipoproteins from LPLase-deficient subjects, maintained on a fat-free diet, was impaired little or not at all. This difference may reflect the injection of a population of smaller VLDL than we have used. As suggested by Nicoll and Lewis (25), the component TGs of small VLDL may be hydrolyzed by hepatic lipase as well as LPLase. Alternatively, these particles might be recognized by the hepatic LDL receptor without preceding lipolysis. The current results are consistent with our earlier observation of grossly impaired catabolism of TG of large VLDL in a LPLase-deficient patient (22) and with the observation that LPLase-deficient patients maintained on low fat diets may develop severe hypertriglyceridemia during pregnancy (1). During pregnancy, VLDL-TG secretion is thought to be increased, and large VLDL particles may then be produced. Finally, our results demonstrate that large nascent VLDL may not be converted to LDL at all. This was the case not only in LPLase-deficient subjects but also in normolipidemic ones, in whom remnant particles presumably were formed efficiently. The mechanisms by which LDL are normally produced is unclear. LDL-like particles can be produced from VLDL by the action of LPLase in vitro (26), but other evidence suggests that the liver participates in the conversion of remnant VLDL to LDL (5, 27). It has been evident for some time that the conversion of VLDL to LDL must involve more than hydrolysis of VLDL TG and associated transfer of surface components to other lipoproteins, such as high density lipoproteins. VLDL particles, especially those of large size, may contain more molecules of cholesteryl esters than are found in LDL, particularly in hypertriglyceridemic subjects in whom VLDL have long residence times (12). The larger amount of cholesteryl esters in VLDL of hypertriglyceridemic subjects may result from the continuing process of transfer of these esters from the site of esterification (12). Our results show that remnant VLDL evidently are not only produced rapidly from such large VLDL but are rapidly cleared from the blood, presumably by LDL receptors in the liver (28). Under these circumstances, there is little opportunity for the further processing to take place of cholesteryl esters and TG that may be required to yield LDL.$ All three of our normolipidemic recipients had low concentrations of LDL cholesterol, and one had an unusually low level of VLDL TG as well. This could reflect a high activity of hepatic LDL receptors, and a lower than average fractional conversion of VLDL to LDL. In any event, our findings indicate that VLDL as well as chylomicrons may have substantial capacity to transfer plasma cholesteryl esters to the liver. Our results may help to explain the large species differences in the extent to which VLDL are converted to LDL. In the rat, in which less than 5% is converted (29),11 the median diameter of VLDL is about 450 A. In guinea pigs, in which 10-15% is converted, diameters are somewhat smaller (about 400 A) (30); similar values for conversion and diameter have been found in rabbits (28, 31). Human VLDL normally are smaller still. Their median diameter is about 350 A (1, 12), and substantially more is converted to LDL, with reported normal values ranging from % (32). The wide variation reported may reflect in part differences in the size of VLDL particles used as tracers and, consequently, the efficiency with which the remnant products are removed by the liver. In addition, smaller remnant VLDL which are largely converted to LDL may constitute a larger fraction of $Consistent with this interpretation, the fraction of VLDL converted to LDL is increased in LDL receptor-deficient rabbits, either genetically determined (28) or acquired during a 9-day fast (unpublished observations). 1The presence of botn apo B-48 and apo B-100 in rat VLDL may also contribute to this low value.

5 Medical Sciences: Stalenhoef et al VLDL from humans than the other mammals studied. Several reports suggest that the extent of conversion of VLDL to LDL is reduced in human hypertriglyceridemia (4, 32, 33). This could reflect the large size of VLDL particles present in some hypertriglyceridemic persons (12). Furthermore, dependence of conversion upon particle size may explain in part the frequently observed inverse relationship between the concentrations of VLDL and LDL in various disease states and during successful treatment of hypertriglyceridemia (1). In both situations, a crucial determinant of LDL formation may be the size of the nascent VLDL secreted by liver. We thank Robert Hamilton for preparing the electron micrographs of lipoprotein fractions and Bess Fung and Kyee Yeo for technical assistance. A.F.H.S. was the recipient of a North Atlantic Treaty Organization Science Fellowship awarded by the Netherlands Organization for Advancement of Pure Research (Nederlandse Organisatie voor Zuiver-Wettenscheppelijk Onderzoek). This research was supported by a grant from the National Institutes of Health (HL , Arteriosclerosis SCOR) and was carried out in the General Clinical Research Center, University of California, San Francisco, with funds provided by the Division of Research Resources, RR-79, U.S. Public Health Service. 1. Havel, R. J., Goldstein, J. L. & Brown, M. S. (1980) in Metabolic Control and Disease, eds. Bondy, P. K. & Rosenberg, L. E. (Saunders, Philadelphia), 8th Ed., pp Kane, J. P. (1983) Annu. Rev. Physiol. 45, van't Hooft, F. M., Hardman, D. A., Kane, J. P. & Havel, R. J. (1982) Proc. Natl. Acad. Sci. USA 79, Sigurdsson, G., Nicoll, A. & Lewis, B. (1976) Eur. J. Clin. Invest. 6, Havel, R. J. (1980) Ann. N. Y. Acad. Sci. 348, Sigurdsson, G., Noel, S.-P. & Havel, R. J. (1978) J. Lipid Res. 19, Rush, R., Leon, L. & Turrell, J. (1970) in Advances in Automated Analysis-Technicon International Congress, ed. Barton, E. C. (Media, Tarrytown, NY), Vol. 1, pp Huang, H. S., Kuan, J. W. & Guilbault, G. G. (1975) Clin. Chem. 21, Stewart, C. P. & Hendry, E. B. (1935) Biochem. J. 29, Lowry, 0. H., Rosebrough, N. J., Farr, A. L. & Randall, R. J. (1951) J. Biol. Chem. 193, Proc. NatL Acad Sci. USA 81 (1984) Kane, J. P., Hardman, D. & Paulus, H. E. (1980) Proc. Natl. Acad. Sci. USA 77, Sata, T., Havel, R. J. & Jones, A. L. (1972) J. Lipid Res. 13, Hamilton, R. L., Havel, R. J., Kane, J. P., Blaurock, A. & Sata, T. (1971) Science 172, Havel, R. J., Kita, T., Kotite, L., Kane, J. P., Hamilton, R. L., Goldstein, J. L. & Brown, M. S. (1982) Arteriosclerosis 2, Havel, R. J., Eder, H. & Bragdon, J. (1955) J. Clin. Invest. 34, Holmquist, L., Carlson, K. & Carlson, L. A. (1978) Anal. Biochem. 88, Havel, R. J. & Gordon, R., Jr. (1960) J. Clin. Invest. 39, Malloy, M. J. & Kane, J. P. (1981) Pediatr. Res. 15, 635 (abstr.). 19. Schaefer, E. J., Jenkins, L. L. & Brewer, H. B., Jr. (1978) Biochem. Biophys. Res. Commun. 80, Pagnan, A., Havel, R. J., Kane, J. P. & Kotite, L. (1977) J. Lipid Res. 18, Quarfordt, S. H. & Goodman, D. S. (1966) Biochim. Biophys. Acta 116, Havel, R. J. & Kane, J. P. (1975) Fed. Proc. Fed. Am. Soc. Exp. Biol. 34, Olivecrona, T. & Bengtsson, G. (1983) in The Adipocyte and Obesity, eds. Angel, A., Hollenberg, C. H. & Roncari, D. A. K. (Raven, New York), pp Nestel, P. J., Billington, T. & Fidge, N. H. (1983) Biochim. Biophys. Acta. 751, Nicoll, A. & Lewis, B. (1980) Eur. J. Clin. Invest. 10, Deckelbaum, R. J., Eisenberg, S., Oschry, Y., Butbul, E., Sharon, J. & Olivecrona, T. (1982) J. Biol. Chem. 257, Turner, P. R., Miller, N. E., Cortese, C., Hazzard, W., Cottart, J. & Lewis, B. (1981) J. Clin. Invest. 67, Kita, T., Brown, M. S., Bilheimer, D. W. & Goldstein, J. L. (1982) Proc. Natl. Acad. Sci. USA 79, Faergeman, O., Sata, T., Kane, J. P. & Havel, R. J. (1975) J. Clin. Invest. 56, Barter, P., Faergeman, 0. & Havel, R. J. (1977) Metabolism 26, Ghiselli, G. (1982) Biochim. Biophys. Acta 711, Kesaniemi, Y. A., Vega, G. & Grundy, S. M. (1982) in Lipoprotein Kinetics and Modeling, eds. Berman, M., Grundy, S. M. & Howard, B. V. (Academic, New York), pp Eaton, R. P., Allen, R. C. & Schade, D. S. (1983) J. Lipid Res. 24,