Effects of metal ions and hydrogen peroxide on the phenotype of yeast hom6Δ mutant.

UNLABELLED: HOM6 is a major gene in the aspartate pathway which leads to biosynthesis of threonine and methionine. The phenotypes of the gene deletion mutant (hom6∆) in a variety of cultural conditions have previously provided meaningful insights into the biological roles of HOM6 and its upstream intermediate metabolites. Here, we conducted a survey on a spectrum of metal ions for their effect on the aspartate pathway and broader sulphur metabolism. We show that manganese (Mn(2+) ) promoted the growth of hom6∆ under both anaerobic and aerobic conditions. Unexpectedly, 4 mmol l(-1) hydrogen peroxide (H2 O2 ), a dose normally causing temporary cell growth arrest, enhanced the growth of hom6∆ under the anaerobic condition only, while it had no effect on the wild type strain BY4743. We propose that Mn(2+) and H2 O2 promote the growth of hom6∆ by reducing the accumulation of the toxic intermediate metabolite-aspartate ß-semialdehyde, via directing the aspartate pathway to the central sugar metabolism-tricarboxylic acid cycle. SIGNIFICANCE AND IMPACT OF THE STUDY: This study focuses on the yeast strain which lacks homoserine dehydrogenase encoded by HOM6 gene in aspartate metabolism. The HOM6-deletion mutant (hom6Δ) was analysed in the context of varying environmental parameters such as metal ions and oxidants, under anaerobic and aerobic conditions. We demonstrated that both manganese and hydrogen peroxide can promote the growth of hom6Δ, with the latter exerting such effect only under anaerobic condition. The findings are relevant to the research areas of ageing and anti-fungal drug development. It highlights the importance of interactions between gene expression and environmental factors as well as culture conditions.

Transcriptomic and biochemical evidence for the role of lysine biosynthesis against linoleic acid hydroperoxide-induced stress in Saccharomyces cerevisiae.

Amino acid biosynthesis forms part of an integrated stress response against oxidants in Saccharomyces cerevisiae and higher eukaryotes. Here we show an essential protective role of the l-lysine biosynthesis pathway in response to the oxidative stress condition induced by the lipid oxidant-linoleic acid hydroperoxide (LoaOOH), by means of transcriptomic profiling and phenotypic analysis, and using the deletion mutant dal80∆ and lysine auxotroph lys1∆. A comprehensive up-regulation of lysine biosynthetic genes (LYS1, LYS2, LYS4, LYS9, LYS12, LYS20 and LYS21) was revealed in dal80Δ following the oxidant challenge. The lysine auxotroph (lys1∆) exhibited a significant decrease in growth compared with that of BY4743 upon exposure to LoaOOH, albeit with the sufficient provision of lysine in the medium. Furthermore, the growth of wild type BY4743 exposed to LoaOOH was also greatly reduced in lysine-deficient conditions, despite a full complement of lysine biosynthetic genes. Amino acid analysis of LoaOOH-treated yeast showed that the level of cellular lysine remained unchanged throughout oxidant challenge, suggesting that the induced lysine biosynthesis leads to a steady-state metabolism as compared to the untreated yeast cells. Together, these findings demonstrate that lysine availability and its biosynthesis pathway play an important role in protecting the cell from lipid peroxide-induced oxidative stress, which is directly related to understanding environmental stress and industrial yeast management in brewing, wine making and baking.

Transcriptomic insights into the molecular response of Saccharomyces cerevisiae to linoleic acid hydroperoxide.

Eukaryotic microorganisms are constantly challenged by reactive oxygen species derived endogenously or encountered in their environment. Such adversity is particularly applied to Saccharomyces cerevisiae under harsh industrial conditions. One of the major oxidants to challenge S. cerevisiae is linoleic acid hydroperoxide (LoaOOH). This study, which used genome-wide microarray analysis in conjunction with deletion mutant screening, uncovered the molecular pathways of S. cerevisiae that were altered by an arresting concentration of LoaOOH (75 µM). The oxidative stress response, iron homeostasis, detoxification through PDR transport and direct lipid ß-oxidation were evident through the induction of the genes encoding for peroxiredoxins (GPX2, TSA2), the NADPH:oxidoreductase (OYE3), iron uptake (FIT2, ARN2, FET3), PDR transporters (PDR5, PDR15, SNQ2) and ß-oxidation machinery (FAA2, POX1). Further, we discovered that Gpx3p, the dual redox sensor and peroxidase, is required for protection against LoaOOH, indicated by the sensitivity of gpx3Δ to a mild dose of LoaOOH (37.5 µM). Deletion of GPX3 conferred a greater sensitivity to LoaOOH than the loss of its signalling partner YAP1. Deletion of either of the iron homeostasis regulators AFT1 or AFT2 also resulted in sensitivity to LoaOOH. These novel findings for Gpx3p, Aft1p and Aft2p point to their distinct roles in response to the lipid peroxide. Finally, the expression of 89 previously uncharacterised genes was significantly altered against LoaOOH, which will contribute to their eventual annotation.

Regulation of ethanolamine and choline phosphatide biosynthesis in isolated rat intestinal villus cells.

The effects of ethanolamine, choline, and different fatty acids on phospholipid synthesis via the CDP-ester pathways were studied in isolated rat intestinal villus cells. The incorporation of [14C]glucose into phosphatidylethanolamine was stimulated severalfold by the addition of ethanolamine and long-chained unsaturated fatty acids, while the addition of lauric acid inhibited the incorporation of radioactivity into phosphatidylethanolamine. At concentrations of ethanolamine higher than 0.2 mM, phosphoethanolamine accumulated, but the concentrations of CDP-ethanolamine and the incorporation of radioactivity into phospatidylethanolamine did not increase further. The incorporation of [14C]glucose into phosphatidylcholine responded in a way similar to that of phosphatidylethanolamine, except that a 10-fold higher concentration of choline was required for maximal stimulation. CCC inhibited the incorporation of choline into phosphatidylcholine. In contrast with hepatocytes, villus cells did not form phosphatidylcholine via phospholipid N-methylation. The data indicate that, in intestinal villus cells, the cytidylyltransferase reactions are rate limiting in the synthesis of phosphatidylethanolamine and probably also of phosphatidylcholine. The availability of diacylglycerol and its fatty acid composition may also significantly affect the rate of phospholipid synthesis.

1,25-Dihydroxyvitamin D3 increases the activity of the intestinal phosphatidylcholine deacylation-reacylation cycle.

The activity of the intestinal phosphatidylcholine deacylation-reacylation cycle has been found to be stimulated by 1,25-dihydroxy-vitamin D3. The stimulation of this cycle thus provides a possible mechanism for the reported retailoring of the fatty acid composition of phosphatidylcholine in intestinal cell membranes by 1,25-dihydroxy-vitamin D3 and its analogue, 1alpha-hydroxyvitamin D3.

The importance of the steric configuration of lysophosphatidylcholine in the lymphatic transport of fat.

The importance of the steric configuration of lysophosphatidylcholine in the lymphatic transport of fat was investigated in bile fistula rats. It was found that the feeding of 1-palmitoyl-sn-glycero-3-phosphocholine increased the lymphatic output of phosphatidyl choline and triacylglycerol, while the feeding of 3-palmitoyl-sn-glycero-1-phosphocholine had no effect. In intestinal microsomes of the bile fistula rats, it was found that the lysophosphatidylcholine acyltransferase was stereospecific in acylating the 1-acyl-sn-glycero-3-phosphocholine enantiomer. The significance of these findings is briefly discussed.

Steric requirements for the stimulation of glycosyltransferase activity by lysophosphatidylcholine.

1-Palmitoyl-sn-glycerol-3-phosphocholine and 3-palmitoyl-sn-glycerol-1-phosphocholine have been found to be equipotent in the stimulation of membrane-bound glycosyltransferases in microsomes of rat intestinal villus cells. This indicates that the stimulatory effect of lysophosphatidylcholine is not stereospecific, but that it may be related to a specific detergent property dependent upon the peculiar balance of hydrophilic and hydrophobic components in the molecule.

Lack of effect of vitamin D and its metabolites on intestinal phosphate transport in familial hypophosphatemia of mice.

Intestinal calcium and phosphate transport was studied in normal and hypophosphatemic mice fed a variety of dietary regimens with and without vitamin D. Regardless of dietary phosphorus levels, the genetic hypophosphatemic mice had drastically reduced levels of serum inorganic phosphate and intestinal phosphate transport while showing only slightly reduced serum calcium and intestinal calcium transport levels. The inclusion of vitamin D in the diet did not increase the low serum phosphorus levels and low rates of intestinal phosphate transport in the genetic hypophosphatemic mice, while it did increase serum calcium and intestinal calcium transport levels. The administration of 1,25-dihydroxyvitamin D3 to the hypophosphatemic mice stimulated intestinal calcium transport but had no effect on intestinal phosphate transport. In contrast, the 1,25-dihydroxyvitamin D3 stimulated both phosphate and calcium transport in the intestine of normal mice. The results obtained are consistent with the hypothesis that the primary metabolic disturbance in familial hypophosphatemia involves a defect in phosphate transport mechanisms.

Ribosome structure and chylomicron formation in rat intestinal mucosa.

Sucrose gradient sedimentation and electron micrographic studies were made on the ribosomes of the cells of intestinal mucosa isolated from control, bile fistula, and puromycin-treated rats. In comparison to controls, there was a 40 to 60 per cent decrease in polysome content of the cells following administration of puromycin or deprivation of luminal choline by creation of a bile fistula. Feeding of lysolecithin of choline to the bile fistula rats or addition to the isolated cells in vitro resulted in a complete restoration of the polysome profile along with lipprotein synthesis and chylomicron release. Addition of lysolecithin to isolated cells treated with puromycin in vitro also brought about a reaggregation of the ribosomes and a reactivation of phospholipid biosynthesis. Choline had no detectable effect on phospholipid synthesis or ribsosme aggregation when fed to ppuromycin-treated rats or when added to puromycin-treated cells in vitro. The results suggest that chylomicron formation and the release by the muscosal cells depend upon intact rough endoplasmic reticulum and an active protein adn phospholipid biosynthesis. The role of lysolecithin in this process is ratonalized on the basis of its ability to supply a precursor of lecithin as well as a surfactant which affects the aggregation, or membrane rebinding of ribosomes, or both.

Glycerolipid biosynthesis in isolated rat intestinal epithelial cells.

Intestinal epithelial cells were prepared from fasted rats by dispersion with collagenase (EC 3.4.24.3). The structural and metabolic integrity of the cells was verified by electron microscopy, a high percentage of Trypan Blue exclusion, a low degree of release of lactate dehydrogenase (EC 1.1.1.27) in the medium, and by the retention of sensitivity to agents known to modify metabolic and transport activity in everted sacs of intestinal mucosa. The isolated intestinal epithelial cells were used to study glycerolipid biosynthesis from glucose, glycerol, 2-monoacylglycerol, and free fatty acids. The cells actively incorporated the labeled precursors into glycerolipids without specific cofactor requirements. Addition of fatty acids stimulated the incorporation of both glucose and glycerol into triacylglycerols and glycerophospholipids, the greatest effect being observed with palmitate. The stimulation of monoacylglycerol acylation appeared to depend on both the nature of the monoacylglycerol and fatty acid supplied. Stereospecific analyses of the diacylglycerols formed from 2-monoacylglycerols and free fatty acids showed that 1,2-diacyl-sn-glycerols (62-70%) were the major and that 2,3-diacyl-sn-glycerols (30-38%) the minor intermediates in triacylglycerol biosynthesis. The data indicate that isolated intestinal epithelial cells exhibit a total capacity of glycerolipid synthesis and a stereochemical course of reaction which is comparable to that observed for triacylglycerol formation in everted sacs of intestinal mucosa, but much less specific than that seen in microsomal preparations of intestinal mucosa.