Sucrose metabolism during endosperm development in wheat (Triticum aestivum)

This work reports changes in sucrose synthase and invertase activities throughout endosperm development in wheat, together with the associated substrates and metabolites, sucrose, UDP, glucose, fructose and UDP-glucose. Throughout endosperm development, sucrose synthase had consistently higher activity than invertase and indeed invertase activity did not change appreciably. The observed variation in pattern and amounts of glucose and fructose present during the mid- and late stages of endosperm development confirmed the suggestion that invertase was not the preferred pathway of sucrose catabolism. Kinetic parameters for sucrose synthase were determined in crude extracts. Estimates of UDP and sucrose concentrations suggest that sucrose synthase is unlikely to achieve its potential maximum velocity. This limitation may however be overcome in part by the apparent excess catalytic activity measured during endosperm development.     

The calcium hypothesis of cystic fibrosis

Data have been presented which suggests that various CF cell types show evidence of alterations in calcium homeostasis. The significance of these observations and the exact nature of the putative calcium defect in CF remains to be elucidated. It must also be determined whether this possible defect is primary, or is secondary or tertiary to some more basic lesion. The data reviewed suggests that altered calcium homeostasis may play some focal role in the aetiology or the pathogenesis of CF.     

Suppression of the immune response to immunotoxins with anti-CD4 monoclonal antibodies.

Treatment of normal mice with a mAb to CD4 (GK1.5) was explored as a means of inhibiting the antibody response to an immunotoxin. Three days of pretreatment with 200 micrograms of GK1.5 completely abrogated the primary antibody response to a 3-micrograms dose of a mutant diphtheria toxin conjugated to an anti-transferrin receptor antibody. The same dose and schedule of anti-CD4 antibody significantly reduced and delayed, but did not prevent, the anamnestic antitoxin response in animals that had been previously primed to the immunotoxin. Three daily injections of anti-CD4 antibodies followed by weekly doses of immunotoxin resulted in a 3-wk delay in the development of antitoxin antibodies, and the kinetics of the antitoxin response correlated with the kinetics of recovery of CD4+ T cells in the spleen and lymph nodes. The antitoxin response to repeated doses of immunotoxin was completely abrogated when anti-CD4 antibodies were given every 2 wk throughout the course of immunotoxin treatment. Thus, transient depletion of Th cells during treatment can block the immune response to an immunotoxin. There was no evidence of tolerance induction with this regimen.     

The mutagenic potency of 1,8-dinitropyrene in cultured mouse lymphoma cells

Although non-toxic, 1,8-dinitropyrene (1,8-DNP) was mutagenic for mouse lymphoma L5178Y cells when assayed for induced resistance to 6-thioguanine, methotrexate, ouabain and 1-β-D-arabinofuranosyl cytosine. In bacteria, nitropyrenes are potent inducers of frame-shift mutations, and the induction of ouabain-resistant mutants, believed to be due to base-pair substitutions, suggests that the mechanism of action may be different in mouse cells and bacteria. Long treatment times were required to detect 1,8-DNP-induced mutants in L5178Y cells, suggesting the possibility of an inducible activation system. 4-Nitroquinoline 1-oxide was both toxic and mutagenic to these same 4 mutation assays after short (2 h) treatment times. The dilemma that exists when comparing the mutagenic potential of test chemicals when concentration of mutagen, treatment times and toxicity are markedly different, is discussed.     

Chemiluminescence in Wounded Root Tissue : EVIDENCE FOR PEROXIDASE INVOLVEMENT

Root and stem segments from soybean (Merrill cv. `Bragg') showed an enhanced chemiluminescence upon mechanical injury. Roots emitted more light than did stems. Light emission was diminished by CN⁻ and N₃⁻ but was not affected by rotenone and antimycin A. Catalase quenched chemiluminescence in wounded root segments as did ascorbic acid and hydroquinone. Superoxide dismutase addition resulted in a small diminution in light emission, but mannitol, an OH· scavenger, was without effect. The addition of H₂O₂ to wounded root segments markedly elevated chemiluminescence in the presence of air as well as under N₂. It is concluded that peroxidases, found abundantly in roots, predominantly contribute to light emission in wounded plant tissue.     

Assays for the fidelity of DNA polymerases in cell-free extracts of Escherichia coli are complicated by contaminating nucleoside triphosphatases.

In the presence of DNA and a divalent cation, an enzyme activity in cell-free extracts of Escherichia coli readily hydrolyses dATP to dADP. dGTP is degraded to a smaller extent, dCTP and dTTP being hardly affected. The artificial template primers poly(dC) . oligo(dG) and poly(dT) . oligo(dA) are also effective cofactors for this triphosphatase activity. As a consequence, assays measuring the misincorporation, by cell-free extracts, of dATP and dGTP into these defined templates are difficult to interpret, since the triphosphate substrate is being rapidly degraded during the polymerase reaction. A partial characterization of the dATPase activity was performed, demonstrating that the optimal conditions for its activity resemble those commonly used for assaying polymerase activity. Thus in crude extracts both polymerase and dATPase compete for the same substrate. The inclusion of an ATP-generating system in the reaction mixture maintains the levels of deoxynucleoside triphosphates and changes the kinetics of misincorporation of dAMP into poly(dC) . oligo(dG). No reproducible difference in such misincorporation has been found between lysates prepared from tif-1 cells grown at either permissive or restrictive temperature.     

Acute regulation of pyruvate kinase activity in rat epididymal adipose tissue by insulin.

1. Evidence is presented that exposure of epididymal fat-pads from fed rats to insulin leads to a marked diminution in the Km for phosphoenolpyruvate of pyruvate kinase. Effects of insulin may be readily demonstrated in experiments both in vivo and in vitro and are not secondary to the activation by the hormone of glucose transport. No effect of insulin is apparent in tissues from 48 h-starved animals. 2. The mechanism of the effect of insulin on pyruvate kinase was not established. The observed changes in Km do not appear to be the result of alterations in the amounts of bound effectors such as fructose 1,6-bisphosphate and alanine. Rather, as the effect persists in incubated extracts, it appears that a change in the degree of phosphorylation or some other covalent modification of the enzyme may be involved.