Phospholipid substrate-specificity of the L-serine base-exchange enzyme in rat liver microsomal fraction.
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ABSTRACT:
SUBMITTER: Bjerve KS
PROVIDER: S-EPMC1153544 | biostudies-other | 1984 May
REPOSITORIES: biostudies-other
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Project description:Phospholipid methyltransferase, the enzyme that converts phosphatidylethanolamine into phosphatidylcholine with S-adenosyl-L-methionine as the methyl donor, was purified to apparent homogeneity from rat liver microsomal fraction. When analysed by SDS/polyacrylamide-gel electrophoresis only one protein, with molecular mass about 50 kDa, is detected. This protein could be phosphorylated at a single site by incubation with [alpha-32P]ATP and the catalytic subunit of cyclic AMP-dependent protein kinase. A less-purified preparation of the enzyme is mainly composed of two proteins, with molecular masses about 50 kDa and 25 kDa, the 50 kDa form being phosphorylated at the same site as the homogeneous enzyme. After purification of both proteins by electro-elution, the 25 kDa protein forms a dimer and migrates on SDS/polyacrylamide-gel electrophoresis with molecular mass about 50 kDa. Peptide maps of purified 25 kDa and 50 kDa proteins are identical, indicating that both proteins are formed by the same polypeptide chain(s). It is concluded that rat liver phospholipid methyltransferase can exist in two forms, as a monomer of 25 kDa and as a dimer of 50 kDa. The dimer can be phosphorylated by cyclic AMP-dependent protein kinase.
| S-EPMC1147047 | biostudies-other
Project description:After force-feeding a protein-free diet to male rats for 5-7 days a substantial (2.4-fold) increase in the specific activity of the liver microsomal enzyme UDP-glucuronyltransferase (EC 2.4.1.17) was observed. A similar activation of the enzyme occurred when rats were fed on a low-protein (5%, w/w, casein) diet for 60 days. Although both the short- and long-term protein-deficient diets decreased the contents of microsomal protein and phospholipid in liver tissue they did not significantly alter the ratio of these major membrane components. Protein deficiency profoundly altered the phospholipid composition of microsomal membranes. The most striking difference in microsomal phospholipid composition between control and protein-deficient rats was their content of lysophosphatides. Whereas microsomal membranes from protein-deficient rats contained significant proportions of lysophosphatidylcholine and lysophosphatidylethanolamine very little or no lysophosphatides were detected in control preparations. Pretreatment of microsomal fractions from normal rats with phospholipase A markedly increased their UDP-glucuronyltransferase activity as did their pretreatment with lysophosphatidylcholine. It is concluded that the quantities of lysophosphatides present in microsomal membranes from protein-deficient rats were sufficient to have caused the increased UDP-glucuronyltransferase activities of these preparations. Evidence is presented suggesting that these changes in microsomal phospholipid composition and UDP-glucuronyltransferase activity caused by protein deficiency reflect changes that occur in vivo. The possible physiological significance of these findings is discussed.
| S-EPMC1166158 | biostudies-other
Project description:More than 80% of the phospholipid component of guinea-pig liver microsomal membranes (prepared with 154mM-KCl) was removed by treatment with phospholipase A followed by extraction of the lysophosphatides and fatty acids produced with albumin. Delipidation strongly inactivated the highly active UDP-glucuronyltransferase of these preparations and activity was restored by mixtures of phosphatidylcholine and lysophosphatidylchlone. However, small quantities of lysophosphatides were still associated with the delipidated fractions after extraction with albumin and might have influenced the inactivation and re-activation observed. To eliminate these uncertainties, microsomal proteins and phospholipids were separated by gel filtration on Sephadex G-150 in the presence of cholate. This technique also strongly inactivated the enzyme but did not generate membrane-active phospholipid degradation products. High transferase activity was again restored to the delipidated protein by choline glycerophosphatides. These results confirm the view that the fully active form of microsomal UDP-glucuronyltransferase is phospholipid-dependent.
| S-EPMC1164666 | biostudies-other
Project description:DcsE, one of the enzymes found in the d-cycloserine biosynthetic pathway, displays a high sequence homology to l-homoserine O-acetyltransferase (HAT), but it prefers l-serine over l-homoserine as the substrate. To clarify the substrate specificity, in the present study we determined the crystal structure of DcsE at a 1.81-Å resolution, showing that the overall structure of DcsE is similar to that of HAT, whereas a turn region to form an oxyanion hole is obviously different between DcsE and HAT: in detail, the first and last residues in the turn of DcsE are Gly(52) and Pro(55), respectively, but those of HAT are Ala and Gly, respectively. In addition, more water molecules were laid on one side of the turn region of DcsE than on that of HAT, and a robust hydrogen-bonding network was formed only in DcsE. We created a HAT-like mutant of DcsE in which Gly(52) and Pro(55) were replaced by Ala and Gly, respectively, showing that the mutant acetylates l-homoserine but scarcely acetylates l-serine. The crystal structure of the mutant DcsE shows that the active site, including the turn and its surrounding waters, is similar to that of HAT. These findings suggest that a methyl group of the first residue in the turn of HAT plays a role in excluding the binding of l-serine to the substrate-binding pocket. In contrast, the side chain of the last residue in the turn of DcsE may need to form an extensive hydrogen-bonding network on the turn, which interferes with the binding of l-homoserine.
| S-EPMC3624550 | biostudies-literature
Project description:1. A new and rapid continuous assay of rat liver microsomal UDP-glucuronyltransferase (EC 2.4.1.17) has been developed. It is based on measurement of UDP production from UDP-glucuronate during the glucuronidation reaction; UDP production was continuously measured by coupling it to the conversion of NADH into NAD+ through pyruvate kinase and lactate dehydrogenase. This assay is independent of the acceptor substrate used; several findings confirm its applicability. 2. The glucuronidation rate of a series of phenol derivatives was determined with this assay, by using a Triton X-100-activated microsomal preparation as enzyme source. Conjugation of a series of nitrophenol derivatives was also investigated by the 'classical' assay (measurement of disappearance of the yellow colour of the nitrophenol during glucuronidation). The substrate with the highest conversion rate was 3-methyl-2-nitrophenol. 3. Both electron releasing and electron withdrawing ring substituents increased the glucuronidation rate of the phenol derivatives, as compared with phenol. 4. Lipid solubility seems important for determining the conversion rate: poorly lipid-soluble substrates were glucuronidated only at a low rate and high lipid solubility seems to be a prerequisite for high conversion rate. Glucuronidation of poorly lipid-soluble compounds may be limited by diffusion. 5. The consequences of these findings for the interpretation of studies on heterogeneity of the enzyme are discussed.
| S-EPMC1172334 | biostudies-other
Project description:Phospholipid oxidation generates several bioactive aldehydes that remain esterified to the glycerol backbone ('core' aldehydes). These aldehydes induce endothelial cells to produce monocyte chemotactic factors and enhance monocyte-endothelium adhesion. They also serve as ligands of scavenger receptors for the uptake of oxidized lipoproteins or apoptotic cells. The biochemical pathways involved in phospholipid aldehyde metabolism, however, remain largely unknown. In the present study, we have examined the efficacy of the three mammalian AKR (aldo-keto reductase) families in catalysing the reduction of phospholipid aldehydes. The model phospholipid aldehyde POVPC [1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphocholine] was efficiently reduced by members of the AKR1, but not by the AKR6 or the ARK7 family. In the AKR1 family, POVPC reductase activity was limited to AKR1A and B. No significant activity was observed with AKR1C enzymes. Among the active proteins, human AR (aldose reductase) (AKR1B1) showed the highest catalytic activity. The catalytic efficiency of human small intestinal AR (AKR1B10) was comparable with the murine AKR1B proteins 1B3 and 1B8. Among the murine proteins AKR1A4 and AKR1B7 showed appreciably lower catalytic activity as compared with 1B3 and 1B8. The human AKRs, 1B1 and 1B10, and the murine proteins, 1B3 and 1B8, also reduced C-7 and C-9 sn-2 aldehydes as well as POVPE [1-palmitoyl-2-(5-oxovaleroyl)-sn-glycero-3-phosphoethanolamine]. AKR1A4, B1, B7 and B8 catalysed the reduction of aldehydes generated in oxidized C(16:0-20:4) phosphatidylcholine with acyl, plasmenyl or alkyl linkage at the sn-1 position or C(16:0-20:4) phosphatidylglycerol or phosphatidic acid. AKR1B1 displayed the highest activity with phosphatidic acids; AKR1A4 was more efficient with long-chain aldehydes such as 5-hydroxy-8-oxo-6-octenoyl derivatives, whereas AKR1B8 preferred phosphatidylglycerol. These results suggest that proteins of the AKR1A and B families are efficient phospholipid aldehyde reductases, with non-overlapping substrate specificity, and may be involved in tissue-specific metabolism of endogenous or dietary phospholipid aldehydes.
| S-EPMC1925154 | biostudies-literature
Project description:Neurotransmitters activate a phospholipase D that is though to specifically hydrolyse phosphatidylcholine. This enzyme has a unique property known as transphosphatidylation: in the presence of an appropriate nucleophilic receptor such as an alcohol, phospholipase D will catalyse the production of phosphatidyl-alcohol. We have studied phospholipase D using an in vitro assay that uses [3H]butanol of high specific radioactivity (15 Ci/mmol) as an acceptor. In the presence of [3H]butanol and phosphatidylcholine, a microsomal membrane fraction from rat brain catalysed the production of phosphatidyl[3H]butanol. Phospholipase D activity was dependent upon the presence of a detergent; the optimal sodium oleate concentration was between 4 and 6 mM. The RF of the phosphatidyl[3H]butanol on t.l.c. was identical to the RF of the phosphatidylbutanol formed when [3H]phosphatidylcholine was incubated with 100 mM butanol. These data confirm the identity of phosphatidyl[3H]butanol. One important advantage of this assay is that the substrate does not need to be labelled. We have used this advantage to examine the substrate specificity of phospholipase D. Microsomal phospholipase D appears to hydrolyse phosphatidylcholine most efficiently. There is a relatively small but significant activity against phosphatidylethanolamine and phosphatidylserine, and there is no significant activity against phosphatidylinositol. As the head-group becomes more like choline, the phospholipid becomes a better substrate for phospholipase D. The addition of one methyl group leads to a large increase in activity. Fatty acid composition does not play a role in determining the substrate specificity. This assay should be useful in furthering our understanding of this important enzyme.
| S-EPMC1134631 | biostudies-other
Project description:Neuropsin is one of serine proteases mainly found at the hippocampus and the amygdala, where it contributes to the long-term potentiation and memory acquisition by rebuilding of synaptic connections. Despite of the importance of neuropsin, the substrate specificity and regulation mechanisms of neuropsin have been unclear. Thus, we investigated the substrate specificity and the catalytic activity of neuropsin by the protein-ligand docking and molecular dynamics (MD) simulations and succeeded to reproduce the trend of the experimental results. Our study revealed that the substrate specificity and the activity of neuropsin depended on multiple factors: the substrate charge, the substrate orientation, the hydrogen bond network within the catalytic triad and the substrate, and the formation of the oxyanion hole. The apo neuropsin was not reactive without proper alignment of catalytic triad. The substrate binding induced the reactive alignment of catalytic triad. Then the substrate-neuropsin interaction forms the oxyanion hole that stabilizes the transition state and reduces the free-energy barrier of the following scission reaction.
| S-EPMC9592888 | biostudies-literature
Project description:1. Isolated rat liver mitochondria do not synthesize labelled phosphatidylcholine from CDP-[(14)C]choline or any phospholipid other than phosphatidic acid from [(32)P]phosphate. The minimal labelling of phosphatidylcholine and other phosphoglycerides can be attributed to microsomal contamination. However, when mitochondria and microsomes are incubated together with [(32)P]phosphate, the phosphatidylcholine, phosphatidylinositol and phosphatidylethanolamine of the reisolated mitochondria become labelled, suggesting a transfer of phospholipids between the two fractions. 2. When liver microsomes or mitochondria containing labelled phosphatidylcholine are independently incubated with the opposite un-labelled fraction, there is a substantial and rapid exchange of the phospholipid between the two membranes. Exchange of phosphatidylinositol also occurs rapidly, whereas phosphatidylethanolamine and phosphatidic acid exchange only slowly. There is no corresponding transfer of marker enzymes. The transfer of phosphatidylcholine does not occur at 0 degrees , and there is no requirement for added substrate, ATP or Mg(2+), but the omission of a heat-labile supernatant fraction markedly decreases the exchange. 3. After intravenous injection of [(32)P]phosphate, short-period labelling experiments of the individual phospholipids of rat liver microsomes and mitochondria in vivo give no evidence for a similar exchange process. However, the incubation of isolated microsomes and mitochondria with [(32)P]phosphate also fails on reisolation of the fractions to demonstrate a precursor-product relationship between the individual phospholipids of the two membranes. 4. The intraperitoneal injection of [(32)P]phosphate results in a far greater proportion of the dose entering the liver than does intravenous administration. After intraperitoneal administration of [(32)P]phosphate the specific radioactivities of the individual phospholipids are in the order microsomes > outer mitochondrial membrane > inner mitochondrial membrane. 5. The incorporation of (32)P into cardiolipin is very slow both in vivo and in vitro. After labelling in vivo the radioactivity in the cardiolipin persists compared with that of the other phospholipids, whose specific radioactivities in the microsomes and mitochondrial fragments decay at a similar rate to that of the acid-soluble phosphate pool. 6. The possibility of phospholipid exchange processes occurring in the liver cell in vivo is discussed, and it is suggested that only a small but highly labelled part of the endoplasmic-reticulum lipoprotein pool is involved in the transfer.
| S-EPMC1187645 | biostudies-other
Project description:Members of the same protease family show different substrate specificity, even if they share identical folds, depending on the physiological processes they are part of. Here, we investigate the key factors for subpocket and global specificity of factor Xa, elastase, and granzyme B which despite all being serine proteases and sharing the chymotrypsin-fold show distinct substrate specificity profiles. We determined subpocket interaction potentials with GRID for static X-ray structures and an in silico generated ensemble of conformations. Subpocket interaction potentials determined for static X-ray structures turned out to be insufficient to explain serine protease specificity for all subpockets. Therefore, we generated conformational ensembles using molecular dynamics simulations. We identified representative binding site conformations using distance-based hierarchical agglomerative clustering and determined subpocket interaction potentials for each representative conformation of the binding site. Considering the differences in subpocket interaction potentials for these representative conformations as well as their abundance allowed us to quantitatively explain subpocket specificity for the nonprime side for all three example proteases on a molecular level. The methods to identify key regions determining subpocket specificity introduced in this study are directly applicable to other serine proteases, and the results provide starting points for new strategies in rational drug design.
| S-EPMC4724848 | biostudies-literature