UDP-Glucose: Sterol Glucosyltransferase and a Steryl Glucoside Acyltransferase Activity in Amyloplast Membranes from Potato Tubers

Envelope membranes were isolated from potato tuber amyloplastby a discontinuous sucrose density gradient and high speed centrifugation.These membranes catalyzed the transfer of [¹⁴C]glucose fromUDP-[¹⁴C]glucose to endogenous sterol acceptors and, in turn,catalyzed the esterification of steryl glucosides with fattyacids from an endogenous acyl donor. The synthesis of sterylglucosides was stimulated in the presence of Triton. X-100,while formation of acyl steryl glucosides was inhibited by thedetergent. However, in the presence of an added sterol acceptorand Triton X-100, the inhibition of acyl steryl glucoside synthesiswas overcome by the addition of phosphatidylethanolamine. Theenzyme involved in steryl glucoside formation was solubilizedby treatment of the envelope membranes with 0.3% Triton X-100.The solubilized enzyme had an almost absolute requirement forsterol acceptors. Key words: Solanum tuberosum, Sterol glucosylation, Steryl glucoside acylation, Amyloplast membrane     

Metabolism of conjugated sterols in eggplant. Part 2. Phospholipid : steryl glucoside acyltransferase

A membrane-bound phospholipid : steryl glucoside acyltransferase from Solanum melongena leaves was partially purified and its specificity and molecular as well as kinetic properties were defined. Among the steryl glycosides tested (e.g. typical plant steryl glucosides, steryl galactosides and cholesteryl xyloside) the highest activity was found with cholesteryl glucoside, but some structurally related compounds such as sito- and stigmasteryl glucoside or galactoside as well as cholesteryl galactoside were also acylated, albeit at lower rates. The investigated enzyme was able to use all classes of phosphoglycerolipids (phosphatidylethanolamine, phosphatidylcholine, phosphatidylserine, phosphatidylinositol, phosphatidylglycerol) as an acyl source for biosynthesis of acylated steryl glucoside. Among them 1,2-dimirystoylphosphatidylic acid appeared to be the best acyl donor. Apart from phosphoglycerolipids, 1,2-diacylglycerols were also used as acyl donor for steryl glucoside acylation, although at a distinctly lower rate. The acyl moiety was transferred from the C-1 position of phospholipid molecule. The investigated acyltransferase activity was stimulated by 2-mercaptoethanol, Triton X-100, 1-monoacylglycerols and inhibited in the presence of divalent cations such as Ca(2+), Mn(2+), Zn(2+) or Co(2+), some lipids (MDGD, ceramide), detergents (Tween 20, 40, 60 and 80, Tyloxapol, sodium deoxycholate) and high ionic strength.     

Steryl glucoside biosynthesis in the alga Prototheca zopfii

A particulate enzyme fraction from the Chlorophyta Prototheca zopfii catalysed the transfer of glucose-[U-¹⁴C]from UDP-Glc-[U-¹⁴C] to endogenous sterol acceptors and the esterification of steryl glucosides with fatty acids from an endogenous acyl donor. Glucose was the only sugar present, and it appeared to have the β-configuration. In the acylated derivatives the glucose-acyl linkage appeared in the C-6 position of glucose, as indicated by periodate oxidation. UDP-Glc:sterol glucosyltransferase was solubilized with detergent and purified 34-fold. The solubilized enzyme showed no specificity for the sterol but a high affinity for the sugar nucleotide UDP-Glc. Time-course incorporation into steryl glucoside (SG) and the acylderivative (ASG) indicated that SG was the precursor of ASG and that phosphatidyl ethanolamine stimulated the formation of the latter compound, presumably acting as acyl donor. A high sterol glucosylating activity was found in the Golgirich fraction. All this evidence indicates that steryl glucosides and their acylated derivatives were synthesized by algae. The early assumption that these compounds were not present in algae must be revised.     

Acylated Steryl Glycoside Synthesis in Seedlings of Nicotiana tabacum L

In tobacco seedlings (Nicotiana tabacum L.), glucose from supplied uridine diphosphate-[U-¹⁴C]glucose was first incorporated into steryl glycosides and later into acylated steryl glycosides. However, when [¹⁴C]cholesterol was used as substrate, the acylated steryl glycosides became labeled earlier than the steryl glycosides. With [¹⁴C]cholesteryl glucoside as substrate, most of the radioactive label was recovered as free sterol, and the acylated steryl glycosides were not readily labeled; however, palmitoyl [¹⁴C]cholesteryl glucoside was rapidly converted to steryl glycoside. In feeding experiments with free sterol, an unknown, highly radioactive steroid component was isolated. Incorporation of radioactivity into the unknown occurred before the acylated steryl glycosides were labeled.     

Acyl composition and biosynthesis of acylated steryl glucosides in Calendula officinalis.

Fatty acids C12-C22 are components of acylated steryl glucosides in Calendula officinalis. Various particulate fractions from 14-day-old seedlings catalyze the esterification of the steryl glucosides with utilization of endogenous acyl donors. The activity seems to be associated mainly with the membranous structures being fragments of Golgi complex, as it has previously been suggested for UDPG: sterol glucosyltransferase. Succesive treatment of the particulate enzyme fraction with Triton X-100 and acetone affords a soluble acyltransferase preparation partly depleted of endogenous lipids. As a source of acyl groups for the synthesis of steryl acylglucosides this preparation utilizes various phospholipids obtained from the same plant in the following sequence: phosphatidylinositol greater than phosphatidylethanolamine greater than phosphatidylcholine. It does not utilize triacylglycerols and monogalactosyldiacylglycerols.     

Steryl glucoside and acyl steryl glucoside formation in the amyloplast membrane during the development of potato tuber

Steryl glucoside (SG) and acylated steryl glucoside (ASG) synthesis was investigated in amyloplast membranes from young, intermediate and mature potato tubers. The synthesis ratio SG/ASG was lowest in young tubers (3:2) and highest in mature tubers (6:1).About a 3-fold stimulation of [¹⁴C] glucose incorporation into a mixture of SG was observed in amyloplast membranes from mature tubers in the presence of β-sitosterol, while radioactivity incorporation in young tubers was unaffected, thus indicating that different availabilities of endogenous acceptors occur in the membranes.The enzymes involved in sterol modification exhibit a different behavior towards Triton X-100, depending on the developmental stage of the tubers. Low concentrations of the detergent (0.045%) are required to stimulate the enzymes present in young tuber membranes (2-fold). On the other,hand, 0.15% of Triton increased the enzymatic activity in mature tubers 5-fold. These results, together with those obtained after studies of pH dependence, could be related to the lipidic structure of the vesicles formed at different developmental stages of the tubers.It is concluded that the major changes in the enzymatic activities occur as a consequence of the sterol acceptors and acyl donor content during potato tuber growth.     

Subcellular distribution of steryl ester biosynthesis in spinach leaves

Higher steryl ester biosynthetic activities were obtained with Triton X-100-phosphatidylcholine-cholesterol mixed micelles than with Tween 80-phosphatidylcholine-cholesterol mixed micelles when incubated with spinach leaf (Spinacia oleracea L.) acetone powder preparations. The best incorporation of [4-¹⁴C]cholesterol into [4-¹⁴C]cholesteryl ester was obtained with a Triton X-100-phosphatidylcholine-cholesterol (10:1:1, w/w) mixed micelle system. This mixed micelle system, however, required 1,2-dipalmitin and fatty acid-free bovine albumin for optimal activity. The reaction exhibited a diglyceride specificity since the dipalmitin requirement could be replaced with neither 1-monopalmitin nor tripalmitin. Significant amounts of steryl ester biosynthetic activity were detected in the chloroplast (1,000g pellet), mitochondrial (3,000g pellet), and microsomal (20,000g and 88,000g pellet) fractions. Little activity was detected in the water-soluble (88,000g supernatant) fraction. The highest specific activity occurred in the 88,000g pellet. The 88,000g supernatant contained a heatstable, water-soluble substance that was required for optimal steryl ester biosynthesis in all of the pellet fractions. This factor was not lost during extensive dialysis but was destroyed by ashing, indicating that it was large and organic. Silver nitrate thin layer chromatography indicated that 60% of the biosynthesized steryl esters contained saturated fatty acids in the absence of 1,2-dipalmitin and that 83% contained saturated fatty acids in the presence of 1,2-dipalmitin.     

Biosynthesis of man-β-G1cNAc-G1cNAc-pyrophosphoryl-polyprenol by a solubilized enzyme from aorta

The particulate enzyme fraction from pig aorta was treated with Triton X-100 or Nonidet P-40 to yield a soluble enzyme preparation. This solubilized enzyme catalyzed the transfer of mannose from GDP-[¹⁴C]mannose, but not from [¹⁴C]mannosyl-phosphoryl-polyprenol, to G1cNAc-G1cNAc-pyrophosphoryl-polyprenol to form the trisaccharide-lipid, Man-β-GlcNAc-GlcNAc-pyrophosphoryl-polyprenol. The trisaccharide-lipid formed in these reactions was isolated by solvent fractionation and was subjected to mild acid hydrolysis to release the [¹⁴C]trisaccharide. Essentially all of the radioactivity was released from this trisaccharide as mannose upon treatment with β-mannosidase while α-mannosidase had no effect.     

Selective Acylation of Plasma Membrane Proteins of Mycoplasma mycoides subsp. mycoides SC,the Contagious Bovine Pleuropneumonia Agent

The plasma membrane of Mycoplasma mycoides subsp. mycoides SC (strain KH₃J) contains over 160 polypeptides with apparent molecular masses ranging from 14 to 125 kDa and isoelectric point values (pIs) from 5 to 9. In vivo labeling with [¹⁴C]-fatty acids revealed about 35 acylated polypeptides including the two major components p42 and p65 and displaying pIs between 5.5 and 9.0, with a majority between 6.5 and 8. The amphiphilic nature of most of these acyl proteins was confirmed by Triton X-114 phase partitioning. Gas-liquid chromatography analyses showed that the order of preference for protein acylation was 16:0 > 18:2c > 18:1c > 18:0 > 14:0, with 16:0 being the major O-ester-bound fatty acyl chain and 18:2c the major N-linked chain. The presence of S-glycerylcysteine and a ratio of [O-ester-bound acyl chains + N-linked chains]/O-ester bound chains of ≈ 1.2 in M. mycoides subsp. mycoides SC membrane proteins are consistent with a lipid modification similar to that occurring in lipoproteins of Gram-negative eubacteria that contain an N-terminal acyl S-glycerylcysteine.     

Molecular mechanisms of olfactory reception IV. Some biochemical characteristics of the camphor receptor from rat olfactory epithelium

Some parameters of the receptor element from the rat olfactory epithelium are evaluated; it is characterized by a high affinity for camphor (K_D = 1.5 · 10^?9 M). Triton X-100 has no marked effect on the binding of [³H]camphor. Neither RNAase nor phospholipase C affected [³H]camphor-binding activity. Pronase and trypsin abolished [³H]camphor binding activity by 65 and 40%, respectively. Sulfhydryl reagents decrease the binding of [³H]camphor by a factor of 5–8. The isoelectric point of the receptor solubilized with Triton X-100 is 4.8, as determined by isoelectric focusing. The molecular weight of the receptor as determined by gel electrophoresis is about 120 000. It is proposed that the camphor receptor is a membrane protein containing sulfhydryl groups and playing a key role in olfactory reception.     

Long-chain acyl CoA synthetase in microsomes from rat brain gray matter and white matter

Long-chain acyl coenzyme A (CoA) synthetase in homogenates and microsomes from rat brain gray and white matter was studied. The formation of the thioesters of CoA was studied upon addition of [1-¹⁴C]-labeled fatty acids. The maximal activities were seen with linoleic acid, followed by arachidonic, palmitic, and docosahexaenoic acids in both gray and white matter homogenates and microsomes. The specific activities in microsomes were 3–5 times higher than in homogenates. The presence of Triton X-100 in the assay system enhanced the activity of long-chain acyl CoA synthetase in homogenates. The effect was more pronounced in palmitic and docosahexaenoic acid activation. The apparentK _m values andV _max values for palmitic and docosahexaenoic acids were much lower than for linoleic and arachidonic acids. The presence of Triton X-100 in the medium caused a definite decrease in the apparentK _m and Vmax values for all the fatty acid except palmitic acid in which case the reverse was true. There were no significant differences observed in the kinetic measurements between gray and white matter microsomes. These findings are similar to those resulting from the known interference of Triton X-100 in the measurement of kinetic variables of long-chain acyl CoA synthetase of liver microsomes. In this work, no correlation was observed between the fatty acid composition of gray and white matter and the capacity of these tissues for the activation of different fatty acids.     

The interaction of insulin with phospholipids

1. A simple two-phase chloroform–aqueous buffer system was used to investigate the interaction of insulin with phospholipids and other amphipathic substances. 2. The distribution of ¹²⁵I-labelled insulin in this system was determined after incubation at 37°C. Phosphatidic acid, dicetylphosphoric acid and, to a lesser extent, phosphatidylcholine and cetyltrimethylammonium bromide solubilized ¹²⁵I-labelled insulin in the chloroform phase, indicating the formation of chloroform-soluble insulin–phospholipid or insulin–amphipath complexes. Phosphatidylethanolamine, sphingomyelin, cholesterol, stearylamine and Triton X-100 were without effect. 3. Formation of insulin–phospholipid complex was confirmed by paper chromatography. 4. The two-phase system was adapted to act as a simple functional system with which to investigate possible effects of insulin on the structural and functional properties of phospholipid micelles in chloroform, by using the distribution of [¹⁴C]glucose between the two phases as a monitor of phospholipid–insulin interactions. The ability of phospholipids to solubilize [¹⁴C]glucose in chloroform increased in the order phosphatidylcholine<sphingomyelin<phosphatidylethanolamine<phosphatidic acid. Insulin decreased the [¹⁴C]glucose solubilized by phosphatidylcholine, phosphatidylethanolamine and phosphatidic acid, but not by sphingomyelin. 5. The significance of these results and the molecular requirements for the formation of insulin–phospholipid complexes in chloroform are discussed.     

Identification of a 51-kilodalton polypeptide fatty acyl chain acceptor in soluble extracts from mouse cardiac tissue

We have identified a protein in the soluble fraction from mouse cardiac tissue extracts which is rapidly and selectively acylated by myristyl CoA. This protein was partially purified by anion-exchange chromatography and gel filtration, and the acylation reaction was measured using [3H]myristyl CoA as substrate, followed by sodium dodecyl sulfate - polyacrylamide gel electrophoresis to resolve [3H]fatty acyl polypeptides. The [3H]acyl protein migrated as heterogeneous bands corresponding to relative masses (MrS) of 42,000-51,000 under nonreducing conditions or as a single polypeptide of Mr 51,000 in the presence of reducing agents. Fatty acyl chain incorporation into protein was very rapid and already maximum after 30 s of incubation, whereas no acylation was detected using heat-denatured samples or when the reaction was stopped immediately after initiation. Only the acyl CoA served as fatty acyl chain donor. No incorporation into protein occurred when myristyl CoA was substituted by myristic acid, ATP, and CoA. A time-dependent reduction in the level of [3H]fatty acyl polypeptide was observed upon addition of excess unlabeled myristyl CoA, indicating the ability of the labeled acyl moiety of the protein to turn over during incubation. The saturated C10:0, C14:0, and C16:0 acyl CoAs were more effective to chase the label from the [3H]acyl polypeptide than the C18:0 and C18:1 acyl CoAs. These results provide evidence for a 51-kilodalton polypeptide which serves as an acceptor for fatty acyl chains and could represent an important intermediate in fatty acyl chain transfer reactions in cardiac tissue.     

Intramyelinic conversion of cerebrosides into acylgalactosylceramides

Acylgalactosylceramide (AGC) synthesis was measured in vivo, and in a cell free system. 24 hours post-injection of [³H]palmitic acid into rat brain, more than 60% of the AGC radioactivity was associated with an ester linkage. Isolated rat myelin was incubated in the presence of [¹⁴C]palmitic acid, 2mM ATP, 50 M CoA and 10 mM MgCl₂ and acylation of myelin cerebrosides occurred at a linear rate for at least 60 min. Incubation of isolated myelin under standard conditions with [³H] cerebrosides and [¹⁴C]palmitic acid produced double labeled AGC. Labeling of AGC was maximum at pH 7.5 and 37°C and appeared to be enzyme mediated inasmuch as it was reduced by myelin incubation with trypsin and drastically reduced by preheating the myelin for 5 min at 80°C. Omission of ATP, CoA, MgCl₂ or all three did not reduce fatty acid incorporation into AGC when compared to the values in the complete system. Addition of Triton X100 or Sodium Dodecyl Sulfate had little or no effect on the acylation of cerebrosides. Pulse chase experiments indicated that the reaction involved the net addition of fatty acid to the cerebrosides, rather than a rapid fatty acid exchange.     

Role of the cell wall in the ability of tobacco protoplasts to form callus

Cellular membranes from dark grown hypocotyls of Phaseolus aureus Roxb. were separated by centrifugation on a continuous sucrose gradient. Each gradient fraction was monitored for activity of inosine diphosphatase (EC 3.6.1.6) and the ability to transfer glucose from UDP-[¹⁴C]glucose to endogenous lipids in vitro. The highest incorporation of radioactivity into lipids occurred in a particulate fraction correlated with the Golgi apparatus, sedimenting at sucrose densities of 31.5–33% w/w. Three endogenous lipids were glucosylated in vitro. The two main lipids were characterized as steryl glucoside and acylated steryl glucoside; data from chromatography and hydrolysis of the third lipid suggests that it is dolichyl-monophosphate-glucoside. Steryl glucoside was found to be the main glucoside synthesized, but the proportion of the acylated form increased with time. The results are discussed in the context of the role of the Golgi apparatus as a centre of membrane modification within the plant cell.Abbreviations DMP-mannose dolichyl monophosphate mannose - ER endoplasmic reticulum - GA Golgi apparatus - ID-Pase inosine diphosphatase     

Membrane-bound D-gluconate dehydrogenase from Pseudomonas aeruginosa. Purification and structure of cytochrome-binding form.

A membrane-bound D-gluconate dehydrogenase [EC 1.1.99.3] was solubilized from membranes of Pseudomonas aeruginosa and purified to a homogeneous state with the aid of detergents. The solubilized enzyme was a monomer in the presence of at least 0.1% Triton X-100, having a molecular weight of 138,000 on polyacrylamide gel electrophoresis or 124,000--131,000 on sucrose density gradient centrifugation. In the absence of Triton X-100, the enzyme became dimeric, having a molecular weight of 240,000--260,000 on sucrose density gradient centrifugation. Removal of Triton X-100 caused a decrease in enzyme activity. Enzyme activity was stimulated by addition of phospholipid, particularly cardiolipin, in the presence of Triton X-100. The enzyme had a cytochrome c1, c-554(551), which might be a diheme cytochrome, and it also contained a covalently bound flavin but not ubiquinone. In the presence of sodium dodecyl sulfate, the enzyme was dissociated into three components with molecular weights of 66,000, 50,000, and 22,000. The components of 66,000 and 50,000 daltons corresponded to a flavoprotein and cytochrome c1, respectively, but that of 22,000 dalton remained unclear as to its function.     

Steryl glucoside and acyl glucoside biosynthesis in maturing pea seeds

Sterol glucosyltransferase activity was found in a particulate fraction of pea seeds. The activity was stimulated by Ca²⁺ and Mg²⁺ and inhibited by Zn²⁺, Cu²⁺, Hg²⁺, EDTA and EGTA. Iodoacetamide was without effect but p-chloromercuribenzoate completely inhibited the enzyme. N -Ethylmaleimide gave 60–70 % inhibition over a wide range of concentrations. The activity was stimulated by ATP in the presence of Mg²⁺. Under such conditions, steryl acyl glucoside was formed. The acyl derivative was barely detectable in the presence of Ca²⁺ either with or without ATP. Both oleyl CoA and palmityl CoA stimulated acyl glucoside synthesis. Of the four nucleoside triphosphates, ATP, GTP, UTP and CTP both ATP and CTP stimulated acylation in the presence of Mg²⁺. The observations suggest that acyl donors other than digalactosyl diglyceride and phospholipids may function in steryl acyl glucoside synthesis in plants.     

Synthesis of Long-Chain Acyl-CoA in Chloroplast Envelope Membranes

The chloroplast envelope is the site of a very active long-chain acylcoenzyme A (CoA) synthetase. Furthermore, we have recently shown that an acyl CoA thioesterase is also associated with envelope membrane (Joyard J, PK Stumpf 1980 Plant Physiol 65: 1039-1043). To clarify the interacting roles of both the acyl-CoA thioesterase and the acyl-CoA synthetase, the formation of acyl-CoA in envelope membranes was examined with different techniques which permitted the measurement of the actual rates of acyl-CoA formation. Using [¹⁴C]ATP or [¹⁴C]oleic acid as labeled substrates, it can be shown that the envelope acyl-CoA synthetase required both Mg²⁺ and dithiothreitol. Triton X-100 slightly stimulated the activity. The specificity of the acyl-CoA synthetase was determined either with [¹⁴C]ATP or with [³H]CoA as substrates. The results obtained in both cases were similar, that is, as substrates, the unsaturated fatty acids were more effective than saturated fatty acids, the velocity of the reaction increased from lauric acid to palmitic acid, and the maximum velocity was obtained with unsaturated C₁₈ fatty acids.     

Studies on a phospholipase B from Penicillium notatum. Purification, properties, and mode of action.

1. Phospholipase B which hydrolyzes both the acyl ester bonds of diacylphospholipids (diacyl-hydrolase) and the acyl ester bond of monoacylphospholipids or lysophospholipids, [monoacyl-hydrolase or lysophospholipase, EC 3.1.1.5] was purified from Penicillium notatum about 2000-fold over the crude extract. The final preparation was homogeneous on disc electrophoresis. The apparent molecular weight, determined by gel filtration on Sephadex G-200, was about 116,000. The isoelectric point was pH 4.0. 2. The purified enzyme was a glycoprotein. The carbohydrate content was approximately 30%, consisting of mannose, glucose, and glucosamine. The amino acid composition was also determined. 3. The ratio of monoacyl-hydrolase to diacyl-hydrolase activities was influenced by the physical state of the substrate in the assay system. It was about 1 : 1 or 100 : 1 in the presence of absence of Triton X-100, respectively, and the latter value remained constant throughout the purification procedures. 4. Both enzyme activities had the same pH optimum, 4.0, and were heat-labile. None of the metals tested had any effect on either activity except for Fe2+ and Fe3+. Diisopropyl fluorophosphate at relatively high concentrations completely inhibited both enzyme activities. 5. The Michaelis-Menten constants (Km) of the enzyme for egg lecithin were about 1.5 and 25 mM in the absence and presence of Triton X-100, respectively. The Km value for dicaproyllecithin was 9.8 mM in the absence of Triton X-100. 6. Using a mixture of 1-[14C]stearoyl-lecithin and 2-[14C]oleoyl-lecithin in the presence of Triton X-100 as a substrate, it was found that the P. notatum phospholipase B attacked the acyl ester bonds sequentially, first the 2-acyl and then 1-acyl groups.     

Analysis of Acyl Fluxes through Multiple Pathways of Triacylglycerol Synthesis in Developing Soybean Embryos

The reactions leading to triacylglycerol (TAG) synthesis in oilseeds have been well characterized. However, quantitative analyses of acyl group and glycerol backbone fluxes that comprise extraplastidic phospholipid and TAG synthesis, including acyl editing and phosphatidylcholine-diacylglycerol interconversion, are lacking. To investigate these fluxes, we rapidly labeled developing soybean (Glycine max) embryos with [¹⁴C]acetate and [¹⁴C]glycerol. Cultured intact embryos that mimic in planta growth were used. The initial kinetics of newly synthesized acyl chain and glycerol backbone incorporation into phosphatidylcholine (PC), 1,2-sn-diacylglycerol (DAG), and TAG were analyzed along with their initial labeled molecular species and positional distributions. Almost 60% of the newly synthesized fatty acids first enter glycerolipids through PC acyl editing, largely at the sn-2 position. This flux, mostly of oleate, was over three times the flux of nascent [¹⁴C]fatty acids incorporated into the sn-1 and sn-2 positions of DAG through glycerol-3-phosphate acylation. Furthermore, the total flux for PC acyl editing, which includes both nascent and preexisting fatty acids, was estimated to be 1.5 to 5 times the flux of fatty acid synthesis. Thus, recycled acyl groups (16:0, 18:1, 18:2, and 18:3) in the acyl-coenzyme A pool provide most of the acyl chains for de novo glycerol-3-phosphate acylation. Our results also show kinetically distinct DAG pools. DAG used for TAG synthesis is mostly derived from PC, whereas de novo synthesized DAG is mostly used for PC synthesis. In addition, two kinetically distinct sn-3 acylations of DAG were observed, providing TAG molecular species enriched in saturated or polyunsaturated fatty acids.