Stereospecificity of tripeptide utilization in a methionine auxotroph of Escherichia coli K-12

The stereospecificity of peptide utilization in Escherichia coli K-12 4212, a methionine auxotroph, was investigated using diastereomers of trimethionine and trimethionine methyl ester. Of the eight stereoisomers examined, only l-Met-l-Met-l-Met, l-Met-l-Met-d-Met, and d-Met-l-Met-l-Met and the corresponding methyl esters serve as growth substrates. Triornithine-resistant mutants of strain 4212 were isolated which failed to transport d-Met-l-Met-l-Met. These results provide evidence that an oligopeptide containing a d residue at its amine terminus can enter E. coli by the oligopeptide transport system.     

Mechanism of hydrolysis of dicholine esters with long polymethylene chain by human butyrylcholinesterase

Human butyrylcholinesterase hydrolyzes long chain dicholine esters more rapidly than short chain dicholine esters. The active site of butyrylcholinesterase is deeply buried within the enzyme molecule and there is limited space for binding of large compounds. Our goal was to understand how butyrylcholinesterase accommodates long chain dicholine esters to make them better substrates than short chain dicholine esters. For this purpose we studied the rate of hydrolysis of adipyldicholine (n=4) and sebacyldicholine (n=8) with mass spectrometry, a method that allowed monitoring the dicholine substrates, the monocholine intermediates, the dicarboxylic acid and choline products. It was shown that hydrolysis of adipyldicholine involves two consecutive steps, dicholine ester hydrolysis followed by relatively slow monocholine ester hydrolysis. However, sebacyldicholine was hydrolyzed at both choline ester sites, though hydrolysis of dicholine was faster than hydrolysis of monocholine. Sebacyldicholine was completely converted to sebacic acid and choline within 90 min, whereas only 15% of the adipyldicholine was converted to adipic acid in this time. Molecular modeling indicated that these dicholine esters can bind to butyrylcholinesterase in two energetically equivalent alternative conformations that may theoretically lead to hydrolysis. The long chain dicholine ester makes closer contact than the short chain ester between one of its carbonyl carbons and the catalytic Ser198, thus explaining why long-chain dicholine esters are hydrolyzed more rapidly by butyrylcholinesterase.     

Inhibitors of neutral cholesteryl ester hydrolase

p-Nitrophenyl N-butyl, N-octyl, and N-dodecyl carbamates and a newly synthesized diethyl phosphate compound were studied as potential inhibitors of the cholesteryl ester hydrolases of Fu5AH rat hepatoma cells. Whole homogenates of Fu5AH cells were used as an enzyme source for the assay of cholesteryl ester hydrolase activity. All four compounds led to marked inhibition (70-80%) of neutral cholesteryl ester hydrolase activity (assayed at pH 7) at concentrations where the activity of acid cholesteryl ester hydrolase (assayed at pH 4) was unaffected. Cholesteryl ester hydrolysis was also evaluated in intact cultured cells induced to accumulate cholesteryl esters in cytoplasmic lipid droplets by exposure to cholesterol-rich phospholipid dispersions. Hydrolysis was then assessed during subsequent incubations in the presence of an inhibitor of cholesterol esterification. All compounds caused significant inhibition of cholesterol ester hydrolysis with the diethyl phosphate being the most effective. At a concentration that caused greater than 90% inhibition of the hydrolysis of cytoplasmic cholesteryl esters, the compound had only a minimal effect on lysosomal hydrolysis of cholesteryl esters. These results suggest that diethyl phosphates and N-alkylcarbamates may be of value in future studies on the substrate specificities, regulation, and physiological role(s) of cholesteryl ester hydrolases.     

Comparison of the kinetics and mechanism of the papain-catalyzed hydrolysis of esters and thiono esters

The kinetic constants for the papain-catalyzed hydrolysis of the methyl thiono esters of N-benzoylglycine and N-(beta-phenylpropionyl)glycine are compared with those for the corresponding methyl ester substrates. The k2/Ks values for the thiono esters are 2-3 times higher than those for the esters, and both show bell-shaped pH dependencies with similar pKa's (approximately 4 and 9). The k3 values for the thiono esters are 30-60 times less than those for the esters and do not exhibit a pH dependency. Solvent deuterium isotope effects on k2/Ks and k3 were measured for the ester and thiono ester substrates of both glycine derivatives. Each thiono ester substrate showed an isotope effect similar to that for the corresponding ester substrate. Moreover, use of the proton inventory technique indicated that, as for esters, one proton is transferred in the transition state for deacylation during reactions involving thiono esters and the degree of heavy atom reorganization in the transition state is very similar in both cases. The k3 values for the hydrolysis of a series of para-substituted N-benzoylglycine esters were found to correlate with the k3 values for the corresponding para-substituted thiono esters [Carey, P. R., Lee, H., Ozaki, Y., & Storer, A. C. (1984) J. Am. Chem. Soc. 106, 8258-8262], showing that the rate-determining step for the deacylation of both thiolacyl and dithioacyl enzymes probably involves the disruption of a contact between the substrate's glycinic nitrogen atom and the sulfur of cysteine-25. It is concluded that the hydrolysis of esters and thiono esters proceeds by essentially the same reaction pathway. Due to an oxygen-sulfur exchange process the product released in the case of the N-(beta-phenylpropionyl)glycine thiono ester substrate is the dioxygen acid; however, for the N-benzoylglycine thiono ester substrate, the thiol acid is the initial product. This thiol acid then acts as a substrate for papain and reacylates the enzyme to eventually give the dioxygen acid product. It is shown that thiol acids are excellent substrates for papain.     

Mechanism-controlled stereospecificity. Acylation of subtilisin with enantiomeric alkyl and nitrophenyl ester substrates.

The activation parameters of acylation of subtilisin with alkyl and p-nitrophenyl esters of N-acylamino acid enantiomers were determined. It was found that (1) the activation entropy is much higher with the nitrophenyl esters than with the corresponding methyl esters, (2) the difference in rate constants between enantiomers is 10(4)--10(5) with methyl esters whereas it is only of the order of 10 with nitrophenyl esters. The results indicate that the catalytic mechanism is simpler for nitrophenyl esters than for alkyl esters. The simple mechanism requires only general base catalysis, and thus permits more freedom of motion in the transition state, whereas the complex mechanism involves both general base and general acid catalysis. Furthermore, the strikingly low enantiomeric specificity with nitrophenyl esters indicates that not only binding but also the catalytic mechanism is an important factor in determining the stereospecificity of an enzyme. The activation parameters for enantiomeric nitrophenyl ester reactions suggest that structurally related substrates can be transformed by the enzyme in different conformations which may be energetically similar or not. The energetically different conformations may account for the activation enthalpy-entropy compensation.     

Macrophage esterase: identification, purification and properties of a chymotrypsin-like esterase from lung that hydrolyses and transfers nonpolar amino acid esters.

A chymotrypsin-like esterase was purified from beef lung. This lysosomal enzyme, not previously characterized, seemed to be composed of two or more forms with molecular weights of about 52 000. It hydrolysed N-benzoyl-DL-phenylalanine beta-naphthol ester at acid and neutral pH; it polymerized L-phenylalanine methyl ester(Phe-OMe) at neutral pH; and it transferred the Phe-residue from Phe-OMe to hydroxylamine at neutral pH. Phenylmethanesulfonyl fluoride, an inhibitor of hydrolytic enzymes with serine in their catalytic site, inhibited this enzyme, but pepstatin, the cathepsin D (EC 3.4.4.23) inhibitor, did not. Sulfhydryl reagents were not required for activity. Macrophages, especially pulmonary alveolar macrophages, were a rich source of this esterase, so it is likely that the enzyme purified from lung came from its macrophages. The esterase hydrolysed and transferred monoamino acid esters, especially those of the aromatic type. Cathepsin C, the dipeptidyl peptide hydrolase (EC 3.4.14.1), acted only on dipeptide esters and amides. Pancreatic chymotrypsin acted on both monoamino acid and dipeptide esters. The chymotrypsin-like esterase did not hydrolyse hemoglobin, casein, or plasma albumin. Thus its proteolytic activity, if present, must be limited to specific substrates, as yet unknown.     

Role of lysosomal acid lipase in the metabolism of plasma low density lipoprotein. Observations in cultured fibroblasts from a patient with cholesteryl ester storage disease.

The hydrolysis of cholesteryl esters contained in plasma low density lipoprotein was reduced in cultured fibroblasts derived from a patient with cholesteryl ester storage disease, an inborn error of metabolism in which lysosomal acid lipase activity is deficient. While these mutant cells showed a normal ability to bind low density lipoprotein at its high affinity cell surface receptor site, to take up the bound lipoprotein through endocytosis, and to hydrolyze the protein component of the lipoprotein in lysosomes, their defective lysosomal hydrolysis of the cholesteryl ester component of the lipoprotein led to the accumulation within the cell of unhydrolyzed cholesteryl esters, the fatty acid distribution of which resembled that of plasma lipoprotein. When the cholesteryl ester storage disease cells were incubated with low density lipoprotein, the reduced rate of liberation of free cholesterol by these mutant cells was associated with a delay in the occurrence of two lipoprotein-mediated regulatory events, suppression of 3-hydroxy-3-methylglutaryl coenzyme A reductase activity, and activation of endogenous cholesteryl ester formation. In contrast to their defective hydrolysis of exogenously derived lipoprotein-bound cholesteryl esters, the choleseryl ester storage disease cells showed a normal rate of hydrolysis of cholesteryl esters that had been synthesized within the cell. These data lend support to the concept that in cultured human fibroblasts cholesteryl esters entering the cell bound to low density lipoprotein are hydrolyzed within the lysosome and that one of the functions of this intracellular organelle is to supply the cell with free cholesterol.     

Rupture of rat liver lysosomes mediated by l-amino acid esters

Treatment of rat liver lysosome suspensions with 0.5–20 mM α-l-amino acid esters results in a progressive loss of latency of lysosomal enzyme activity. The increase in available acid phosphatase activity in lysosomal suspensions is correlated with the decrease of turbidity of these suspensions. Ester mediated turbidity decrease is dependent upon ester concentration, and the pH and ionic strength of the suspending medium. d-Stereoisomers of amino acid esters do not exhibit comparable capacity to damage lysosomes.α-l-Amino acid esters were found to be substrates for neutral lysosomal esterase and transpeptidase activity. The d-stereoisomers are degraded at much lower rates. These data support the hypothesis that ester dependent lysosomal rupture is mediated by the specific interaction of the ester with a structural or functional lysosomal protein.     

Cholesteryl ester hydrolysis in rat liver lysosomes: different response to female sex hormones

The regulation of the hydrolysis of cholesteryl oleate by female sex hormones was studied in the lysosomal fraction of rat liver. Cholesterol ester hydrolase activity was determined at pH 5.0 with an acetone-dissolved cholesteryl [1-14C]oleate substrate preparation. The administration of a single dose of progesterone decreased the enzyme activity during a 3- to 24-hr period following hormone injection. This effect was not correlated to changes in the lysosomal protein synthesis rate. The lysosomal hydrolysis of cholesteryl esters was also inhibited in a noncompetitive manner by the addition of progesterone at concentrations higher than 100 microM. The esterase failed to respond to the estradiol in vivo as well as in vitro. The findings of the present paper suggest that the lysosomal breakdown of cholesteryl esters in rat liver may be under selective hormonal regulation and that the inhibitory effect of progesterone on the enzyme activity might be, at least in part, responsible for the liver cholesterol ester accumulus produced by the administration of the hormone.     

Substrate specificity of an α-amino acid ester hydrolase produced by Acetobacter turbidans A.T.C.C. 9325

A partially purified preparation of an alpha-amino acid ester hydrolase was obtained from Acetobacter turbidans A.T.C.C. 9325, which catalyses synthesis of 7-(d-alpha-amino-alpha-phenylacetamido)-3-cephem-3-methyl-4- carboxylic acid (cephalexin) from methyl d-alpha-aminophenylacetate and 7-amino-3-deacetoxycephalosporanic acid. The enzyme preparation catalysed both cephalosprin synthesis from 7-amino-3-deacetoxycephalosporanic acid and suitable amino acid esters (e.g. methyl d-alpha-aminophenylacetate, l-cysteine methyl ester, glycine ethyl ester, d-alanine methyl ester, methyl dl-alpha-aminoiso-butyrate, l-serine methyl ester, d-leucine methyl ester, l-methionine methyl ester) and the hydrolysis of such esters. The substrate specificity of the enzyme preparation for the hydrolysis closely paralleled the acyl-donor specificity for cephalosporin synthesis, even to the reaction rates. Only alpha-amino acid derivatives could act as acyl donors. The hydrogen atom on the alpha-carbon atom was not always required by acyl donors. The hydrolysis rate was markedly diminished by adding 7-amino-3-deacetoxycephalosporanic acid to reaction mixtures, but no effect on the total reaction rate (the hydrolysis rate plus synthesis rate) was observed with various concentrations of 7-amino-3-deacetoxycephalosporanic acid. Both the hydrolytic and the synthetic activities of the enzyme preparation were inhibited by high concentrations of some acyl donors (e.g. methyl d-alpha-aminophenylacetate, ethyl d-alpha-aminophenylacetate). The enzyme preparation hydrolysed alpha-amino acid esters much more easily than alpha-amino acid derivatives with an acid-amide bond.     

Cholesteryl ester hydrolysis in J774 macrophages occurs in the cytoplasm and lysosomes

The relationship of cholesteryl ester hydrolysis to the physical state of the cholesteryl ester in J774 murine macrophages was explored in cells induced to store cholesteryl esters either in anisotropic (ordered) inclusions or isotropic (liquid) inclusions. In contrast to other cell systems, the rate of cholesteryl ester hydrolysis was faster in cells containing anisotropic inclusions than in cells containing isotropic inclusions. Two contributing factors were identified. Kinetic analyses of the rates of hydrolysis are consistent with a substrate competition by co-deposited triglyceride in cells with isotropic inclusions. In addition, hydrolysis of cholesteryl esters in cells with anisotropic droplets is mediated by both cytoplasmic and lysosomal lipolytic enzymes, as shown by using the lysosomotropic agent, chloroquine, and an inhibitor of neutral cholesteryl ester hydrolase, umbelliferyl diethylphosphate. In cells containing anisotropic inclusions, hydrolysis was partially inhibited by incubation in media containing either chloroquine or umbelliferyl diethylphosphate. Together, chloroquine and umbelliferyl diethylphosphate completely inhibited hydrolysis. However, when cells containing isotropic inclusions were incubated with umbelliferyl diethylphosphate, cholesteryl ester hydrolysis was completely inhibited, but chloroquine had no effect. Transmission electron microscopy demonstrated a primarily lysosomal location for lipid droplets in cells with anisotropic droplets and both non-lysosomal and lysosomal populations of lipid droplets in cells with isotropic droplets.These results support the conclusion that there is a lysosomal component to the hydrolysis of stored cholesteryl esters in foam cells.     

How pure is your thiosialoside? A reinvestigation into the HPLC purification of thioglycosides of N-acetylneuraminic acid

The synthesis of thiosialosides as potential biological probes for investigations involving the use of sialic acid-recognising proteins has been reinvestigated. It has been found that the most efficient method for the preparation of thiosialosides free from any 2,3-didehydro sialic acid contaminants involves an intermediate HPLC purification of thiosialosides as their methyl esters. Subsequent methyl ester hydrolysis provides thiosialosides (eg. 6 and 14) which are suitable for studies involving the use of sialic acid-recognising proteins.     

Effects of secondary interactions on the kinetics of peptide and peptide ester hydrolysis by tissue kallikrein and trypsin

Kinetic constants for the hydrolysis by porcine tissue beta-kallikrein B and by bovine trypsin of a number of peptides related to the sequence of kininogen (also one containing a P2 glycine residue instead of phenylalanine) and of a series of corresponding arginyl peptide esters with various apolar P2 residues have been determined under strictly comparative conditions. kcat and kcat/Km values for the hydrolysis of the Arg-Ser bonds of the peptides by trypsin are conspicuously high. kcat for the best of the peptide substrates, Ac-Phe-Arg-Ser-Val-NH2, even reaches kcat for the corresponding methyl ester, indicating rate-limiting deacylation also in the hydrolysis of a peptide bond by this enzyme. kcat/Km for the hydrolysis of the peptide esters with different nonpolar L-amino acids in P2 is remarkably constant (range 1.7), as it is for the pair of the above pentapeptides with P2 glycine or phenylalanine. kcat for the ester substrates varies fivefold, however, being greatest for the P2 glycine compounds. Obviously, an increased potential of a P2 residue for interactions with the enzyme lowers the rate of deacylation. In contrast to results obtained with chymotrypsin and pancreatic elastase, trypsin is well able to tolerate a P3 proline residue. In the hydrolysis of peptide esters, tissue kallikrein is definitely superior to trypsin. Conversely, peptide bonds are hydrolyzed less efficiently by tissue kallikrein and the acylation reaction is rate-limiting. The influence of the length of peptide substrates is similar in both enzymes and indicates an extension of the substrate recognition site from subsite S3 to at least S'3 of tissue kallikrein and the importance of a hydrogen bond between the P3 carbonyl group and Gly-216 of the enzymes. Tissue kallikrein also tolerates a P3 proline residue well. In sharp contrast to the behaviour of trypsin is the very strong influence of the P2 residue in tissue-kallikrein-catalyzed reactions. kcat/Km varies 75-fold in the series of the dipeptide esters with nonpolar L-amino acid residues in P2, a P2 glycine residue furnishing the worst and phenylalanine the best substrate, whereas this exchange in the pentapeptides changes kcat/Km as much as 730-fold. This behaviour, together with the high value of kcat/Km for Ac-Phe-Arg-OMe of 3.75 X 10(7) M-1 s-1, suggests rate-limiting binding (k1) in the hydrolysis of the best ester substrates.(ABSTRACT TRUNCATED AT 400 WORDS)     

Plasma Membrane Repair Is Regulated Extracellularly by Proteases Released from Lysosomes

Eukaryotic cells rapidly repair wounds on their plasma membrane. Resealing is Ca^2+-dependent, and involves exocytosis of lysosomes followed by massive endocytosis. Extracellular activity of the lysosomal enzyme acid sphingomyelinase was previously shown to promote endocytosis and wound removal. However, whether lysosomal proteases released during cell injury participate in resealing is unknown. Here we show that lysosomal proteases regulate plasma membrane repair. Extracellular proteolysis is detected shortly after cell wounding, and inhibition of this process blocks repair. Conversely, surface protein degradation facilitates plasma membrane resealing. The abundant lysosomal cysteine proteases cathepsin B and L, known to proteolytically remodel the extracellular matrix, are rapidly released upon cell injury and are required for efficient plasma membrane repair. In contrast, inhibition of aspartyl proteases or RNAi-mediated silencing of the lysosomal aspartyl protease cathepsin D enhances resealing, an effect associated with the accumulation of active acid sphingomyelinase on the cell surface. Thus, secreted lysosomal cysteine proteases may promote repair by facilitating membrane access of lysosomal acid sphingomyelinase, which promotes wound removal and is subsequently downregulated extracellularly by a process involving cathepsin D.     

Neutral and acid retinyl ester hydrolases associated with rat liver microsomes: relationships to microsomal cholesteryl ester hydrolases

We recently reported the presence of a neutral, bile salt-independent retinyl ester hydrolase (REH) activity in rat liver microsomes and showed that it was distinct from the previously studied bile salt-dependent REH and from nonspecific carboxylesterases (Harrison, E. H., and M. Z. Gad. 1989. J. Biol. Chem. 264: 17142-17147). We have now further characterized the hydrolysis of retinyl esters by liver microsomes and have compared the observed activities with those catalyzing the hydrolysis of cholesteryl esters. Microsomes and microsomal subfractions enriched in plasma membranes and endosomes catalyze the hydrolysis of retinyl esters at both neutral and acid pH. The acid and neutral REH enzyme activities can be distinguished from one another on the basis of selective inhibition by metal ions and by irreversible, active site-directed serine esterase inhibitors. The same preparations also catalyze the hydrolysis of cholesteryl esters at both acid and neutral pH. However, the enzyme(s) responsible for the neutral REH activity can be clearly responsible for the neutral REH activity can be clearly differentiated from the neutral cholesteryl ester hydrolase(s) on the basis of differential stability, sensitivity to proteolysis, and sensitivity to active site-directed reagents. These results suggest that the neutral, bile salt-independent REH is relatively specific for the hydrolysis of retinyl esters and thus may play an important physiological role in hepatic vitamin A metabolism. In contrast to the neutral hydrolases, the activities responsible for hydrolysis of retinyl esters and cholesterol esters at acid pH are similar in their responses to the treatments mentioned above. Thus, a single microsomal acid hydrolase may catalyze the hydrolysis of both types of ester.(ABSTRACT TRUNCATED AT 250 WORDS)     

The use of phospholipid vesicles for in vitro studies on cholesteryl ester hydrolysis.

Radiolabeled cholesteryl oleate was incorporated into vesicles prepared from egg yolk lecithin and utilized as a substrate for studies of sterol ester hydrolases present in rat liver homogenates. The cholesteryl oleate was shown to be associated with vesicles (unilamellar liposomes) using Sepharose 4B chromatography. With this substrate, two different cholesteryl ester hydrolytic enzymes were demonstrated in subcellular fractions from the liver homogenates. In the lysosome-rich fraction an acid hydrolase was present, while in the cytosol fraction (150,000 g supernatant), hydrolytic activity was shown to occur with an optimum pH between 8 and 8.5. The substrate was characterized by Sepharose chromatography both before and after incubation with the liver fraction and was not dramatically altered even by rigorous incubation conditions. The lysosomal enzyme preparation was capable of hydrolyzing almost all the cholesteryl oleate in the vesicles. Hydrolysis of the phospholipid was proportionately much less than that of the cholesteryl oleate. Comparisons were performed between the vesicle preparation and an alternate substrate preparation involving the direct addition of cholesteryl oleate in acetone solution. The vesicles appeared to be a better substrate for the lysosomal enzyme whereas the activity in the cytosol fraction did not distinguish between the two substrate preparations. Unsonicated suspensions of cholesteryl oleate and lecithin did not serve as suitable substrates for the enzymes. These studies demonstrate the applicability of cholesteryl ester-containing vesicles as a useful substrate for studying cholesteryl ester hydrolysis in vitro.     

Synthesis of fatty acid sterol esters using cholesterol esterase from Trichoderma sp. AS59

We recently reported the characterization of novel cholesterol esterase (EC. 3.1.1.13) from Trichoderma sp. and preliminary work on sterol ester synthesis. In the present study, we further examined the enzyme ability to synthesize cholesterol esters from cholesterol and free fatty acids of various chain lengths, and compared the fatty acid specificity in synthesis with that in hydrolysis. The enzyme catalyzed the synthesis of medium- and long-chain fatty acid cholesterol esters, but failed to synthesize short-chain fatty acid esters. The fatty acid specificities in the synthesis and hydrolysis of cholesterol esters were entirely different from each other. Unlike other lipolytic enzymes, the enzyme was largely independent of water content in the synthesis of cholesterol oleate, and it achieved near-complete esterification in the presence of an equimolar excess of oleic acid. Of additional interest is the finding that the addition of n-hexane markedly enhanced the esterification activities on all the medium- and long-chain saturated fatty acids used. Based on these findings, we attempted to synthesize stigmasterol stearate as a food additive to lower cholesterol levels in blood plasma, and found that the enzyme catalyzed effective synthesis of the ester without the need of dehydration during the reaction, indicating the potential utility of the enzyme in the food industry.     

Novel synthesis of optically active γ-carboxyglutamic acid and its derivatives

A novel synthesis of optically active L-γ-carboxyglutamic acid (GLA), found in prothrombin and other vitamin K-dependent factors, is described. This method involves the formation of γ-carbanion of the protected glutamic acid esters by means of a non-nucleophilic base, followed by the treatment with benzyl chloroformate or other electrophilic agents, and deprotection by hydrogenation or hydrolysis. This convenient technique is potentially useful for preparation of not only the γ-carboxyglutamic acid itself, but also of the mixed protected ester and amino functions for further peptide syntheses containing this amino acid.     

Fatty acid specificity of the lysosomal acid cholesterol esterase in intact human arterial smooth muscle cells

The fatty-acid specificity of the lysosomal cholesterol esterase was examined in cultured human arterial smooth muscle cells. The lysosomal compartment of cultured cells was enriched with cholesteryl esters by incubation of cells with 0.2 mg/ml low-density lipoprotein and 50 microM chloroquine for 24 h. The hydrolysis of cholesteryl esters was subsequently induced by incubating cells in a medium containing 5% lipoprotein-deficient serum without chloroquine. Cellular cholesteryl ester mass was markedly reduced after 23 h in the lipoprotein-deficient serum. Fatty-acid analysis of cholesteryl esters in cells before and after the 23 h incubation with lipoprotein-deficient serum revealed that polyunsaturated cholesteryl esters (linoleate and arachidonate) were preferentially hydrolyzed compared to cholesteryl oleate or saturated cholesteryl esters. An increase in the ratio of cholesteryl oleate to cholesteryl linoleate was observed even when the cellular activity of acyl-CoA:cholesterol acyltransferase was inhibited with Sandoz Compound 58-035. We conclude that, in human arterial smooth muscle cells, the lysosomal acid cholesterol esterase preferentially hydrolyzes polyunsaturated cholesteryl esters.     

Functional analyses and application of microbial lactonohydrolases

Microbial lactonohydrolases (intramolecular ester bond-hydrolyzing enzymes) with unique properties were found. The lactonohydrolase fromFusarium oxysporum catalyzes enantioselective hydrolysis of aldonate lactones andd-pantoyl lactone (d-PL). This enzyme is useful for the large-scale optical resolution of racemic PL. TheAgrobacterium tumefaciens enzyme catalyzes asymmetric hydrolysis of PL, but the stereospecificity is opposite to that of theFusarium enzyme. Dihydrocoumarin hydrolase (DHase) fromAcinetobacter calcoaceticus is a bifunctional enzyme, which catalyzes not only hydrolysis of aromatic lactones but also bromination of monochlorodimedon in the presence of H₂O₂ and dihydrocoumarin. DHase also hydrolyzes several linear esters, and is useful for enantioselective hydrolysis of methyldl-β-acetylthioisobutyrate and regioselective hydrolysis of methyl cetraxate.