Fibrate induction of the adrenoleukodystrophy-related gene (ABCD2): promoter analysis and role of the peroxisome proliferator-activated receptor PPARalpha.

X-linked adrenoleukodystrophy (X-ALD) is a neurodegenerative disease due to a defect in the ABCD1 (ALD) gene. ABCD1, and the two close homologues ABCD2 (ALDR) and ABCD3 (PMP70), are genes encoding ATP-binding cassette half-transporters of the peroxisomal membrane. As overexpression of the ABCD2 or ABCD3 gene can reverse the biochemical phenotype of X-ALD (reduced beta-oxidation of very-long-chain fatty acids), pharmacological induction of these partially redundant genes may represent a therapeutic approach to X-ALD. We previously reported that the ABCD2 and ABCD3 genes could be strongly induced by fibrates, which are hypolipidaemic drugs and peroxisome-proliferators in rodents. We provide evidence that the induction is dependent on peroxisome proliferator-activated receptor (PPARalpha) as both genes were not induced in fenofibrate-treated PPARalpha -/- knock-out mice. To further characterize the PPARalpha pathway, we cloned and analysed the promoter of the ABCD2 gene, the closest homologue of the ABCD1 gene. The proximal region (2 kb) of the rat promoter displayed a high conservation with the human and mouse cognate sequences suggesting an important role of the region in regulation of the ABCD2 gene. Classically, fibrate-induction involves interaction of PPARalpha with a response element (PPRE) characterized by a direct repeat of the AGGTCA-like motif. Putative PPRE motifs of the rat ABCD2 promoter were studied in the isolated form or in their promoter context by gel-shift assay and transfection of COS-7 cells. We failed to characterize a functional PPRE, suggesting a different mechanism for the PPARalpha-dependent regulation of the ABCD2 gene.

Rat adrenoleukodystrophy-related (ALDR) gene: full-length cDNA sequence and new insight in expression.

X-linked adrenoleukodystrophy (X-ALD) is an inherited demyelinating disorder due to mutations in the ALD gene, which encodes a peroxisomal ABC half-transporter (ALDP). It has been suggested that ALDP assembles with ALDRP (adrenoleukodystrophy-related protein), a close homologous half-transporter, to form a functional heterodimer. For the first time full-length ALDRP cDNA (5.5 kb) was cloned, and 5' and 3' RACE analysis revealed that alternative usage of polyadenylation sites generates the two transcripts of 3.0 and 5.5 kb observed in the rat in Northern blot analysis. Southern blotting and chromosomal mapping demonstrated one ALDR locus in the rat genome. Characterisation of the 3' flanking region suggested that an ID sequence might be responsible for high expression of the 5.5 kb ALDRP transcript in rat brain. ALDR gene expression was found to be high in the liver of rats before weaning and very low in adult rats; the reverse developmental regulation was observed in the brain. Fenofibrate, which is a potent inducer of the ALDR gene in the liver of adult rats, could not induce the ALDR gene in suckling rats. The exact significance of this result with regard to development of an efficient pharmacological gene therapy for X-ALD is discussed.

Genomic organization, expression analysis, and chromosomal localization of the mouse PEX3 gene encoding a peroxisomal assembly protein.

The peroxin Pex3p has been identified as an integral peroxisomal membrane protein in yeast where pex3 mutants lack peroxisomal remnant structures. Although not proven in higher organisms, a role of this gene in the early peroxisome biogenesis is suggested. We report here the cDNA cloning and the genomic structure of the mouse PEX3 gene. The 2 kb cDNA encodes a polypeptide of 372 amino acids (42 kDa). The gene spans a region of 30 kb, contains 12 exons and 11 introns and is located on band A of chromosome 10. The putative promoter region exhibits characteristic housekeeping features. PEX3 expression was identified in all tissues analyzed, with the strongest signals in liver and in testis, and could not be induced by fenofibrate. The data presented may be useful for the generation of a mouse model defective in PEX3 in order to clarify the yet unknown functional impact of disturbances in early peroxisomal membrane assembly.

The four murine peroxisomal ABC-transporter genes differ in constitutive, inducible and developmental expression.

Four ATP-binding cassette (ABC) half-transporters have been identified in mammalian peroxisomes: adrenoleukodystrophy protein (ALDP), adrenoleukodystrophy-related protein (ALDRP), 70-kDa peroxisomal membrane protein (PMP70) and PMP70-related protein (P70R). Inherited defects in ALDP cause the neurodegenerative disorder X-linked adrenoleukodystrophy (X-ALD). By comparative Northern blot analyses we found each of the four murine peroxisomal ABC transporter mRNA species at maximum abundance only in a few tissues, which differed for each family member. The four genes were also regulated differentially during mouse brain development: ALDP mRNA was most abundant in embryonic brain and gradually decreased during maturation; ALDRP and P70R mRNA accumulated in the early postnatal period; and the amount of PMP70 transcript increased slightly during the second and third postnatal week. The different expression patterns could explain why beta-oxidation is defective in X-ALD, although ALDRP and PMP70 can replace ALDP functionally in fibroblasts. Dietary fenofibrate had no effect on the ALD and P70R genes, but strongly increased expression of the ALDR and PMP70 genes in mouse liver. However, in P-glycoprotein Mdr1a-deficient mice fenofibrate treatment increased ALDR gene expression also in the brain, suggesting that the multidrug-transporter P-glycoprotein restricts entry of fenofibrate to the brain at the blood-brain barrier. Analysis of the promoter sequences revealed a cryptic nuclear hormone receptor response element of the DR+4 type in the ALDR promoter and a novel 18-bp sequence motif present only in the 5' flanking DNA of the ALDR and PMP70 genes. The mouse ALDR gene uses a single transcription start site but alternative polyadenylation sites. These data are of importance for the use of ALDP-deficient mice as a model in pharmacological gene therapy studies.

The mouse gene encoding the peroxisomal membrane protein 1-like protein (PXMP1-L): cDNA cloning, genomic organization and comparative expression studies.

PXMPI-L (synonyms: PMP69, P70R) is a peroxisomal protein that belongs to the ABC-transporter superfamily. Its closest homolog is the peroxisomal membrane protein 1 (PMP70). We have cloned the mouse PXMP1-L gene. It encodes a 606 amino acid protein. In contrast to the human and the rat, mouse PXMP1-L is predominantly expressed in the liver. The mouse PXMP1-L gene consists of 19 exons and spans 21 kb of genomic sequence. No obvious peroxisome proliferator response element has been found in 1.1 kb of the putative promoter region. No coordination of constitutive or fenofibrate-induced expression of PXMP1-L with other peroxisomal ABC transporters was observed so that an obligate exclusive heterodimer formation is not likely to occur. The data presented will be particularly useful for the generation of a mouse model defective in PXMP1-L in order to elucidate the yet unknown function of this protein.

Copurification of dihydroxyacetone-phosphate acyl-transferase and other peroxisomal proteins from liver of fenofibrate-treated rats.

Dihydroxyacetone-phosphate acyl-transferase (DHAP-AT), a peroxisomal membrane-bound enzyme that catalyzes the first step of ether-glycerolipid synthesis, was purified from liver of rats treated with fenofibrate, a peroxisome proliferator. The protocol first included isolation of peroxisomes, their purification through a discontinuous gradient and solubilization of membranes in CHAPS. DHAP-AT was further purified by four chromatographic steps, namely low-pressure size-exclusion, cation-exchange, hydroxylapatite and chromatofocusing. The chromatofocusing step led to a 4000-fold increase in the specific activity of DHAP-AT with respect to the liver homogenate with a yield of about 0.2%. Trypsin digestion of a 64-kDa protein band upon SDS-PAGE resulted in a peptide sequence unknown in databases. A corresponding degenerated oligonucleotide was used as a probe in Northern blotting, and a transcript of 3.3 kb was detected in some rat tissues. Moreover, the overall procedure allowed co-purification of four major peroxisomal enzymes: urate-oxidase, catalase, multifunctional enzyme and palmitoyl-CoA oxidase, respectively.

Microscale synthesis of phosphatidyl-[3H]choline from 1,2-diacylglycerol. Assessment of isomerization by reversed-phase high-performance liquid chromatography.

The synthesis of rac-1-palmitoyl-2-oleoylglycero-3-phospho-[3H]choline of high specific activity was carried out on a microscale by making 7 mumol of rac-1-palmitoyl-2-oleoylglycerol react first with an equimolar amount of POCl3 and then of [3H]choline. After purification by thin-layer chromatography and normal-phase high-performance liquid chromatography and normal-phase high-performance liquid chromatography (HPLC), the yield of the synthesis of [3H]phosphatidylcholine (120 microCi/mumol) was 22%. rac-1-Palmitoyl-2-oleoylglycerol was purified before use by reversed-phase HPLC under conditions which were nonisomerizing and allowed the separation of 1,2- and 1,3-isomers of diacylglycerol. Ethanol, but not benzene, was shown to cause isomerization of long-chain diacylglycerol and, therefore, was not used for drying the substrate before reaction. A rapid and complete separation of 1,2- and 1,3-isomers of long-chain phosphatidylcholine was obtained by reversed-phase HPLC using 20 mM choline chloride in methanol/acetonitrile/water (50:50:1, by vol) isocratically as the mobile phase. Under these conditions, analysis of the synthesized rac-1-palmitoyl-2-oleoylglycero-3-phospho-[3H]choline showed a total absence of 1,3-isomer.

Composition of molecular species of triacylglycerols in bovine milk fat.

Triacylglycerols from bovine milk fat were fractionated by reversed-phase liquid chromatography. The fatty acid and triacylglycerol compositions of each fraction were determined by capillary gas chromatography. These data were used to determine the accurate proportions of 223 individual molecular species of even-numbered triacylglycerols, accounting for 80% of total triacylglycerols (all percentages are expressed as moles per 100 mol). The three major triacylglycerols were butyroylpalmitoylacylglycerols, namely butyroylpalmitoyloleoylglycerol (4.2%), butyroyldipalmitoylglycerol (3.2%), and butyroylmyristoylpalmitoylglycerol (3.1%). Twenty-two triacylglycerols (> 1%) contained at least two of the four major long-chain fatty acids (C14:0, C16:0, C18:0, and C18:1). Among them were eight butyroyldiacylglycerols, the proportions of which reached 19% in total but only 12% when calculated on the basis of a random distribution of the fatty acids in the triacylglycerol molecules. More generally, most of the triacylglycerols that are composed of a short-chain fatty acid (C4:0 or C6:0) and two fatty acids in the range of C12 to C18 are preferentially synthesized by the mammary gland; their proportions (36% in total) were higher than the corresponding random values (24% in total). Conversely, the total amounts of simple (.4%) and mixed (2.9%) saturated long-chain (C14:0 to C18:0) triacylglycerols were much lower than those expected from random calculation (1.9 and 6.1%, respectively).

Effects of two peroxisome proliferators (ciprofibrate and fenofibrate) on peroxisomal membrane proteins and dihydroxyacetone-phosphate acyl-transferase activity in rat liver.

The effects of ciprofibrate and fenofibrate, which are more potent peroxisome proliferators than clofibrate, on the activities of dihydroxyacetone-phosphate acyl-transferase (DHAP-AT) and glycerol-3-phosphate acyl-transferase (G3P-AT) were studied at the two pH optima 5.5 and 7.4 in subcellular fractions of rat liver, and in solubilized peroxisomal membranes (PMP) as well. Protein was also analyzed by gel electrophoresis. 1) Under the conditions of the specific activity of peroxisomal acyl-CoA oxidase (CN(-)-ACO) being increased (8 to 9-fold), there was no specific induction of the DHAP-AT activity when measured at pH 5.5 in purified peroxisomes and PMP. However, the total activities of DHAP-AT in these two fractions were increased by 6 to 11 times, as a result of hepatomegaly and peroxisome proliferation. In contrast, they were only slightly enhanced (x 1.1 to 2.2-fold) when determined at pH 7.4. The magnitude of the effects of a fibrate treatment was, therefore, dependent on the pH of the incubation medium. 2) Experiments of reversibility of enzyme induction reinforced the finding that the peroxisomal DHAP-AT activity is not specifically induced by ciprofibrate and fenofibrate. 3) Our results suggest the existence of a peroxisomal G3P-AT, non-inducible by fibrates, in the rat liver. 4) Induction of peroxisomal membrane-associated polypeptides with apparent molecular masses of 26- and 36-kDa was evidenced in stained electrophoretic gels of protein.

Proteins and enzymes of the peroxisomal membrane in mammals.

Proteins of the peroxisomal membrane can be schematically divided into two groups, one being made up of more or less characterized proteins with generally unknown functions and the other consisting of enzyme activities of which the corresponding proteins have not been characterized. In the present report, these proteins and enzymes are described with the addition of unpublished results regarding their induction by peroxisome proliferators at the post-transcriptional level. Integral membrane proteins (IMPs) can be isolated using an alkaline solution of sodium carbonate. A dozen of preponderant IMPs can be seen on sodium dodecyl sulfate polyacrylamide gel electrophoresis, and the major band corresponds to a 70 kDa IMP, of which the corresponding rat cDNA is known. Some IMPs have been characterized by immunoblot analysis. Recently, a cDNA has been cloned for a peroxisome assembly factor (35 kDa IMP). Functions have also been proposed for some IMPs but are not yet firmly settled. Some IMPs (450/520, 70 and 26 kDa) are strongly induced by peroxisome proliferators. Our results extend to cipro- and fenofibrate the observation that the 70 kDa IMP mRNA level is strongly increased in di(2-ethylhexyl)phtalate-treated rats. All the enzyme activities associated with the peroxisomal membrane are involved in lipid metabolism: activation of substrates (fatty acids), ether lipid biosynthesis, and formation of precursors (fatty alcohols). It is believed that the same long-chain acyl-CoA synthetase occurs in the peroxisome as well as in the outer mitochondrial membrane and the endoplasmic reticulum. However, two highly homologous but different cDNAs encoding rat liver and brain long-chain acyl-CoA synthetases have been isolated recently. Evidence has been accumulated for a distinct synthetase that specifically activates very-long chain fatty acids. The first two steps of ether lipid biosynthesis require dihydroxyacetone-phosphate (DHAP) acyltransferase and alkyl-DHAP synthetase, the active sites of which are located on the inner surface of the membrane. In contrast, the catalytic site of the acyl/alkyl-DHAP reductase, which generates sn-1-alkyl-glycerol-3-phosphate, is located on the outer surface. Long-chain fatty alcohols, which are obligate precursors of ether lipids and wax esters, are biosynthetized by the reduction of the corresponding acyl-CoAs via the action of an acyl-CoA reductase. Peroxisome proliferators do not appear to stimulate these enzyme activities specifically. However, we report that feno- and ciprofibrate treatments increase six-fold the palmitoyl-CoA synthetase mRNA level in the rat liver.

Determination of bovine butterfat triacylglycerols by reversed-phase liquid chromatography and gas chromatography.

Triacylglycerols (TGs) from a sample of summer butterfat (bovine milk) were analysed and fractionated by reversed-phase liquid chromatography (RPLC). Fatty acid and TG composition of eac of the 47 RPLC fractions ranging from 0.1 to 6.9% were determined by capillary gas chromatography. The data were used together to determine the quantitative composition of the molecular species of TGs. A large number of TG species, accounting for 80% of the total, could be unequivocally identified and individually determined. The combination of the chromatographic methods used proved to be a powerful and accurate approach for the determination of molecular species of TGs in a complex fat, but also a difficult and time-consuming task.

Metabolic fate of sphingomyelin of high-density lipoprotein in rat plasma.

The metabolic fate of high density lipoprotein (HDL) sphingomyelin in plasma was studied in rats over a 24-hr period after injection of HDL containing sphingomyelin which was 14C-labeled in the stearic (18:0) or lignoceric acid (24:0) moiety and 3H-labeled in the choline methyl groups. Decay of label in plasma followed three phases. The first two phases were similar for both isotopes and both types of sphingomyelin (t1/2 approximately 10 and 110 min). However, during the third phase (from 10 hr after injection), 3H label disappeared more slowly than 14C label from 18:0 sphingomyelin, whereas the 3H/14C ratio remained relatively constant when 24:0 sphingomyelin was used. Intact, doubly-labeled 18:0 sphingomyelin disappeared from HDL rapidly (t1/2 = 38 min) by tissue uptake and by transfer to very low density lipoprotein (VLDL). VLDL contained up to 12% of the sphingomyelin 1 hr after injection. This is the first demonstration of a transfer in vivo of sphingomyelin from HDL to VLDL. A similarly rapid transfer was also observed in vitro. Some nontritiated, [14C]18:0 or [14C]24:0 sphingomyelin was redistributed more slowly into HDL. Doubly-labeled phosphatidylcholine appeared in VLDL and HDL within 1 hr after injection and reached 1.8 and 2.1% of the injected 14C and 3H in VLDL at 1 hr, and 4.8 and 6.9% in HDL at 3 hr, respectively.

Time-course of utilization of [stearic or lignoceric acid]sphingomyelin from high-density lipoprotein by rat tissues.

Utilization of stearic and lignoceric acids supplied by high-density lipoprotein (HDL) sphingomyelin to different tissues was followed for 24 h after rats were injected with HDL containing [[1-14C]stearic (18:0) or [1-14C]lignoceric (24:0) acid [Me-3H]choline]sphingomyelin. Both isotopes reached a maximum in tissue lipids 3-12 h after injection and were recovered mainly in the liver (30%) and small intestine (3%), whereas the other tissues contained approx. 1% or less of the injected dose. All the tissues were able to take up some intact sphingomyelin from HDL and hydrolyze it. In the lung and erythrocytes, the 3H:14C ratio of sphingomyelin remained unchanged throughout the studied period, while an increase in the isotopic ratio was observed in the kidney due to the 3H choline moiety re-used for synthesis of new sphingomyelin. Conversely, the isotopic ratio of sphingomyelin decreased in the liver, indicating a saving of the 14C-labelled fatty acids, especially 24:0. Furthermore, [24:0]ceramide in the liver remained at a high level (6% of the injected dose), whereas [18:0]ceramide decreased to 1%. When the tissues were examined 24 h after injection, the proportion of the 14C linked to sphingomyelin in the total 14C was always higher for both kinds of sphingomyelin than the molar proportion of sphingomyelin in the whole of lipid classes. However, in the majority of the extra-hepatic tissues, more [14C]18:0 than [14C]24:0 was recovered in sphingomyelin, and more 14C radioactivity from 18:0 than from 24:0 was redistributed in the other lipids. The choline moiety from both kinds of sphingomyelin was re-used to synthesize phosphatidylcholine, especially in the liver (up to 20% of the injected dose). All these results show that utilization of sphingomyelin from HDL by tissues normally occurs in vivo and that this phenomenon should be taken into account in the study of the phospholipid turnover of cell membranes. They also show that metabolism of sphingomyelin from HDL in the liver and other tissues is dependent on the sphingomyelin acyl moiety.

Utilization of high-density lipoprotein sphingomyelin by the developing and mature brain in the rat.

Utilization of very long chain saturated fatty acids by brain was studied by injecting 20-day-old and adult rats with high-density lipoprotein containing [stearic or lignoceric acid-14C, (methyl-3H)choline]sphingomyelin. Labeling was followed for 24 h. Very small amounts of 14C were recovered in the brain of all rats, and there was no preferential uptake of lignoceric acid. Approximately 20% of the entrapped 14C was located in the form of unchanged sphingomyelin 24 h after injection. This result shows that the rat brain utilizes very little very long chain fatty acids (greater than or equal to 20 C atoms) from high-density lipoprotein sphingomyelin, even during the myelinating period. The [3H]choline moiety from sphingomyelin was recovered in brain phosphatidylcholine in a higher proportion in comparison with the 14C uptake. The brain 3H increased throughout the studied period in all experiments, but was much higher in the myelinating brain than in the mature brain. From the radioactivity distribution in liver and plasma lipids, it is clear that the choline 3H in the brain originates from either double-labeled phosphatidylcholine of lipoproteins or tritiated lysophosphatidylcholine bound to albumin, both synthesized by the liver.

In vivo incorporation of lauric acid into rat adipose tissue triacylglycerols.

An in vivo approach was taken to examine fatty acid esterification in adipose tissue using a coconut oil-enriched diet. Rats were fed a diet containing coconut oil (50% lauric acid) for six weeks. Triacylglycerols from perirenal adipose tissue were fractionated by silver nitrate-thin layer chromatography and, then, preparative gas chromatography. The distribution of 169 triacylglycerol types accounting for 97% of total triacylglycerols was determined. There was evidence for a very high content of mixed triacylglycerols composed of intermediate (12:0 and 14:0) and long acyl moieties. No significant differences were observed between the experimental distribution of triacylglycerol types and the random distribution, calculated from the total fatty acid composition. This indicated that most long chain triacylglycerols stored before coconut oil feeding would have been rearranged after the six weeks of coconut oil feeding. The experimental proportion of trilauroylglycerol reached 2%, as expected from its random proportion, and the proportions of dilauroylacylglycerols were slightly higher than the random values. Present results were compared with those previously obtained from triacylglycerols of adipose tissue of rats fed a low-fat standard diet. From our results and those of other authors, it is suggested that lauric acid is a good substrate for sn-glycero-3-phosphate acyltransferase and diacylglycerol acyltransferase in rat adipose tissue.

Turnover and uptake of double-labelled high-density lipoprotein sphingomyelin in the adult rat.

Rat HDL containing [stearic acid-14C, (methyl-3H)choline]sphingomyelin was prepared by incubating labelled sphingomyelin liposomes with serum. HDL was then separated by ultracentrifugation and purified by gel-filtration chromatography. The maximum transfer was reached when 1.5 microliter sphingomyelin was incubated in the presence of 1 ml of serum at 37 degrees C for 1 h. When transfer was limited to a 5-7% increase in HDL mass, no significant change was observed in the HDL electrophoretic pattern, and rats could therefore be injected with this type of HDL under physiological conditions. Plasma radioactivity decay was followed for 24 h, and the recovery of both isotopes in 11 tissues was studied 24 h after the injection. The decay in plasma of both isotopes followed three exponential phases. During the first two phases, both isotopes disappeared with the same velocity (t1/2 = 12.8 and 98-105 min for the first and second phases, respectively). 10 h after injection, 3H had disappeared more slowly than 14C (t1/2 = 862 and 502 min for 3H and 14C, respectively) and 24 h after injection, only 1.5% of 14C and 2.5% of 3H remained in the plasma. This radioactivity was located mainly in HDL (80-85% for 3H and 14C), with a 3H/14C ratio close to that of injected sphingomyelin, and in VLDL, with the same isotopic ratio as that of liver lipids. Some 3H was associated with non-lipoprotein proteins. 17.5% of 3H and 23.4% of 14C were recovered in the liver, 1.6% of each isotope in erythrocytes, and 1.4% of 3H and 0.6% of 14C in kidney. Less than 1% of each isotope was recovered in each of the other tissues. Phosphatidylcholine was the lipid most labelled, and in several tissues sphingomyelin had a 3H/14C ratio close to that of injected sphingomyelin, showing an uptake without prior hydrolysis.

Effects of preincubation of primary monolayer cultures of rat hepatocytes with low- and high-density lipoproteins on the subsequent binding and metabolism of human low-density lipoprotein.

1. There are two distinct binding sites (Site 1 and Site 2) for human low-density lipoprotein (LDL) on rat hepatocytes in monolayer culture [Salter, Saxton & Brindley (1986) Biochem. J. 240, 549-557]. 2. Binding of 125I-LDL to Site 1, but not to Site 2, is up-regulated between 20 and 44 h in culture by preincubation of the cells with human high-density lipoprotein 3 (HDL3). 3. A similar preincubation with HDL2 had no significant effect on binding to either site. 4. Preincubation with human LDL led to a partial down-regulation of subsequent binding of 125I-LDL to Site 1. Since binding after incubation with LDL was measured at 37 degrees C, binding to Site 2 could not be distinguished from LDL that had been internalized by the cells. 5. Hepatocytes were shown to degrade 125I-LDL, resulting in the accumulation of [125I]iodotyrosine in the medium. Evidence was found that iodotyrosine may be further degraded by deiodinase produced by the cells. 6. Regulation of binding to Site 1 by preincubation with LDL or HDL3 was found to lead to a parallel regulation of LDL degradation. 7. It is concluded that rat hepatocytes not only bind but also metabolize human LDL and that these processes are under metabolic regulation.

Occurrence, absorption and metabolism of short chain fatty acids in the digestive tract of mammals.

Short chain fatty acids (SCFA) also named volatile fatty acids, mainly acetate, propionate and butyrate, are the major end-products of the microbial digestion of carbohydrates in the alimentary canal. The highest concentrations are observed in the forestomach of the ruminants and in the large intestine (caecum and colon) of all the mammals. Butyrate and caproate released by action of gastric lipase on bovine milk triacylglycerols ingested by preruminants or infants are of nutritional importance too. Both squamous stratified mucosa of rumen and columnar simple epithelium of intestine absorb readily SCFA. The mechanisms of SCFA absorption are incompletely known. Passive diffusion of the unionized form across the cell membrane is currently admitted. In the lumen, the necessary protonation of SCFA anions could come first from the hydration of CO2. The ubiquitous cell membrane process of Na+-H+ exchange can also supply luminal protons. Evidence for an acid microclimate (pH = 5.8-6.8) suitable for SCFA-protonation on the surface of the intestinal lining has been provided recently. This microclimate would be generated by an epithelial secretion of H+ ions and would be protected by the mucus coating from the variable pH of luminal contents. Part of the absorbed SCFA does not reach plasma because it is metabolized in the gastrointestinal wall. Acetate incorporation in mucosal higher lipids is well-known. However, the preponderant metabolic pathway for all the SCFA is catabolism to CO2 except in the rumen wall where about 80% of butyrate is converted to ketone bodies which afterwards flow into bloodstream. Thus, SCFA are an important energy source for the gut mucosa itself.

Loss of stereospecificity of phospholipases C and D upon introduction of a 2-alkyl group into rac-1,2-diacylglycero-3-phosphocholine.

rac-1-[1-14C]Lauroyl-2-oleylglycero-3-phospho[methyl-3H]choline and rac-1-lauroyl-2-[1-14C]oleoylglycero-3-phospho[methyl-3H]choline along with rac-1-palmitoyl-2-oleylglycero-3-phosphocholine and sn-1-palmitoyl-2-oleylglycero-3-phosphocholine were synthesized and subjected to hydrolysis with phospholipase C (EC 3.1.4.3) from Clostridium perfringens and phospholipase D (EC 3.1.4.4) from cabbage. Kinetics of hydrolysis of the radioactive substrates were determined by measuring the 3H radioactivity retained in the aqueous phase due to free choline and phosphocholine and the 3H and 14C radioactivity recovered in the organic phase due to the released diacylglycerols and phosphatidic acids and the residual phosphatidylcholines. The rate of hydrolysis of the unlabelled substrates by phospholipase C was determined by thin-layer chromatography and gas-liquid chromatography of the methanolysis products. The relative initial rates of hydrolysis of sn-1,2,- and sn-2,3-enantiomers were 100-200:1 for phospholipase C and 40-50:1 for phospholipase D using rac-1-lauroyl-2-oleoylglycero-3-phosphocholine as the substrate. The substitution of the 2-acyl group by an alkyl group resulted in a loss of stereospecificity, which was partial for phospholipase C (relative rates equal to 8-13:1) and total for phospholipase D. There was a parallel dramatic decrease (500-1000-fold) in the initial rate of hydrolysis with phospholipase C but the activity of phospholipase D was only moderately reduced (18-fold). These findings are consistent with the earlier observed loss of the stereospecificity of lipoprotein lipase following introduction of a 2-alkyl group into triacylycerols, and point to a general unsuitability of 2-alkyl-linked acylglycerols as substrates for the assay of the stereospecificity of lipases, as well as for the isolation of enantiomeric 2-alkylacylglycerols by means of stereospecific lipases.