Search PubMed⌕ Search

Biomedical subjects

D H Northcote

Publications and source records attributed to D H Northcote.

At least 55 records · Page 3Linked to original sources
PubMed1976

Phospholipid composition of the plasma membrane of the green alga, Hydrodictyon africanum.

A plasma-membrane fraction was isolated from the alga Hydrodictyon africanum by micro-dissection and its phospholipid components were analysed. Phosphatidylcholine was the major phospholipid of the preparation. Both phosphatidylserine and diphosphatidylglycerol were enriched in the fraction compared with the whole cell, but the relative amount of phosphatidylglycerol present was less than that in the whole cell. Phosphatidylinositol was absent from the plasma-membrane preparation.

Cell Membrane↗
PubMed1976

The influence of gibberellic acid and abscisic acid on cell and tissue differentiation of bean callus.

Bean callus was induced to form roots (tissue differentiation) and vascular nodules (cell differentiation) by lowering the ratio of auxin to cytokinin in the growth medium. Both types of differentiation were inhibited by the addition of abscisic acid (at concentrations greater than I muM) to induction medium. Initiation of differentiation was inhibited, but its subsequent development was not, and the inhibition was not affected by the addition of gibberellic acid. Addition of gibberellic acid (GA) alone to induction medium stimulated tissue differentiation, although cell differentiation was unaffected (30 muM GA) or inhibited (45 muM GA) and its onset was delayed at both concentrations. Root initiation was also stimulated by gibberellic acid (0.I-45 muM) at an auxin-to-kinin ratio 10 times that normally optimal for cell differentiation. The phenylalanine ammonia lyase (PAL) activity of the calluses was closely correlated with the amount of cell differentiation which had occurred, and measurement of this confirmed that gibberellic acid delayed the initiation of cell differentiation. The increase and subsequent decline of PAL and betaI leads to 3 glucan synthetase activities, normally induced by transfer to induction medium, was abolished by abscisic acid. Addition of gibberellic acid did not affect the betaI leads to 3 glucan synthetase activity.

Abscisic Acid↗
PubMed1975

The formation of oligoglucans linked to lipid during synthesis of beta-glucan by characterized membrane fractions isolated from peas.

Membrane fractions were obtained from peas roots by using a method that permitted the isolation of a fraction rich in relatively intact dictyosome stacks. No chemical fixatives were used. The method involved incubation of the roots with cellulase, followed by gentle homogenization and sucrose-density-gradient fractionation of the homogenate. The fractions were characterized by electron microscopy. All fractions were enzymically active in incorporating glucose from UDP-glucose into water-insoluble glycolipids containing both single glucose residues and glucose oligosaccharides. Some or all of the linkages of glucose to lipid were through phosphate esters. A substance containing glucose oligosaccharides attached to or very strongly adsorbed on to protein was also formed. The membrane fractions also incorporated glucose from UDP-glucose into alkali-soluble and alkali-insoluble beta-glucans, which like the oligosaccharides contained beta(1leads to 3) and beta-(1leads to4) linkages. The distribution of the enzymic activities and the chemical properties of the lipid-linked and protein-linked oligosaccharides suggest that they may be intermediates in beta-glucan synthesis. The synthetic activity is associated with smooth-membrane vesicles which may be derived from the plasma membrane.

Amylases↗
PubMed1975

The breakdown of lipid reserves in the endosperm of germinating castor beans.

1. Lipid extracts were obtained from castor-bean endosperm tissue at various times during germination and, after purification, the total lipid content was determined. Quantitative measurements of the triglyceride and phospholipid content together with the fatty acid composition were made. 2. The total lipid content of the endosperm rapidly decreased during germination; after 10 days less than 20% of the original weight of lipid remained. In contrast, the phospholipid content (initially less than 0.5% of the total lipid) increased slightly during this time. The fatty acid composition and the relative proportions of the triglyceride species of the total lipid extract remained constant during 10 days of germination. 3. Gibberellic acid (0.3 mM) markedly stimulated the rate of lipid breakdown but did not alter either the fatty acid composition or the relative proportion of triglyceride species. 4. The embryo had little effect on lipid metabolism in the endosperm tissue; only after 6 days of germination were differences observed in the rate of fat utilization in the presence and absence of the embryo.

Ricinus communis↗
PubMed1975

Pectin synthesis during the wall regeneration of plasmolysed tobacco leaf cells.

1. Discs of tobacco leaf lamina were floated on media containing D-[U-14C]glucose. Glucose uptake was mainly through the cut edges, and diffusion through the veins and intracellular spaces was slow. 2. Radioactivity was detected in all polysaccharide fractions extracted from the discs, including those associated with the wall. 3. Plasmolysis in sorbitol or KCL decreased the incorporation of radioactive material into all fractions. 4. Incorporation of arabinose into pectins was increased or unaffected by plasmolysis, but the incorporations of other sugars were decreased. Removal of the lower epidermis did not affect this result. 5. Seperate mechanisms for arabinan and polygalacturonorhamman syntheses must exist, and these must differ in their responses to the physiochemical, structural and organizational changes that accompany plasmolysis.

Arabinose↗
PubMed1975

Some enzymes present in the walls of mesophyll cells of tobacco leaves.

Cell-wall enzymes were assayed by the difference between enzyme activities in the whole cell and the protoplast. Both peroxidase (85.2%) and acid phosphatase (21.9%) were located in the wall. However, malate dehydrogenase was found only in the protoplast. A study of the time-course of the release of peroxidase and malate dehydrogenase into the incubation medium from cells either treated with cellulase or untreated, also indicated that peroxidase and not malate dehydrogenase was located in the wall. Only two anodic isoenzymes of peroxidase were present in the cell wall. These were more negatively charged than those of horseradish peroxidase.

Acid Phosphatase↗
PubMed1975

The induction of enzyme activity in the endosperm of germinating castor-bean seeds.

Endosperm extracts were prepared at various times during germination from intact castor-bean seeds and from seeds from which the embryos had been removed. The sterilized seeds were incubated either on solid water agar or on agar containing 0.3 mM-gibberellic acid. 2. Isocitrate lyase and 3-hydroxyacyl-CoA dehydrogenase had very low activities in the mature seeds, but increased 44-fold and 27-fold respectively during germination. In contrast, the extracts of mature seeds had considerable acid and alkaline lipase activity and this only increased two- to three-fold during the incubation period. 3. Incubation of the seeds with gibberellic acid accelerated the rate of appearance of isocitrate lyase and 3-hydroxyacyl-CoA dehydrogenase. It also increased the total activity attained. However, the application of hormone had, in comparison, little effect on the development of lipase activity. 4. The removal of the embryo had little influence on the development of enzyme activity in the endosperm tissue; only with isocitrate lyase was a decrease in activity observed in the absence of the embryo.

Ricinus communis↗
PubMed1975

Quantitative measurement of the course of bean callus differentiation.

Two strains of callus have been isolated from bean hypocotyl and grown on a defined maintenance medium supplemented with 2 mg/l. 2:4-dichlorophenoxyacetic acid (2:4D) and 2% sucrose. Root initiation was observed in one strain and formation of nodules containing xylem and phloem in both strains after transfer to an induction medium supplemented with 1 mg/l. naphthyleneacetic acid, 0-2 mg/l. kinetin and 3% sucrose, after 3 transfers to maintenance medium. The number of nodules per gramme increased 10-fold between 6 and 12 days after transfer, and thereafter remained constant. Phenylalanine ammonia lyase (PAL) activity rose to a maximum value when the rate of nodule formation was greatest, and decreased after the maximum nodule concentration was reached. The final constant value for PAL activity was above that of callus grown on maintenance medium. Beta I leads to 3 glucan synthetase activity rose to a maximum 15 days after transfer, and then fell gradually to a level above that measured in callus on maintenance medium. Callus was transferred from maintenance medium after 3, 4, 5 and 6 transfers. The concentration of nodules after 21 days on induction medium decreased as the callus was kept in culture. No further differentiation could be induced after 6 transfers. The fall in nodule formation was paralleled by a decrease in PAL and betaI leads to 3 glucan synthetase activities measured 21 days after transfer.

2,4-Dichlorophenoxyacetic Acid↗
PubMed1974

The relationship of root-cap slimes to pectins.

1. The patterns of incorporation of radioactivity from d-[U-(14)C]glucose into the pectic components of sections of sycamore roots changed so that sections nearer the tip incorporated relatively more label into arabinose and galactose compared with uronic acid. 2. Radioactive maize root-cap slime was prepared and found to contain three water-soluble component polymers which were electrophoretically (i) neutral, (ii) weakly acidic and (iii) strongly acidic at pH6.5. The neutral component was a glucan. The other components, which could be degraded by trans-elimination, consisted of an acidic backbone chain composed of galacturonic acid and glucose, attached to which were different proportions of neutral sugars. Arabinose, galactose and fucose, the main neutral sugars of the weakly and strongly acidic materials, were absent from the neutral fraction. 3. Fucose was a major sugar in maize-root slime and in a slime of similar composition synthesized by a maize callus of shoot origin. Only trace amounts were found in sycamore, pea and wheat root tips, and in pectin prepared from maize roots and coleoptiles. A high proportion of fucose is therefore a chemical characteristic of maize slime, and slime synthesis indicated a state of differentiation of the tissue. 4. The similarity between the slime and pectin is discussed; slime is a form of pectin modified in such a way as to provide a hydrated protective coating around the root tip.

Arabinose↗
PubMed1974

The amounts and rates of export of polysaccharides found within the membrane system of maize root cells.

1. Maize seedling roots were incubated in vivo with d-[U-(14)C]glucose for 2, 5, 10, 15, 30 and 45min. The total incorporation of radioactivity into polysaccharide components in isolated fractions was investigated, and the pattern of incorporation into different polysaccharide components within the rough endoplasmic reticulum, Golgi apparatus and exported material was analysed. 2. The membrane compartments reached a saturation value of radioactivity in polysaccharide components by 30min incubation. Radioactivity in exported polysaccharide continued to increase after that time. The latter was formed and maintained by a steady-state turnover of polysaccharide synthesis and transport from the membrane system. 3. If the only access of the slime polysaccharide to the cell surface is via dictyosome-derived vesicles, the amount of slime components in the Golgi apparatus would have to be displaced every 0.3min in order to maintain the observed rates of increase in slime. This is in contrast with a displacement time of about 2.5min that is necessary for polysaccharide components in the Golgi apparatus to produce the observed increase in cell-wall material. The activity of the membrane system in the production of maize root slime is 8 times as great as that of the membrane system involved in cell-wall synthesis. 4. If the amount of polysaccharide material in the Golgi apparatus is maintained only by inflow of polymeric material from the rough endoplasmic reticulum the total amount of slime components in the rough endoplasmic reticulum would have to be displaced every 7min to maintain a constant amount in the Golgi apparatus. If the endoplasmic reticulum contributed directly to the cell surface in the synthesis of cell-wall material, displacement times necessary to maintain the observed rate of polymer production would be very slow.

Arabinose↗
PubMed1973

A hydroxyproline-containing glycopeptide released from the walls of sycamore tissue-culture cells by hydrazinolysis.

Degradation of the sycamore cell wall by hydrazinolysis yielded soluble glycopeptides. The peptide portion of the predominant glycopeptide was shown to be cyclic (4-trans-hydroxy-l-prolyl)-4-trans-hydroxy- l-proline. From this structure and the composition of the major glycopeptide it was deduced that only short oligosaccharides are attached to the hydroxyprolyl residues of the wall glycoprotein. Detailed evidence for the structure of the peptide is given in a supplementary paper that has been deposited as Supplementary Publication SUP 50020, at the National Lending Library for Science and Technology, Boston Spa, Yorks. LS23 7BQ, U.K., from whom copies can be obtained on the terms indicated in Biochem. J. (1973) 131, 5.

Arabinose↗