Crassulacean acid metabolism: plastic, fantastic

The occurrence, activity and plasticity of the CAM pathway is described from an introductory viewpoint, framed by the use of the four "Phases" of CAM as comparative indicators of the interplay between environmental constraints and internal molecular and biochemical regulation. Having described a number of "rules" which seem to govern the CAM cycle and apply uniformly to most species, a number of key regulatory points can then be identified. These include temporal separation of carboxylases, based on the circadian expression of key genes and their control by metabolites. The role of a circadian oscillator and interplay between tonoplast and nuclear control are central to maintaining the CAM cycle. Control of reserve carbohydrates is often neglected, but the importance of daily partitioning (for growth and the subsequent night-time CAM activity) and use at night is shown to drive the CAM cycle. Finally, it is shown that the genotypic and phenotypic plasticity in patterns of CAM expression is mediated partly by environmental conditions and molecular signalling, but also by diffusive constraints in succulent tissues. A transformation system is now required to allow these key areas of control to be elucidated.     

Sequence and expression of the gene for N10-formyltetrahydrofolate synthetase from Clostridium cylindrosporum.

Sau3 A and Hind III restriction fragments of Clostridium cylindrosporum genomic DNA were used to isolate clones containing 80% of the N10-H4folate synthetase gene in a 5' fragment and the remaining 20% of the gene in the 3' fragment. These fragments were joined at a common SnaB I restriction site and expressed in Escherichia coli at a level equivalent to what is normally found in C. cylindrosporum. Sequence comparisons show a large degree of homology with genes from two other clostridial species, including a thermophile. Certain conserved sequences found in the three clostridial proteins and in the N10-H4folate synthetase portion of eukaryotic C1-H4folate synthases may represent consensus sequences for nucleotide and H4folate binding.     

The effects of altered sterol composition on the mitochondrial adenine nucleotide transporter of Saccharomyces cerevisiae.

1. The membrane sterol composition of mitochondria of the ole-3 mutant of Saccharomyces cerevisiae was manipulated by growing the organism in the presence of Tween 80 (1%, W/V) plus defined supplements o- delta-aminolaevulinate. 2. Changes in mitochondrial sterol content induced considerable changes in the adenine nucleotide transporter. 3. As the sterol content was decreased, the affinity of the transporter for ATP did not alter significantly, but the rate of ATP uptake was greatly decreased, the total number of atractylate-sensitive binding sites diminished, and the proportion of high-affinity binding sites was decreased. 4. Since sterol depletion also uncouples oxidative phosphorylation [Astin & Haslam (1977) Biochem. J., 166, 287-298] and prevents the intramitochondrial generation of ATP, the decrease in the rate of ATP uptake by sterol-depleted mitochondria will cause a decrease in intramitochondrial ATP concentrations in vivo. This probably explains the inhibition of mitochondrial macromolecular synthesis that has previously been reported in lipid-depleted yeast mitochondria.     

Structural studies of N-glycans of filarial parasites. Conservation of phosphorylcholine-substituted glycans among species and discovery of novel chito-oligomers.

N-Type glycans containing phosphorylcholine (PC-glycans), unusual structures found in the important human pathogens filarial nematodes, represent a novel target for chemotherapy. Previous work in our laboratories produced compositional information on the PC-glycan of ES-62, a secreted protein of the rodent parasite Acanthocheilonema viteae. In particular, we established using fast atom bombardment mass spectrometry (MS) analysis that PC was attached to a glycan with a trimannosyl core, with and without core fucosylation, carrying between one and four additional N-acetylglucosamine residues. In the present study, we demonstrate that this structure is conserved among filarial nematodes, including the parasite of humans, Onchocerca volvulus, for which new drugs are most urgently sought. Furthermore, by employing a variety of procedures, including collision-activated dissociation MS-MS analysis and matrix-assisted laser desorption MS analysis, we reveal that surprisingly, filarial nematodes also contain N-linked glycans, the antennae of which are composed of chito-oligomers. To our knowledge, this is the first report of such structures in a eukaryotic glycoprotein.     

Estimating the number of viable animal cells in multi-well cultures based on their lactate dehydrogenase activities

A method is described for estimating the numbers ofanimal cells in multi-well culture by simultaneouslymeasuring the lactate dehydrogenase activity of thetotal culture and the medium. The difference betweenthe two reflects the dehydrogenase content of thecells and correlates with cell number. This LDH/INTmethod was tested using several lines of normal andtransformed suspension and adherent cells. Thelactate dehydrogenase activities of duplicate cultureswere determined colourimetrically using reactioncocktails containing lactate, NAD⁺, diaphorase,and p-iodonitrotetrazolium violet, with and withoutTriton X-100. The difference in absorbance at 490 nm(A₄₉₀ = A_{490, test} – A_{490, control}) was used to calculate the lactatedehydrogenase activity of the total culture (+ Triton)and the medium (– Triton). The cellular lactatedehydrogenase activity (difference between totaland medium dehydrogenaseactivities) was proportional to viable cell number. The effects on cell growth of four metabolicinhibitors, sodium azide, actinomycin D,cycloheximide, and taxol, were determined using theLDH/INT assay and direct cell counting. The inhibitorconcentrations that caused decreases in the LDHactivity and cell number by 50% were similar. TheLDH/INT assay is quick and sensitive, works equallywell for adherent and suspension cells, and providesinformation about LDH activities of both the mediumand cells. It is particularly useful for screeningpotential cell-growth inhibitors.     

Translocation of pleckstrin requires its phosphorylation and newly formed ligands

Pleckstrin is the major substrate of protein kinase C (PKC) in platelets. We sought to determine whether pleckstrin phosphorylation is sufficient to target the soluble protein to binding sites. Permeabilization of platelets by streptolysin O (SLO) was used to separate bound and soluble pleckstrin. Platelets were incubated with phorbol 12-myristate 13-acetate (PMA) and/or guanosine 5'-[gamma-thio]triphosphate (GTP[S]) in the presence of [gamma-(32)P]ATP and SLO. PMA stimulated pleckstrin phosphorylation, but this pleckstrin diffused from permeabilized platelets. Addition of GTP[S] with PMA caused up to 40-50% of pleckstrin to be retained within platelets and enhanced secretion of platelet 5-hydroxytryptamine. PKC alpha pseudosubstrate peptide inhibited pleckstrin phosphorylation, the binding of pleckstrin and secretion. After extraction of permeabilized platelets containing bound pleckstrin with Triton X-100, the protein was solubilized. Thus, phosphorylated pleckstrin was retained in platelets only after activation of GTP-binding proteins that stimulate the formation of membrane-bound pleckstrin ligands. Translocation of pleckstrin may facilitate the associated secretion.     

Estrogen and estrogen plus progestin act directly on the mammary gland to increase proliferation in a postmenopausal mouse model

Hormone replacement therapy (HRT) with ovarian hormones is an important therapeutic modality for postmenopausal women. However, a negative side effect of HRT is an increased risk of breast cancer. Surgical induction of menopause by ovariectomy (OVX) in mice is an experimental model that may provide insights into the effects of hormone replacement therapy on the human breast. We have developed a mouse model of early and late postmenopausal states to investigate the effects of HRT on the normal mammary gland. The purpose of this study was to determine if HRT-induced proliferation was due to the direct action of the hormones on the mammary gland, or mediated systemically by hormones or growth factors produced elsewhere in the body. Estrogen (E) or E plus the synthetic progestin, R5020, were implanted directly into the mammary glands of early (1 week post OVX) and late (5 week post OVX) postmenopausal mice instead of administration by injection. We report that responses of early and late postmenopausal mice to implanted hormones were the same as those observed previously with systemically administered hormones. Implanted E conferred an enhanced proliferative response in the late postmenopausal gland characterized morphologically by enlarged duct ends. E+R5020 implants induced similar degrees of cell proliferation in both postmenopausal states but the morphological responses differed. Ductal sidebranching was observed in early postmenopausal mice, whereas duct end enlargement was observed in late postmenopausal mice. The differences in morphological response to E+R5020 in 5 week post OVX were associated with an inability of E to induce progesterone receptors (PR) in the late postmenopausal gland. The responses of the late postmenopausal glands to E and E+P were very similar to that observed previously in immature pubertal glands in ovary-intact mice. In pubertal mice, PR cannot be induced by E unless the mammary gland is pre-treated with EGF-containing implants. Similarly, herein pre-treatment of the late postmenopausal mammary gland with EGF-containing implants restored PR induction by E. Thus, EGF may determine the sensitivity of the mammary gland to E and E+P in late postmenopause and at puberty.     

Isolation and structural determination of novel sulfated hexasaccharides from squid cartilage chondroitin sulfate E that exhibits neuroregulatory activities

Squid cartilage chondroitin sulfate E (CS-E) exhibits various biological activities, including anticoagulant activities, lymphoid regulatory activities, and neuroregulatory activities [Ueoka, C., Kaneda, N., Okazaki, I., Nadanaka, S., Muramatsu, T., and Sugahara, K. (2000) J. Biol. Chem. 275, 37407-37413]. These activities are expressed through molecular interactions with specific proteins, including heparin cofactor II, selectins, CD44, chemokines, and the heparin-binding growth factor midkine. Hence, the sugar sequence information is essential for a better understanding of the CS-E functions. Previously, several novel tetrasaccharides containing the unreported 3-O-sulfated glucuronic acid (GlcA) were isolated after digestion of squid cartilage CS-E with testicular hyaluronidase. In this study, hexasaccharides were isolated to obtain more detailed sequence information, especially around the GlcA(3-O-sulfate) residue, and were characterized by fast atom bombardment mass spectrometry and 500 or 600 MHz (1)H NMR spectroscopy. The findings demonstrate one tetrasulfated and five pentasulfated hexasaccharide sequences, five of them being novel. They were composed of three disaccharide building units of either A [GlcA(beta1-3)GalNAc(4-O-sulfate)], E [GlcA(beta1-3)GalNAc(4,6-O-disulfate)], K [GlcA(3-O-sulfate)(beta1-3)GalNAc(4-O-sulfate)], L [GlcA(3-O-sulfate)(beta1-3)GalNAc(6-O-sulfate)], or M [GlcA(3-O-sulfate)(beta1-3)GalNAc(4,6-O-disulfate)], forming E-A-A, M-A-A, K-L-A, E-E-A, K-K-A, and A-M-A hexasaccharide sequences. The K-L tetrasaccharide sequence is to date unreported. The isolated sequences appear to indicate the occurrence of an unreported GlcA 3-O-sulfotransferase specific for chondroitin sulfate. The obtained sequence information will be useful for investigating the structure-function relationship and biosynthesis of CS-E.     

A limited survey of aflatoxins and fumonisins in retail maizebased products in the UK using immunoassay detection

A total of 27 maize-based products destined for human consumption were collected from retail outlets within the city of Glasgow in the UK and were analysed for the presence of aflatoxins using immunoaftinity column chromatography with fluorescence detection and for fumonisins by competitive ELISA. Aflatoxins were detected at a trace level below 4 in eight (30%) of the 27 samples tested, no sample contained aflatoxins at a high level although one sample of sweetcorn did contain aflatoxins at a level of 5-10 Fumonisins were detected in eight (30%) of the samples at levels from 1 to 8mgkg^-1 and a further eight samples contained fumonisin at a level below 1 mgkg^-1 but above the detectable level. The highest concentration of fumonisins was found in a sample of fine corn meal at 8-12mgkg^-1.