Characterization of the transacylase activity of rat liver 60-kDa lysophospholipase-transacylase. Acyl transfer from the sn-2 to the sn-1 position

Structural and functional diversity of asparaginases: Overview and recommendations for a revised nomenclature

Asparaginases (ASNases) are a large and structurally diverse group of enzymes ubiquitous amongst archaea, bacteria and eukaryotes, that catalyze hydrolysis of asparagine to aspartate and ammonia. Bacterial ASNases are important biopharmaceuticals for the treatment of acute lymphoblastic leukemia, although some patients experience adverse allergic side effects during treatment with these protein therapeutics. ASNases are currently divided into three families: plant-type ASNases, Rhizobium etli-type ASNases and bacterial-type ASNases. This system is outdated as both bacterial-type and plant-type families also include archaeal, bacterial and eukaryotic enzymes, each with their own distinct characteristics. Herein, phylogenetic studies allied to tertiary structural analyses are described with the aim of proposing a revised and more robust classification system that considers the biochemical diversity of ASNases. Accordingly, based on distinct peptide domains, phylogenetic data, structural analysis and functional characteristics, we recommend that ASNases now be divided into three new distinct classes containing subgroups according to structural and functional aspects. Using this new classification scheme, 25 ASNases were identified as candidates for future new lead discovery.

Novel reactions in acyl editing of phosphatidylcholine by lysophosphatidylcholine transacylase (LPCT) and acyl-CoA:glycerophosphocholine acyltransferase (GPCAT) activities in microsomal preparations of plant tissues

Plants have lysophosphatidylcholine transacylase (LPCT) and acyl-CoA:glycerophosphocholine acyltransferase (GPCAT) activities. The combined action of LPCT and GPCAT provides a novel route of PC re-synthesis after its deacylation. Phosphatidylcholine (PC) is the major lipid in eukaryotic membranes and has a central role in overall plant lipid metabolism. It is also the site of production of polyunsaturated fatty acids in plants. The recently discovered acyl-CoA:glycerophosphocholine acyltransferase (GPCAT) activity in yeast provides a novel route of re-synthesising PC via lysophosphatidylcholine (LPC) after its deacylation. This route does not require the degradation of the glycerophosphocholine (GPC) into free choline, the activation of choline to CDP-choline, nor the utilization of CDP-choline by the CDP-choline:diacylglycerol cholinephosphotransferase. We show here that GPCAT activities also are present in membrane preparations from developing oil seeds of safflower and other species as well as in membrane preparations of roots and leaves of Arabidopsis, indicating that GPCAT activity plays a ubiquitous role in plant lipid metabolism. The last step in formation of GPC, the substrate for GPCAT, is the deacylation of LPC. Microsomal membranes of developing safflower seeds utilized LPC in LPC:LPC transacylation reactions (LPCT activities) creating PC and GPC. The results demonstrate that safflower membranes have LPCT and GPCAT activities that represent novel reactions for PC acyl editing. The physiological relevance of these reactions probably has to await identification of the enzymes catalysing these reactions.

Phosphatidylinositol-4-phosphate 5-kinase isoforms exhibit acyl chain selectivity for both substrate and lipid activator

Phosphatidylinositol 4,5-bisphosphate is mostly produced in the cell by phosphatidylinositol-4-phosphate 5-kinases (PIP5K) and has a crucial role in numerous signaling events. Here we demonstrate that in vitro all three isoforms of PIP5K, α, β, and γ, discriminate among substrates with different acyl chains for both the substrates phosphatidylinositol 4-phosphate (PtdIns4P) and phosphatidylinositol (PtdIns) although to different extents, with isoform γ being the most selective. Fully saturated dipalmitoyl-PtdIns4P was a poor substrate for all three isoforms, but both the 1-stearoyl-2-arachidonoyl and the 1-stearoyl-2-oleoyl forms of PtdIns4P were good substrates. V(max) was greater for the 1-stearoyl-2-arachidonoyl form compared with the 1-stearoyl-2-oleoyl form, although for PIP5Kβ the difference was small. For the α and γ isoforms, K(m) was much lower for 1-stearoyl-2-oleoyl PtdIns4P, making this lipid the better substrate of the two under most conditions. Activation of PIP5K by phosphatidic acid is also acyl chain-dependent. Species of phosphatidic acid with two unsaturated acyl chains are much better activators of PIP5K than those containing one saturated and one unsaturated acyl chain. PtdIns is a poor substrate for PIP5K, but it also shows acyl chain selectivity. Curiously, there is no acyl chain discrimination among species of phosphatidic acid in the activation of the phosphorylation of PtdIns. Together, our findings indicate that PIP5K isoforms α, β, and γ act selectively on substrates and activators with different acyl chains. This could be a tightly regulated mechanism of producing physiologically active unsaturated phosphatidylinositol 4,5-bisphosphate species in the cell.

Cloning of CnLYSO1, a novel extracellular lysophospholipase of the pathogenic fungus Cryptococcus neoformans

We cloned a novel lysophospholipase (CnLYSO1) from Cryptococcus neoformans var. grubii by PCR amplification and a cDNA library screen. The open reading frame (ORF) of 1278 nucleotides coded for a predicted 426-amino-acid protein (CnLyso1p) with two highly conserved GXSXG lipase-specific catalytic motifs and a molecular weight of 48.3 kDa. CnLyso1p exhibited 14% and 21% identity to Arabidopsis thaliana and human lysophospholipases, respectively. Immunoprecipitation and Western blot analyses indicated that CnLyso1p was secreted as a high molecular weight protein of 97-140 kDa. CnLYSO1 expressed in a phospholipase B-null mutant of Saccharomyces cerevisiae demonstrated lysophospholipase and lysophospholipase transacylase activities at pH 4.0. Targeted disruption of CnLYSO1 did not affect growth, melanin or capsule production by C. neoformans. Secreted lysophospholipase and transacylase activities (pH 4.0) were 50% of wild type and CnLyso1p was undetectable on Western blots. Phospholipase B activity was reduced at pH 7.0 (P<0.006) and at pH 4.0 (P=NS). The amount of secreted Plb1p (the gene product of PLB1) was also reduced. Deletion of PLB1 abolished all three secreted activities at pH 4.0 and 7.0. These results are best explained by post-translational interaction, most likely the formation of a functional complex between the independently regulated gene products, CnLyso1p and CnPlb1p.

Phospholipase and lysophospholipase activity of rat eosinophil leukocytes

Previous studies have shown the high lysophospholipase activity of rat eosinophilic leukocytes and used this enzyme to measure the rise in eosinophilic population of peripheral tissues caused by parasitic infections. This report details the methods and results of an investigation showing the presence in the same cells of high phospholipase (PLA) activity. Unfractionated and metrizamide-purified peritoneal eosinophil preparations were assayed using a mixed micelle substrate (6/15 mM lecithin/Triton X-100) at experimentally determined pH (6.4) and ionic strength (I=0.2) optima: the attendant reaction products included free fatty acids and organic P in a 2/1 molar proportion with a correspondent loss in the initial phospholipid concentration. The organic P fragment was further characterized as GPC (glycerylphosphorylcholine) by quantitative precipitation and acid hydrolysis. Estimates of PLA activity averaged 5 micromol/h/10(6) unfractionated eosinophils and metrizamide-purified eosinophil preparations. Paired tests for PLA and LysoPLA on unfractionated and enriched cell preparations, cytosolic extracts, and chromatographic fractions yielded similar activity ratios, supporting the inference of a close association of the two activities which could also be confirmed for the major tissues of eosinophil production and distribution.

Incorporation and distribution of saturated and unsaturated fatty acids into nuclear lipids of hepatic cells

Liver nuclear incorporation of stearic (18:0), linoleic (18:2n-6), and arachidonic (20:4n-6) acids was studied by incubation in vitro of the [1-14C] fatty acids with nuclei, with or without the cytosol fraction at different times. The [1-14C] fatty acids were incorporated into the nuclei as free fatty acids in the following order: 18:0 > 20:4n-6 >> 18:2n-6, and esterified into nuclear lipids by an acyl-CoA pathway. All [1-14C] fatty acids were esterified mainly to phospholipids and triacylglycerols and in a minor proportion to diacylglycerols. Only [1-14C]18:2n-6-CoA was incorporated into cholesterol esters. The incorporation was not modified by cytosol addition. The incorporation of 20:4n-6 into nuclear phosphatidylcholine (PC) pools was also studied by incubation of liver nuclei in vitro with [1-14C]20:4n-6-CoA, and nuclear labeled PC molecular species were determined. From the 15 PC nuclear molecular species determined, five were labeled with [1-14C]20:4n-6-CoA: 18:0-20:4, 16:0-20:4, 18:1-20:4, 18:2-20:4, and 20:4-20:4. The highest specific radioactivity was found in 20:4-20:4 PC, which is a minor species. In conclusion, liver cell nuclei possess the necessary enzymes to incorporate exogenous saturated and unsaturated fatty acids into lipids by an acyl-CoA pathway, showing specificity for each fatty acid. Liver cell nuclei also utilize exogenous 20:4n-6-CoA to synthesize the major molecular species of PC with 20:4n-6 at the sn-2 position. However, the most actively synthesized is 20:4-20:4 PC, which is a quantitatively minor component. The labeling pattern of 20:4-20:4 PC would indicate that this molecular species is synthesized mainly by the de novo pathway.