Apolipoprotein A-I deficiency. Biochemical and metabolic characteristics

Familial HDL deficiencies are associated with variable susceptibility to premature coronary heart disease, but the mechanism underlying this association remains poorly understood. Three homozygotes with isolated complete apo A-I deficiency caused by an autosomal codominant apo A-I Q[-2]X mutation and one heterozygote developed coronary heart disease before age 40 years. We characterized the effects of this mutation on lipoprotein metabolism. LDL FC, phospholipid, and apo B were all significantly higher in homozygotes than in heterozygotes. The HDLs of the heterozygotes were apo A-I poor relative to apo A-II. Lecithin-cholesterol acyltransferase activity was 59% lower in homozygotes than in normal subjects or heterozygotes. Cholesteryl ester transfer activity was increased in a homozygote compared with a normolipidemic control subject. Postprandial lipid metabolism was studied in one homozygote and one heterozygote. Post-prandial TG response in the homozygote was significantly exaggerated, while residual plasma HDL level remained unaffected. The homozygote also had delayed clearance of retinyl ester, a marker of chylomicron remnant metabolism. Thus, homozygosity and heterozygosity for apo A-I Q[-2]X are associated with qualitative, as well as quantitative, disturbances in plasma HDLs, LDLs, lipid-modifying enzyme activities, and postprandial retinyl ester metabolism. The observed elevation of atherogenic lipoproteins and reduction in antiatherogenic lipoproteins in the affected members of the apo A-I Q[-2]X kindred are consistent with the primary deficiency in apo A-I having pleiotropic effects that markedly enhance susceptibility for coronary heart disease.

Constitutive expression of a microinjected glucose-regulated protein (grp78) fusion gene during early Xenopus laevis development

In this study we have found that a rat glucose-regulated protein (grp) 78 chloramphenicol acetyltransferase (CAT) fusion gene deleted to -456 bp at the 5' end and injected into fertilized Xenopus eggs was first expressed in a constitutive manner in late blastula stage embryos and displayed increased expression as the embryos developed to the gastrula and neurula stages. Using a series of internal deletion mutants and linker-scanner mutants of the rat grp78 promoter, we have found that a CCAAT box and CCAAT-like element within the region -129 to -90 were essential for constitutive expression of the chimeric genes in neurula stage embryos. These results suggest conservation of the regulatory sequences within the grp78 promoter between rat and Xenopus. Interestingly, deletion or alteration of sequences between -130 and -149 had a dramatic stimulatory effect on basal promoter activity. This effect, which was not observed previously in rat cells, may be the result of upstream elements that are transcriptionally active in Xenopus and that can compensate for the mutated or deleted sequences. It is also possible that these results indicate the presence of a negative regulatory element that is recognized by the Xenopus transcriptional apparatus.

Kinetics of lipolysis of very low density lipoproteins by lipoprotein lipase. Importance of particle number and noncompetitive inhibition by particles with low triglyceride content

Our objective was to identify the major compositional factor(s) of very low density lipoprotein which determines its properties as a substrate for lipoprotein lipase. Human very low density lipoprotein was fractionated by preparative electrophoresis. The apparent Km was significantly lower for pre-beta very low density lipoprotein compared with beta very low density lipoprotein when calculated on the basis of triglyceride concentration. When the triglyceride concentration was adjusted for the triglyceride/apolipoprotein B ratio, the apparent Km was not different among very low density lipoprotein fractions. This implied that very low density lipoprotein particle number was of primary importance. To test this hypothesis further, rabbit cholesterol-rich very low density lipoprotein and human intermediate density lipoprotein and low density lipoprotein, from a patient with hepatic lipase deficiency, were added to the incubations. Each of these fractions functioned as noncompetitive inhibitors of lipolysis. We speculate that the saturation of lipoprotein lipase by an excess number of particles is a characteristic of human hyperlipoproteinemias that predispose to coronary heart disease and that are commonly classified as familial combined hyperlipoproteinemia or hyperapobetalipoproteinemia.

Apolipoprotein A-I Q[-2]X causing isolated apolipoprotein A-I deficiency in a family with analphalipoproteinemia

We report a Canadian kindred with a novel mutation in the apolipoprotein (apo) A-I gene causing analphalipoproteinemia. The 34-yr-old proband, product of a consanguineous marriage, had bilateral retinopathy, bilateral cataracts, spinocerebellar ataxia, and tendon xanthomata. High density lipoprotein cholesterol (HDL-C) was < 0.1 mM and apoA-I was undetectable. Genomic DNA sequencing of the proband's apoA-I gene identified a nonsense mutation at codon [-2], which we designate as Q[-2]X. This mutation causes a loss of endonuclease digestion sites for both BbvI and Fnu4HI. Genotyping identified four additional homozygotes, four heterozygotes, and two unaffected subjects among the first-degree relatives. Q[-2]X homozygosity causes a selective failure to produce any portion of mature apoA-I, resulting in very low plasma level of HDL. Heterozygosity results in approximately half-normal apoA-I and HDL. Gradient gel electrophoresis and differential electroimmunodiffusion assay revealed that the HDL particles of the homozygotes had peak Stokes diameter of 7.9 nm and contained apoA-II without apoA-I (Lp-AII). Heterozygotes had an additional fraction of HDL3-like particles. Two of the proband's affected sisters had documented premature coronary heart disease. This kindred, the third reported apoA-I gene mutation causing isolated complete apoA-I deficiency, appears to be at significantly increased risk for atherosclerosis.

Identification of disulfide-linked apolipoprotein species in human lipoproteins

We wished to determine whether apolipoprotein C-IIToronto, a mutant form of apolipoprotein C-II that contains a C-terminal cysteine residue, exists as a monomeric species or as multiple disulfide-linked species in plasma lipoproteins. The plasma lipoproteins from a heterozygous carrier and two homozygous carriers of apoC-IIToronto were investigated. The mutant apolipoprotein was found in homodimeric form and as heterodimers with apolipoprotein A-II, apolipoprotein B-100, and apolipoprotein E. Of particular interest was the demonstration of the existence of the disulfide-linked species apolipoprotein B-100:A-II and B-100:C-IIToronto in the very low density and low density lipoproteins in subjects who were carriers of apoC-IIToronto. We also observed that apoE3:C-IIToronto and apoE3:A-II dimers were present in the chylomicrons and very low density lipoproteins of these subjects. The observation of the existence of apolipoprotein B-100:A-II was extended to other hypercholesterolemic and hypertriglyceridemic subjects. The highest proportion of apolipoprotein B-100:A-II was observed in the very low density lipoproteins of hypertriglyceridemic subjects. The concentration of this species was significantly higher in hyperlipidemic subjects than in normolipidemic controls. These results demonstrate that the molecular species of cysteine-containing apolipoproteins are complex and should be considered in studies of human lipoprotein composition and function.

Hepatic lipase deficiency. Clinical, biochemical, and molecular genetic characteristics

Hepatic lipase (HL) is an important enzyme in the metabolism of triglyceride-rich lipoproteins and high density lipoproteins. The clinical syndrome of HL deficiency is rare and difficult to identify. We studied carriers of mutant HL to ascertain whether there are distinctive clinical and/or biochemical characteristics of the heterozygous state. In an Ontario kindred, compound heterozygosity for two HL mutations, S267F and T383M, underlies the clinical syndrome of complete HL deficiency. We report that simple heterozygotes for either HL mutant do not have a discrete lipoprotein abnormality, except for relative triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL. Postheparin HL activity is depressed to a greater degree in carriers of S267F compared with carriers of T383M. Retinyl palmitate loading studies in a compound heterozygote revealed impaired clearance of chylomicron remnants. The dyslipoproteinemia in a compound heterozygote was ameliorated by lovastatin. There was no difference in the quantity and distribution of HL mRNA in the liver of a compound heterozygote when compared with that of a normal subject. Thus, HL deficiency associated with structural variation of the HL gene is characterized by premature atherosclerosis, triglyceride enrichment of lipoprotein fractions with d > 1.006 g/mL, the presence of circulating beta-very low density lipoproteins, and abnormal catabolism of postprandial triglyceride-rich lipoproteins.

Specific types of colonic fermentation may raise low-density-lipoprotein-cholesterol concentrations

To assess the effects of increased colonic fermentation on serum lipids, eight healthy volunteers were placed on two identical 2-wk metabolic diets, one of which was supplemented with lactulose (18-25 g/d). Lactulose raised day-long concentrations of breath hydrogen and serum glutamine as indicators of increased colonic fermentation by 78 +/- 13% (P less than 0.001) and 24.7 +/- 9.5% (P less than 0.05), respectively). Unexpectedly, however, fasting serum total and low-density-lipoprotein cholesterol and apolipoprotein B concentrations were higher at 2 wk by 8.9 +/- 1.5% (P less than 0.001), 10.9 +/- 2.2% (P less than 0.005), and 18.9 +/- 5.9% (P less than 0.02), respectively, compared with the control diet. With lactulose, mean free fatty acid concentrations were reduced over the day by 19.5 +/- 5.9% (P less than 0.02), with no change in mean day-long blood glucose, serum insulin, or C-peptide concentrations. We conclude that certain rapidly fermented substrates may raise rather than lower serum lipids, possibly through increasing the amount of acetate absorbed from the colon.

A hepatic lipase gene mutation associated with heritable lipolytic deficiency

Absent hepatic lipase (HL) activity results in dyslipidemia and premature atherosclerosis. DNA sequencing of the HL gene from subjects with heritable HL deficiency identified a new C to T substitution within exon 8 that in the mature enzyme caused a threonine to methionine change at position 383 (T383M). With a rapid DNA detection method we observed that all 6 individuals with complete HL deficiency from 2 families had the T383M mutation. None of 50 random unrelated unaffected subjects had this mutation. We propose that T383M is specific to families with heritable HL deficiency. Furthermore, structural variation at the HL gene, possibly in combination with other factors, appears to be etiologic in HL deficiency.

Cholesteryl ester and apolipoprotein E transfer between human high density lipoproteins and chylomicrons

The transfer of cholesteryl esters and apolipoprotein E has been studied between plasma HDL and chylomicrons isolated either from ascitic fluid or from the plasma of a patient with type V hyperlipoproteinemia. Whereas apolipoprotein E transfer was rapid and occurred at low temperature, cholesteryl ester transfer was suppressed at 4 degrees C. Apolipoprotein E transfer did not depend upon the presence of cholesteryl ester transfer protein and was in fact inhibited by the partially purified preparation of this protein. Apolipoprotein E transfer was not increased by reduction with dithiothreitol. The transfer of cholesteryl esters increased sharply at a chylomicron to HDL ratio of cholesteryl ester above 1/10, a value which may be of physiological significance at the peak of postprandial lipemia. At this ratio, the transfer of apolipoprotein E was minimal and increased only at ratios above 2/1. From these results, it is concluded that there is no connection between apolipoprotein E and cholesteryl ester transfer from HDL to chylomicrons. It is, therefore, proposed that whereas chylomicron apolipoprotein E is acquired rapidly and mostly in the lymphatic system, the concentration of chylomicron cholesteryl esters increases significantly and independently in the circulation.

Lecithin: cholesterol acyltransferase activity and fatty acid composition of erythrocyte phospholipids in Friedreich's ataxia

In a study of the fatty acid composition of erythrocyte membrane phospholipids in Friedreich's ataxia, a lower percentage of linoleic acid in phosphatidylcholine was demonstrated. An enzyme involving the exchange of lipids between plasma and erythrocyte membrane, lecithin: cholesteryl acyltransferase (LCAT) was also studied. It was found that the LCAT activity had a trend towards low values. However, crossing-over studies indicated that when the LCAT enzyme of patients was exposed to its own substrate it gave low activity values but that the result reverted to normal when control substrate was used.

Heterogeneity of human high density lipoprotein: presence of lipoproteins with and without apoE and their roles as substrates for lecithin:cholesterol acyltransferase reaction

By affinity chromatography on heparin-Sepharose, two classes of lipoproteins were separated from high density lipoproteins (HDL) isolated from patients with primary or secondary lecithin:cholesterol acyltransferase (LCATase; EC 2.3.1.43) deficiency and from normal subjects. The unretained fraction, HDL(A), was characterized by having apoA-I as a major apoprotein; it also contained apoA-II, -C-II, and -C-III but it contained only traces of immunodetectable apoE and no apoB. The retained fraction, HDL(E), had apoE as the major apoprotein; it also contained apoA-I, -A-II, -B, -C-II, and -C-III. The relative concentration of apoA-I increased with increasing density in the HDL(E) subclass. Compared to HDL(A), HDL(E) had a significantly higher cholesterol content and a lower protein concentration. HDL(E) was mainly (90%) contained within the HDL(2) subfraction. Contamination of HDL(E) by low density lipoproteins (LDL) or Lp(a) was minimal on the basis of pre-beta-electrophoretic mobility and absence of albumin, respectively. Contamination by LDL or Lp(a) could be resolved in part by application of HDL(E) to concanavalin A-Sepharose or to heparin-Sepharose with a shallow gradient. When evaluated as substrates for a highly purified LCATase preparation, the initial reaction rates and V(max) obtained with HDL(A) were always higher than those obtained with HDL(E) in any given plasma. However, both HDL subclasses from LCATase-deficient subjects were better substrates than the corresponding HDL subclasses from normal plasma. Also, both HDL(3A) and HDL(3E) isolated from normal HDL(3) were better substrates than the corresponding subclasses isolated from normal HDL(2). The recognition of this compositional and functional heterogeneity within HDL will allow a better understanding of the metabolism of this lipoprotein class.

Transfer of cholesterol esters between human high density lipoproteins and triglyceride-rich lipoproteins controlled by a plasma protein factor

A protein factor from the d greater than 1.25 g/ml plasma fraction controls the transfer of cholesterol esters between high density lipoproteins and very low density lipoproteins. This transfer is time-dependent, and follows saturation kinetics relative to the concentration ratio of acceptor to donor lipoproteins. Although the process is reversible, the transfer rates are faster from high density to very low density lipoproteins and result in a net increase of cholesterol esters in the very low density lipoproteins. Under the same conditions, there is also a net mass transfer of cholesterol esters from high density lipoproteins to chylomicrons. This constitutes the first demonstration of cholesterol ester mass transfer between isolated lipoproteins and contrasts with the equilibrium of cholesterol esters between HDL and LDL which we previously demonstrated [4]. The apparent maximum transfer rate of cholesterol esters from high density to very low density lipoproteins was calculated to be about 80 nmoles cholesterol esters/h/ml plasma, which is very similar to the initial rate of reaction of lecithin:cholesterol acyltransferase in plasma. It is concluded that cholesterol ester formation in high density lipoproteins and their transfer to triglyceride-rich lipoproteins may be closely coupled.

Cholesterol ester exchange between human plasma high and low density lipoproteins mediated by a plasma protein factor

Although unesterified cholesterol and phospholipids exchange freely, a protein factor from the d greater than 1.25 g/ml plasma fraction was found to be necessary for cholesterol esters to transfer from HDL to LDL. This transfer was reversible, time-dependent and a function of the concentration of the d greater than 1.25 fraction, but independent of lecithin : cholesterol acyltransferase reaction. The transfer represented an equilibration of molecules, but no net mass transfer of cholesterol esters could be demonstrated from HDL to LDL.

Antimycin A, a teleocidal antibiotic

Glucose stimulates the high-affinity processes of chloroquine and amodiaquin accumulation in owl monkey erythrocytes infected with a chloroquine-susceptible strain of Plasmodium falciparum. Although these erythrocytes have greater ability to accumulate amodiaquin than chloroquine, glucose has relatively less effect on amodiaquin accumulation than on chloroquine accumulation. In contrast to these findings with chloroquine-susceptible P. falciparum, glucose stimulates amodiaquin but not chloroquine accumulation in erythrocytes infected with chloroquine-resistant P. falciparum. This lack of function of a substrate-dependent component of chloroquine accumulation distinguishes chloroquine-resistant from chloroquine-susceptible P. falciparum.

Agar plate method for detecting microorganisms which produce equilin and other estrogens from various steroids

The intense red color produced by a reagent specific for xi(7)-estrogens is used to directly detect microorganisms which produce these estrogens from various steroids.

11 alpha-hydroxylation of steroids by spores of Aspergillus ochraceus

Spores of A. ochraceus were used to transform a variety of steroids into their 11α-hydroxy derivatives. Physicochemical data of some new 11α-hydroxy derivatives are presented. Specificity of A. ochraceus in relation to steroid transformations is discussed.

Large-scale transformation of steroids by fungal spores

Spores of Aspergillus ochraceus and Septomyxa affinis were produced on a large scale by surface sporulation on moist wheat bran and barley. 11alpha-Hydroxylation of progesterone and Reichstein's compound S by spores of A. ochraceus and 1-dehydrogenation of compound S by spores of S. affinis were carried out in 5-liter fermentors. It was shown that, above a certain minimum, increase in aeration and agitation did not significantly affect steroid conversion. The industrial feasibility of the spore process was further demonstrated by 11alpha-hydroxylation of 6alpha-fluoro-16alpha,17alpha-dihydroxypregn-4-ene-3,20-dione in a modified 200-gal stainless-steel vessel with spores of A. ochraceus. Strict aseptic conditions are not necessary, either during harvesting of spores or during steroid transformation.