Plasma lipids and acyltransferase activities in experimental nephrotic syndrome

Lecithin:cholesterol acyltransferase (LCAT) and lysolecithin acyltransferase (LAT) are two activities carried out by the same plasma enzyme, but require different apoprotein activators. The LCAT reaction takes place primarily on high density lipoproteins (HDL) and is activated by serum albumin, whereas LAT takes place on low density lipoproteins (LDL) and is inhibited by albumin. In nephrotic syndrome (NS), the levels of serum albumin are reduced, whereas the LDL levels are increased, and therefore, the ratio of LAT/LCAT activities should be increased. To test this hypothesis, we estimated the lipid levels and the two enzyme activities in experimental NS induced in rats by the injection of anti-Fx1A antibody (passive Heymann nephritis). As found in other nephrotic conditions, the plasma lipid levels rose progressively as the proteinuria increased and the serum albumin concentration declined. In addition, the ratio of LAT/LCAT activities increased by about fourfold after nine days of induction of nephritis. The LCAT activity correlated positively and the LAT activity negatively with serum albumin levels. The esterified cholesterol correlated positively with LCAT activity in normal rats but negatively in nephrotic animals, indicating that most of the cholesteryl esters in NS may be non-LCAT derived. The free cholesterol/lecithin ratio, a known risk factor for atherosclerosis, increased significantly in nephrotic rats. Furthermore, since the increase in the LAT activity produces more disaturated lecithins, another putative risk factor, the cumulative risk of coronary heart disease may be increased in long-term NS.

Molecular species of phosphatidylcholine in familial lecithin-cholesterol acyltransferase deficiency: effect of enzyme supplementation

We studied the molecular species composition of plasma phosphatidylcholine (PC) in three patients with familial deficiency of lecithin-cholesterol acyltransferase in order to determine whether certain species are increased in the absence of this enzyme activity. Compared to normal plasma, the deficient plasma contained significantly higher percentages of 16:0-18:2 and 18:0-18:2 species and lower percentages of 16:0-20:4 and 18:0-20:4 species. The bulk of the total plasma PC as well as the abnormal composition of molecular species were found in the very-low-density and low-density lipoprotein (VLDL + LDL) fractions. When the deficient plasma was incubated with partially purified enzyme from normal human plasma, there was a significant reduction in the amounts of most major species, mainly in the VLDL + LDL fraction. When the selectivity factors were calculated by dividing the percentage contribution of each species for cholesterol esterification by its percentage concentration, the highest selectivity factors were found for 16:0-18:2, 18:1-18:1 and 18:1-18:2. The order of selectivity of the enzyme for various species was very similar to that obtained earlier using normal HDL as substrate. These results show that lecithin-cholesterol acyltransferase has significant effects on the molecular species composition of plasma PC and the deficiency of the enzyme results in accumulation of certain PC species normally used by the enzyme, as well as in abnormal distribution of these species among the lipoproteins.

Abnormal high-density lipoprotein composition in women with insulin-dependent diabetes

To determine whether alterations in lipoprotein phospholipid composition might be an unrecognized factor that contributes to the unexplained acceleration of atherogenesis and the loss of sex-related protection from the development of coronary heart disease in women with insulin-dependent diabetes mellitus, we have estimated levels of neutral lipids, apolipoproteins (A-I, A-II, B), and free cholesterol (FC) in plasma and the four major phospholipid constituents of the very low-density lipoprotein + low-density lipoprotein and high-density lipoprotein (HDL) fractions in 12 ambulatory female patients with varying degrees of diabetic control. Although levels of triglyceride, cholesterol, HDL-cholesterol, and lipoprotein phospholipids in whole plasma of the patients with diabetes were similar to those in controls, their FC levels and FC/lecithin ratio, a recently described index of cardiovascular risk, both were abnormally increased (p less than 0.01). In the HDL-containing plasma fraction, concentrations of sphingomyelin, lecithin, and lysolecithin all were significantly reduced (p less than 0.05; p less than 0.01, and p less than 0.02, respectively). These compositional changes may be potentially atherogenic, because a reduction in the phospholipid content of HDL may impair its capacity to promote the efflux of cholesterol from cells, and the transfer of cholesterol ester from HDL to the larger apo-B-containing lipoproteins is inhibited when their content of FC is increased relative to phospholipid. These previously unrecognized qualitative defects, which are inapparent in the routine estimation of plasma lipids, may compromise reverse cholesterol transport and thereby promote atherogenesis in women with insulin-dependent diabetes mellitus.

Substrate specificity of human plasma lecithin-cholesterol acyltransferase towards molecular species of phosphatidylcholine in native plasma

The specificity of human plasma lecithin-cholesterol acyltransferase for molecular species of phosphatidylcholine (PC) was studied by determining the molecular species composition of whole plasma before and after incubation at 37 degrees C. Since the disappearance of PC under the conditions employed is entirely due to the activity of lecithin-cholesterol acyltransferase, its specificity can be determined from the decrease in the concentration of each species after the reaction. The selectivity factor for each species was calculated by dividing its observed contribution by its concentration at zero time. The major species contributing to cholesterol esterification in whole plasma were 16:0-18:2 (46%), 18:0-18:2 (16%), 16:0-18:1 (15%), 16:0-20:4 (10%), 18:0-20:4 (5%) and 18:1-18:2 (5%). The specificity, as determined from the selectivity factors for whole plasma, was in the order: 16:0-18:2 greater than 18:1-18:2 greater than 16:0-18:1 greater than 18:0-18:2 greater than 16:0-22:6 greater than 18:0-20:4 greater than 16:0-20:4. The high-density lipoproteins (HDL) contained a significantly higher percentage of 16:0-20:4 and 18:0-20:4 and a lower percentage of 16:0-18:1 and 18:0-18:1 compared to the very-low and low-density lipoproteins. These differences disappeared after incubation of the plasma for 24 h. Using selectivity factors for HDL PCs only, the specificity of the enzyme was found to be in the order: 16:0-18:2 greater than 18:1-18:2 greater than 18:1-18:1 greater than 16:0-22:6 greater than 18:0-18:2 greater than 16:0-18:1 greater than 16:0-20:4. These results indicate that in native plasma, lecithin-cholesterol acyltransferase prefers 16:0 greater than 18:1 greater than 18:0 at the 1-position and 18:2 greater than 18:1 greater than 22:6 greater than 20:4 at the 2-position of PC.

Influence of low estrogen-containing oral contraceptives on lipoprotein phospholipid composition and mononuclear cell membrane fluidity

Since the effects of the new low estrogen-containing oral contraceptive (OC) preparations on lipoprotein phospholipid (PL) composition are unknown, we studied 3 groups of 10 young women before and after 6 months of use of 3 commonly prescribed agents containing almost identical amounts of ethinyl estradiol (0.30-0.35 microgram) and differing progestogens, and correlated these changes with their estrogen to progestin (E/P) ratio. The directional changes in both plasma neutral lipid and PL concentrations tended to correlate with the E/P ratio, with plasma HDL-cholesterol (HDL-C) falling slightly and the low density lipoprotein-cholesterol (LDL-C)/HDL-C ratio increasing in the women taking the OC with the lowest E/P ratio; in contrast, plasma HDL-C increased and the LDL-C/HDL-C ratio fell in those receiving the preparation with the highest E/P ratio. In HDL, the ratio of the 2 principal PL, sphingomyelin and lecithin, an index of lipid fluidity, tended to increase, suggesting that the surface of this lipoprotein class had become more rigid. This change was most apparent in women receiving the agent in which the progestin was predominant; in women receiving the preparations with the higher E/P ratios the sphingomyelin/lecithin ratio actually declined. The membrane fluidity of mononuclear cells obtained from five women taking an OC with a relatively high E/P ratio, however, was significantly increased (P less than 0.007) compared to that in normal women. These findings demonstrate that, even with substantial reductions in their estrogen content, the use of these newer OC is associated with quantitative and qualitative changes in lipoprotein PL composition that parallel their E/P balance and are associated with altered fluidity of mononuclear cell membranes.

Effects of serum from oral contraceptive users on the metabolism of low density lipoprotein

The metabolism of radiolabeled 125I-low density lipoprotein (LDL) was studied in cultured human aortic smooth muscle cells (SMC) to investigate potential mechanisms contributing to heart attack and stroke in young women taking oral contraceptive (OC) preparations. No consistent difference was discerned in the capacity of these cells to bind and internalize LDL following 24-hour exposure to pooled lipoprotein-poor (d greater than 1.25 gm/ml) serum from nonsmoking OC users and controls. However, significantly (p greater than 0.01) less soluble radioactive material was released into the media from dishes containing SMC incubated with OC serum, indicating that their capacity to degrade LDL was impaired. Increased amounts of both cholesterol and cholesterol ester and electron microscopic evidence of lipid accumulation were found in SMC grown for 12 days under standard culture conditions with 10% OC serum. This combination of in vitro findings suggests that OC use may accelerate the development of cardiovascular disease in some women by modifying the metabolism of LDL by cells of the arterial wall.

Glucocorticoid-related impairment in the metabolism of low density lipoprotein by human fibroblasts

The metabolism of radiolabelled 125I-low density lipoprotein (LDL) was studied in cultured human dermal fibroblasts to investigate potential mechanisms contributing to the accelerated development of cardiovascular disease in patients treated chronically with corticosteroids. Fibroblasts exposed for 48 hours to pooled lipoprotein-poor (d greater than 1.25 gm/ml) serum from glucocorticoid-treated patients showed an increased capacity to bind LDL (p less than .001) compared to cells incubated under identical conditions with pooled serum from controls. In addition, a significantly (p less than .001) reduced amount of soluble radioactive material appeared in the media indicating that exposure of fibroblasts to corticosteroid serum also impaired their capacity to degrade LDL. If this tendency of cultured cells to accumulate cholesterol-rich lipoprotein when exposed to constituents of serum from these patients is present in patients receiving long-term treatment with glucocorticoids, it might influence arterial lipid accumulation and accelerate their developing cardiovascular disease.

The measurement of arterial smooth muscle cell mitogens in the blood of oral contraceptive users

We have previously shown that serum from young women receiving the same combined mestranol-norethindrone containing oral contraceptive (OC) preparation accelerated the proliferation of arterial smooth muscle cells (SMC) in tissue culture, and this in vitro effect was not a direct action of either of its estrogenic or progestogenic constituents. To identify the substance(s) which might contribute to this potentially atherogenic action, blood was obtained from 20 OC users and control women for the measurement of growth hormone, insulin, somatomedins (insulin-like growth factor IGF-I and IGF-II), and the platelet alpha-granule constituents platelet-derived growth factor (PDGF), beta-thromboglobulin, and platelet factor 4 (PF4). No difference was demonstrable between OC users and controls in the levels of any of these growth-promoting hormones, nor in plasma concentrations of any of the platelet alpha-granule proteins. These studies indicate that the enhanced mitogenicity found in OC sera is probably not attributable directly to these hormones or PDGF, and may instead result from an in vivo OC-induced alteration in other as yet unidentified mediators of cellular growth.

Substrate specificity of plasma lysolecithin acyltransferase and the molecular species of lecithin formed by the reaction

Human plasma lecithin-cholesterol acyltransferase also converts lysolecithin to lecithin in the presence of low density lipoproteins. To understand the physiological importance of this lysolecithin acyltransferase reaction, we investigated the molecular species of lysolecithin available for acylation in normal plasma and the lecithins which are formed by the acylation of each of these lysolecithins. Palmitate- and stearate-containing lysolecithins were formed by the lecithin-cholesterol acyltransferase reaction, whereas oleate- and linoleate-containing lysolecithins were formed by the action of post-heparin lipase(s). All the natural lysolecithins were esterified at comparable rates by the isolated enzyme. Lyso platelet-activating factor was esterified about 70% as efficiently as the lysolecithins, while lysophosphatidylethanolamine was esterified at about 30% the rate observed with lysolecithin. The 2-acyl isomers of lysolecithin were acylated to the same extent as the 1-acyl isomers, although considerable isomerization of the former took place during the incubation. There were no net changes in the concentrations of lecithin and lysolecithin after 6 h of incubation with the enzyme, although over 10% of the labeled lysolecithin was converted to lecithin, indicating that the endogenous lecithin serves as the acyl donor in the reaction. When the molecular species of lecithin formed were analyzed by high performance liquid chromatography, the same pattern of fatty acid incorporation was observed with all the lysolecithins used. The bulk of the radioactivity was incorporated into molecular species formed by the acylation with linoleic, oleic, and palmitic acids, in decreasing order. However, in each case, the lecithins formed by acylation with palmitic acid had the highest specific radioactivity, followed by those acylated with linoleic and oleic acids. From these results it is postulated that the enzyme alters the molecular species composition of lecithin in plasma without increasing the net amount of total lecithins.

Role of low density lipoprotein in the activation of plasma lysolecithin acyltransferase activity. Effect of chemical and enzymatic modifications of the lipoprotein on enzyme activity

The effect of various chemical and enzymatic modifications of low density lipoprotein (LDL) on its ability to activate the isolated human plasma lysolecithin acyltransferase (LAT) was studied. Removal of all lipids from LDL resulted in the complete loss of LAT activation. Removal of only neutral lipids by extraction with heptane retained up to 50% of the original activity, which was not increased further by reconstitution of the LDL with the extracted lipids. Hydrolysis of the diacylphosphoglycerides of the LDL with phospholipases resulted in complete loss of LAT activation which was partially restored by the addition of egg lecithin. Hydrolysis of more than 4% of LDL protein by trypsin led to a linear decrease in activity with complete loss of activity occurring when about 25% of the LDL protein is hydrolyzed. Modification of the arginine groups of LDL reversibly inhibited the activation of LAT. Modification of lysine residues of LDL by acetylation, acetoacetylation or succinylation also abolished its ability to activate lysolecithin acylation.

Increased lysolecithin acyltransferase activity in the plasma of type II hyperlipoproteinemic patients

Lecithin-cholesterol acyltransferase (LCAT) in human plasma has been shown to acylate lysolecithin to lecithin in presence of low density lipoprotein (LDL). To determine the physiological importance of LDL in activating lysolecithin acyltransferase (LAT) activity, we assayed the LAT activity in 18 hypercholesterolemic (Type II) patients and 15 control subjects. The enzyme activity was about 60% higher in the patients compared with the controls. On the other hand, the LCAT activity, measured by 2 different procedures, as well as enzyme mass, determined by radioimmuno assay, were comparable in the controls and hypercholesterolemics. The LAT activity was highly correlated with LDL levels in the plasma, but the LCAT activity and the enzyme mass had no correlation with the LDL levels. These results show that the plasma LDL is the rate-limiting activator of the enzyme, and pathological conditions, resulting in higher LDL levels, also cause higher LAT activities.

Studies on the cofactor requirement for the acylation and hydrolysis reactions catalyzed by purified lecithin-cholesterol acyltransferase. Effect of low density lipoproteins and apolipoprotein A-i

Because lecithin-cholesterol acyltransferase (LCAT) has been shown to carry out acylation of lysolecithin as well as hydrolysis of lecithin in addition to an esterification of cholesterol, the cofactor requirements of the three reactions catalyzed by the enzyme were studied. The purified enzyme required apolipoprotein A-I (apo A-I) for both the phospholipase A2 activity (release of free fatty acids from lecithin) and cholesterol esterification, whereas, low density lipoprotein (LDL) was required for the acylation of lysolecithin. Apo A-I and lecithin liposomes could not substitute for LDL for the activation of lysolecithin acyltransferase activity. Removal of apo A-I from the LDL preparation by affinity chromatography did not affect the activation of lysolecithin acylation, indicating that the contaminating apo A-I is not responsible for the activation. LDL facilitates cholesterol esterification in presence of labelled lecithin liposomes by providing the unesterified cholesterol. Removal of contaminating apo A-I, however, abolishes this LCAT activity which could be restored by addition of pure apo A-I. Lysolecithin inhibits both phospholipase A2 and LCAT activities, but LDL appeared to attenuate the effects of lysolecithin, in addition to stimulating the acylation of lysolecithin. These results show that apo A-I is not obligatory for all the reactions carried out by the enzyme, and that LDL plays an important role in the regulation of the hydrolysis and acylation reaction carried out by the enzyme.

Host defense mellitus: defective membrane synthesis during phagocytosis

During phagocytosis, polymorphonuclear leukocytes from poorly controlled diabetic patients convert lysolecithin to lecithin at a lower rate than cells from control subjects. This defect is partially reversed by treatment with insulin. By reducing the availability of structural lipid for formation of the phagocytic vacuole which normally takes place during phagocytosis, this impairment may compromise cellular host-defense mechanisms in some diabetic patients.

Polyamines and atherosclerosis. Platelet releasate and other mitogens stimulate putrescine transport in arterial smooth muscle cells

Although polyamine (PA) levels are believed to increase in response to mitogenic stimuli in all cells during growth, their role in arterial smooth muscle cell (ASMC) proliferation, an essential step in atherogenesis, is unknown. To determine whether the arterial wall mitogen, platelet-derived growth factor (PDGF) influences PA metabolism when cell cycle traverse is initiated, we examined its effects on the transport of the PA precursor [3H]putrescine (PUT) in culture in bovine and human ASMC. PUT uptake was stimulated in a dose-response relationship by PDGF-containing human serum (2-10%), and abolished in 24 h without it. Inhibition of this uptake by spermidine and the lack of effect of thymidine, leucine and ornithine indicated that uptake was by a PA-specific mechanism. Without serum, platelet releasate containing PDGF stimulated PUT uptake but not that of leucine or glucose. While platelet-poor plasma alone also promoted PUT uptake, the combination of platelet releasate and platelet-poor plasma was required for maximal DNA synthesis. PUT uptake under these conditions reached a peak at 16 h, while the synthesis of DNA was maximal at 24 h. Supraphysiological concentrations of insulin, and fibroblast and epidermal growth factors, also stimulated the uptake of the labeled PUT both in the absence and presence of serum, but at much lower rates than those observed with platelet releasate. These findings indicate that the early replicative actions of a variety of mitogens for ASMC involve stimulation of PUT uptake and suggest that PA uptake must precede the initiation of DNA synthesis in ASMC during atherogenesis.

Atherosclerosis and oral contraceptive use. Serum from oral contraceptive users stimulates growth of arterial smooth muscle cells

Pooled serum from women taking a combined estrogen-progestin oral contraceptive preparation caused significantly greater cell proliferation and incorporation of 3H-thymidine into DNA in both human arterial smooth muscle cells and dermal fibroblasts in tissue culture than did serum from controls. A portion of this mitogenic effect appears to be related to the presence of a factor(s) that is heat stable, nondialyzable, and contained in the lipoprotein-deficient serum fraction. In vitro addition of varying concentrations of mestranol and norethindrone, the two constituents of the oral contraceptive preparation taken by participants in the study, to control serum did not enhance its mitogenicity, suggesting that the effect observed with intact serum was due either to a metabolite of these compounds or to the production of other growth-promoting substances during oral contraceptive treatment. Since smooth muscle cell proliferation is an integral feature of all atherosclerotic lesions, these findings provide some insights applicable to understanding the pathogenesis of vascular disease associated with oral contraceptive use.

Requirement of low density lipoproteins for the lysolecithin acyl transferase activity in human plasma: assay of enzyme activity in abetalipoproteinemic patients

Previous studies from this laboratory have shown that the lecithin:cholesterol acyl transferase (LCAT) of normal human plasma can convert lysolecithin to lecithin in the presence of low density lipoproteins (LDL). In order to study the importance of LDL for the lysolecithin acyl transferase (LAT) activity, the LCAT and LAT activities were assayed in two patients with abetalipoproteinemia. The plasma from both patients had only 5%-6% of the LAT activity present in normal plasma. This activity was stimulated up to 22-fold by the addition of normal LDL but not very low density lipoproteins (VLDL) or high density lipoproteins. The LAT activity of normal plasma was only stimulated by two-threefold by LDL. The LCAT activity in both patients was reduced to 42%-46% of the control values. This activity was stimulated up to fourfold by the addition of LDL as well as VLDL which is comparable to the activation obtained with control plasma. These results therefore suggest that LDL is physiological activator of LAT reaction in normal human plasma.

Lecithin: cholesterol acyltransferase activity in dialyzed and undialyzed chronic uremic patients

Lecithin cholesterol acyltransferase (LCAT) and lysolecithin acyltransferase (LAT) activities in plasma were significantly lower in undialyzed than in dialyzed uremic patients and controls. Both LCAT and LAT activities were normal in the maintenance dialysis group. The free/total cholesterol ratio was significantly increased in the undialyzed patients only. This ratio and LCAT were inversely related in the dialyzed group, unrelated in the undialyzed patients, and positively correlated in controls. Unlike non-uremic populations with hypertriglyceridemia in which LCAT is increased, no correlation was found between plasma triglyceride and LCAT in the uremic population studied. Our results show that both LCAT and LAT activities are reduced in undialyzed chronic uremic patients but are not significantly different from control levels in patients treated with chronic hemodialysis. These results indicate that the changes in LCAT activity contrast with those described in lipoprotein composition and metabolism which do not appear to be affected by chronic dialysis therapy.

The uptake and acylation of exogenous lysophosphatidylethanolamine by Escherichia coli cells

Escherichia coli envelopes were fractionated to yield inner and outer membrane fractions. Both these fractions were found to convert [14C]lysophosphatidylethanolamine to its diacyl analogue. Intact Escherichia coli cells were capable of absorbing exogenous labelled lysophosphatidylethanolamine and converting it to phosphatidylethanolamine. When the 14C- and 32P-labelled lyso analogue was used, both the absorption process and the conversion to diacyl analogue proceeded without a significant change in isotope ratio either in the presence or in the absence of added inorganic phosphate. The absorption process was not markedly stimulated by Ca2+ in the medium; it proceeded to an amount representing 25--30% of the endogenous membrane lipid and was accompanied by some degradation to water-soluble products which accumulated in the cell mainly, but also in the incubation medium. The absorbed lipid was recovered in both the inner and outer membrane fractions of the cell envelope. The results indicate that Escherichia coli inner and outer membranes are capable of absorbing exogenous lysophosphoglyceride and converting it into structurally useful diacyl analogue.

Low density lipoprotein-activated lysolecithin acylation by human plasma lecithin-cholesterol acyltransferase. Identity of lysolecithin acyltransferase and lecithin-cholesterol acyltransferase

There is in normal plasma an enzyme activity which converts labeled lysolecithin to lecithin by an energy-independent low density lipoprotein-activated pathway. Studies were undertaken to compare the identity of this enzyme with lecithin-cholesterol acyltransferase. During purification of the enzyme by ultracentrifugation and by chromatography on high density lipoprotein affinity column, DEAE-Sepharose column, and hydroxylapatite column, both the lysolecithin acyltransferase activity and the lecithin-cholesterol acyl transferase activity were found in the same fractions and were enriched to the same extent at each step. The final purified preparation which had 16,000- to 24,000-fold higher specific activities than starting plasma gave a single protein band on polyacrylamide gel electrophoresis and this single band contained both the activities. Also, the effects of pH, heat, and chemical inhibitors on the enzyme activities were similar. Plasma from patients with familial lecithin-cholesterol acyltransferase deficiency also lacked lysolecithin acyltransferase activity. These results indicate that a single enzyme carries out both lecithin-cholesterol acyltransferase and lysolecithin acyltransferase activities. The purified enzyme required apolipoprotein A-I for lecithin-cholesterol acyltransferase activity, but required low density lipoprotein for lysolecithin acyltransferase activity.

Studies of membrane formation in Tetrahymena. The identification of membrane proteins and turnover rates in nongrowing cells

Membrane proteins within cell fractions of Tetrahymena have been separated by polyacrylamide gel electrophoresis and the turnover of these proteins has been studied in nongrowing cells using double labeling procedures. Lactoperoxidase iodination and two-dimensional gel separations have revealed over 20 surface-labeled proteins, two of which are responsible for the single peak found in ciliary and nonciliary surface membranes. Turnover measurements show that there is heterogeneity in the rates at which membrane proteins turn over. The data suggest that larger proteins may turn over at higher rates in the microsomal fraction, as previously shown in mammalian cells. This was not found in the surface membrane fractions of Tetrahymena. However, membrane-associated cytoskeletal proteins had lower turnover rates than integral membrane proteins, and significant heterogeneity was found among the integral proteins measured. The implications of these findings for possible modes of membrane biogenesis in Tetrahymena are discussed.

Association of lysolecithin acyltransferase with the high density lipoproteins and its activation by the low density lipoproteins in normal human plasma

The lysolecithin acyltransferase of human plasma is shown to be associated with the high-density lipoprotein fraction. Although the low density lipoproteins do not have intrinsic enzyme activity, their presence activated the enzyme 3--7-fold. This activation is not affected by heat-treatment of the low density lipoproteins, but is abolished by the addition of heparin.

Polyamines: an unrecognised cardiovascular risk factor in chronic dialysis?

The significance of raised polyamine (P.A.) levels in chronic-dialysis patients is unknown. Since these biologically active substances have hormone-like properties and promote cell-growth in plant and animal tissues, it is possible that they stimulate proliferation of arterial smooth-muscle cells (S.M.C.)--a central process in atherogenesis--and thereby contribute to the rapidly accelerated cardiovascular disease observed during dialysis. Such a role for P.A. is supported by tissue-culture studies, which show not only that P.A.-rich serum from dialysis patients stimulates S.M.C. growth, but also that this mitogenic effect is lost when P.A. are selectively removed from uraemic serum and restored by their addition. Although these observations provide new insights into possible mechanisms of atherogenesis, they are not surprising in view of the many known biological actions of P.A.

Interactions of the components of the prothrombinase complex

The interaction of components of the prothrombinase complex, i.e. bovine Factor X or Factor Xa, bovine Factor V or Factor Va, phospholipid, and Ca2+, in various combinations was studied primarily by a gel filtration technique. In experiments, in which phospholipids ranging from those isolated from naturally occurring sources to those long chain (18 : 1) as well as short chain 6 : 0 and 7 : 0 fatty acids prepared by chemical and enzymatic synthesis were used, it was evident that a net negative surface charge on the lipid dispersions was one of the important requirements for interaction. Though the short chain fatty acid phospholipids interacted with the proteins of the prothrombinase complex, there was invariably a diminution in the activity of the enzyme complex. It was established that Factor V or Va did not bind Ca2+ and that the binding of either of these factors with phospholipids (with a net negative charge) was not dependent on Ca2+. However, the interaction of Factor X or Factor Xa with phospholipids with a negative charge required Ca2+. It was shown that Factor X could bind to the same type of lipid of lipid surface as that noted for Factor Xa. Of interest was the apparent difference in the phospholipid binding characteristics of the two variant forms of bovine plasma Factor X, i.e. X1 and X2, which might in part explain the differences in their specific activities. Of importance was the lack of demonstrable complex formation between Factors II, X and V in the absence of phospholipids and/or in the presence or absence of Ca2+. The significance of these results as they might apply to the configuration of the prothrombinase complex and its interaction with prothrombin plus the usefulness of the short chain fatty acid phospholipid in exploring these lipid-protein interactions are discussed.

The mechanism of intestinal absorption of phosphatidylcholine in rats

1. The mechanism of absorption of phosphatidylcholine was studied in rats by injecting into the intestine phosphatidylcholine specifically labelled either in the fatty acid or in the glycerol moiety or with (32)P, when considerable amounts of 1-acyl-lysophosphatidylcholine were found in the intestinal lumen. 2-([(14)C]Acyl)phosphatidylcholine gave markedly more radioactive unesterified fatty acids in the lumen, compared with the 1-([(14)C]acyl) derivative. Some of the radioactivity from either the fatty acid or the glycerol moiety of the injected phosphatidylcholine appeared in the mucosal triacylglycerols. 2. Injection of (32)P-labelled phosphatidylcholine or (32)P-labelled lysophosphatidylcholine led to the appearance of radioactive glycerylphosphorylcholine, glycerophosphate and P(i) in the mucosa. 3. Rat mucosa was found to contain a highly active glycerylphosphorylcholine diesterase. 4. It was concluded that the dietary phosphatidylcholine is hydrolysed in the intestinal lumen by the pancreatic phospholipase A to 1-acylglycerylphosphorylcholine, which on entering the mucosal cell is partly reacylated to phosphatidylcholine, and the rest is further hydrolysed to glycerylphosphorylcholine, glycerophosphate, glycerol and P(i). The fatty acids and glycerophosphate are then reassembled to give triacylglycerols via the Kennedy (1961) pathway.

Acylation of lysolecithin in the intestinal mucosa of rats

1. The presence of an active acyl-CoA-lysolecithin (1-acylglycerophosphorylcholine) acyltransferase was demonstrated in rat intestinal mucosa. 2. ATP and CoA were necessary for the incorporation of free [1-(14)C]oleic acid into lecithin (phosphatidylcholine). 3. The reaction was about 20 times as fast with [1-(14)C]oleoyl-CoA as with free oleic acid, CoA and ATP. 4. With 1-acylglycerophosphorylcholine as the acceptor, both oleic acid and palmitic acid were incorporated into the beta-position of lecithin; the incorporation of palmitic acid was 60% of that of oleic acid. 5. Of the various analogues of lysolecithin tested as acyl acceptors from [1-(14)C]oleoyl CoA, a lysolecithin with a long-chain fatty acid at the 1-position was most efficient. 6. The enzyme was mostly present in the brush-border-free particulate fraction of the intestinal mucosa. 7. Of the various tissues of rats tested for the activity, intestinal mucosa was found to be the most active, with testes, liver, kidneys and spleen following it in decreasing order.

Studies on the phospholipases of rat intestinal mucosa

1. Subcellular distribution and characteristics of different phospholipases of rat intestinal mucosa were studied. 2. The presence of free fatty acid was necessary for the maximal hydrolysis of lecithin (phosphatidylcholine), but there was no accumulation of lysolecithin (1 or 2-acylglycerophosphorylcholine);lysolecithin accumulated when the reaction was carried out in the presence of sodium deoxycholate and at or above pH8.0. 3. The fatty acid-activated phospholipase B as well as lysolecithinase showed optimum activity at pH6.5, whereas for the phospholipase A it was about pH8.6. 4. The bulk of the phospholipase A was present in the microsomal fraction, whereas the phospholipase B and lysolecithinase activities were distributed between the microsomal and soluble fractions of the mucosal homogenate. 5. Phospholipase A was equally distributed between the brush border and brush-border-free particulate fraction, with the brush border having highest specific activity, whereas the other two activities were distributed between the brush-border-free particulate and soluble fractions. 6. Various treatments showed marked differences between the phospholipase A and phospholipase B activities, but not between phospholipase B and lysolecithinase activities. 7. By using (beta[1-(14)C]-oleoyl) lecithin it was shown that the mucosal phospholipase A was specific for the beta-ester linkage of the lecithin molecule.