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Biomedical subjects

J Bennett

Publications and source records attributed to J Bennett.

At least 325 records · Page 18Linked to original sources
PubMed1981

Biosynthesis of the light-harvesting chlorophyll a/b protein. Polypeptide turnover in darkness.

1. When etiolated pea seedlings were exposed to continuous light for 24 h and then returned to darkness, 38% of the chlorophyll a, 74% of the chlorophyll b and 84% of the light-harvesting chlorophyll a/b protein that had accumulated under illumination proved to be unstable in darkness. The unstable chlorophyll displayed a half-life of about 90 min. In contrast, alpha and beta subunits of the chloroplast coupling factor and the large and small subunits of ribulose 1,5-biphosphate carboxylase continued to accumulate in darkness, although at a slower rate than in plants maintained under light. 2. Short-term labelling in vivo with L-[35S]methionine showed that leaves continued to synthesize the light-harvesting protein and the small subunit of ribulose 1,5-biphosphate carboxylase for up to 48 h after transfer of plants from light and darkness. However, after long-term labelling (16 h), the light-harvesting chlorophyll a/b protein was found to be labelled to high specific activity only in illuminated leaves. 3. I conclude that the light-harvesting chlorophyll a/b protein is subject to turnover after transfer of plants from light to darkness. The site of breakdown appears to be the photosynthetic membrane. I suggest that turnover of the protein is part of the normal physiological mechanism for co-ordinating the accumulation of the pigment and protein components of the light-harvesting chlorophyll a/b complex.

Chlorophyll↗
PubMed1981

Biosynthesis of the light-harvesting chlorophyll a/b protein. Control of messenger RNA activity by light.

1. Antibodies raised against the 26000-Mr polypeptides of the light-harvesting chlorophyll a/b proteins of pea leaves specifically immunoprecipitated two 32000-Mr polypeptides synthesized when pea leaf poly(A)-containing RNA was translated in vitro. On the basis of immunochemical relatedness and by comparison of their partial tryptic digestion products, the 32000-Mr products formed in vitro are identified as precursors to the authentic polypeptides of the light-harvesting chlorophyll a/b complex. 2. The specificity of the immunoprecipitation permitted the development of an assay for the cellular levels of translationally active light-harvesting protein mRNA in plants exposed to different light regimes. Low levels of the mRNAs were detectable in dark-grown plants. Exposure to continuous illumination caused these levels to increase by at least ten-fold and led to the appearance of large quantities of the light-harvesting chlorophyll a/b complex. In plants exposed to intermittent illumination (2 min of white light every 2 h for 2 days), the light-harvesting complex did not accumulate, although levels of mRNA specifying the polypeptides of the complex were high (50% of those in continuously illuminated plants). 3. Messenger RNAs encoding the light-harvesting proteins were detected in polysomes of intermittently illuminated leaves. These polysomes were active in a wheat-germ 100 000 X g supernatant "run-off" system, to form light-harvesting protein precursors, under conditions when only nascent polypeptide chains initiated in vivo were elongated and terminated. These results demonstrate that the inability of intermittently illuminated leaves to accumulate the light-harvesting proteins is not due to a selective inhibition of the translation of the corresponding mRNAs. 4. Intermittently illuminated leaves were labelled with [35S]methionine in darkness, and incorporation of radioisotope into the light-harvesting proteins and their precursors was assayed immunologically. No pool of untransported or unprocessed 32000-Mr precursor polypeptides could be detected in the soluble fraction (cytoplasm and stroma). However, low levels of the mature 26000-Mr polypeptides were detected in the membrane fraction. It is concluded that the newly synthesized light-harvesting chlorophyll a/b protein fail to accumulate in intermittently illuminated leaves because they undergo rapid turnover. The site of light-harvesting protein breakdown is probably the thylakoid membrane, and the cause of breakdown is probably the absence of chlorophyll a and chlorophyll b molecules that are required for eventual stabilization of the proteins within the photosynthetic membrane.

Chlorophyll↗
PubMed1980

Effects of sodium butyrate and dimethylsulfoxide on biochemical properties of human colon cancer cells.

Sodium butyrate and dimethylsulfoxide (DMSO) have marked effects on the growth, morphology, and biochemistry of two human colonic adenocarcinoma cell lines in culture. Doubling times were increased between 18% and 660% while cell viability was unaffected. Both cell lines formed colonies in soft agar in the absence of butyrate of DMSO, but no colonies were observed in the presence of these agents. However, no differences in in vivo tumorigenicities, when cells were implanted in athymic mice, were seen following treatment. Gross morphological alterations including cell enlargement, process formation, and cellular flattening occurred during culture in butyrate or DMSO. Acrylamide gel electrophoresis in sodium dodecyl sulfate revealed no change in membrane protein constituents, but autoradiographic analysis of membrane glycoproteins demonstrated differences between treated and untreated cells. Ganglioside compositions were altered, and a sialyltransferase required for the synthesis of GM3 ganglioside was elevated by butyrate. Although cytoplasmic aminooligopeptidase remained unaffected by butyrate or DMSO, brush border-associated activity was enhanced by butyrate. Alkaline phosphatase also rose dramiatically during culture in butyrate but was not enhanced by DMSO.

Adenocarcinoma↗
PubMed1980

Chloroplast phosphoproteins: regulation of excitation energy transfer by phosphorylation of thylakoid membrane polypeptides.

Incubation of isolated chloroplast thylakoid membranes with [gamma-32P]ATP results in phosphorylation of surface-exposed segments of several membrane proteins. The incorporation of 32P is light dependent, is blocked by 3(3,4-dichlorophenyl)-1,1-dimethylurea (diuron, an inhibitor of electron transport), but is insensitive to uncouplers of photophosphorylation. Polypeptides of the light-harvesting chlorophyll a/b-protein complex are the major phosphorylated membrane proteins. Addition of ATP to isolated chloroplast thylakoid membranes at 20 degrees C results in a time-dependent reduction of chlorophyll fluorescence emission; this is blocked by diuron but not by nigericin. ADP could not substitute for ATP. Chlorophyll fluorescence induction transients showed a decrease in the variable component after incubation of the membranes with ATP. Chlorophyll fluorescence at 77 K of phosphorylated thylakoid membranes showed an increase in long-wavelength emission compared with dephosphorylated controls. We conclude that a membrane-bound protein kinase can phosphorylate surface-exposed segments of the light-harvesting pigment-protein complex, altering the properties of its interaction with the two photosystems such that the distribution of absorbed excitation energy increasingly favors photosystem I.

Adenosine Triphosphate↗
PubMed1980

Effect of microinjected N-ethylmaleimide-modified heavy meromyosin on cell division in amphibian eggs.

N-Ethylmaleimide-modified heavy meromyosin (NEM-HMM) microinjected into amphibian eggs inhibits cytokinesis and the cortical contractions associated with wound closure. Injection of NEM-HMM into two-cell Rana pipiens embryos produces a zone of cleavage inhibition around the point of injection. Early furrows followed by time-lapse microcinematography are seen to slow and stop as they enter the NEM-HMM-injected zone. Arrested furrows slowly regress, leaving a large region of cytoplasm uncleaved. Few nuclei are found in these regions of cleavage inhibition. Wound closure is often inhibited by NEM-HMM, especially when this inhibitor is injected just beneath the egg cortex. We observe that the surface of an unfertilized Rana egg is covered with microvilli that disappear during the course of development. The surfaces of NEM-HMM-inhibited zones remain covered with microvilli and resemble the unfertilized egg surface.

Animals↗
PubMed1980

Chloroplast phosphoproteins. Evidence for a thylakoid-bound phosphoprotein phosphatase.

1. Isolated intact pea (Pisum sativum) chloroplasts incorporate [32P]orthophosphate into several thylakoid polypeptides in the light. Transfer of the labelled chloroplasts to darkness results in rapid dephosphorylation of the polypeptides. The most rapidly dephosphorylated phosphoproteins are the 26000-Mr doublet derived from the light-harvesting chlorophyll a/b binding complex. 2. Incubation of isolated 32P-labelled thylakoids in buffer in the absence of stromal components also results in rapid protein dephosphorylation. Again, the most rapidly dephosphorylated phosphoproteins are the 26000-Mr light-harvesting doublet. Dephosphorylation of all thylakoid phosphoproteins is accelerated by addition of up to 10 mM MgCl2. 3. The enzyme responsible for dephosphorylation is a phosphatase rather than a phosphotransferase or the thylakoid protein kinase acting in reverse. The enzyme is specifically and totally inhibited by NaF and does not require phosphoryl group acceptors such as ADP. Unlike the protein kinase, the phosphatase is indifferent to light and the electron transport inhibitor 3(3,4-dichlorophenyl)-1,1-dimethylurea. 4. The phosphorylated regions of the thylakoid phosphoproteins protrude from the outer surface of the membrane and are removed by trypsin treatment.

Chloroplasts↗
PubMed1980

Suppressor cells in vitro: differential effects of indomethacin and related compounds.

In vitro immune response systems known to involve suppressor cell regulation were examined for effects produced by the drug indomethacin and other compounds known to inhibit the cyclooxygenase pathway of arachidonic acid metabolism. In all cases tested where suppressor T cell activity is known to be the dominant inhibitory mechanism, suppression was not blocked by drug addition and was sometimes more pronounced. In the cases tested where suppression could be attributed to a non-T cell, most likely a macrophage-like cell (M0), suppression could be abolished by drug treatment. Indomethacin and related compounds may be useful analytical tools for separation of T cell vs. non-T cell mediated suppression.

Animals↗
PubMed1979

Chloroplast phosphoproteins. Phosphorylation of polypeptides of the light-harvesting chlorophyll protein complex.

When isolated, intact chloroplasts of pea (Pisum sativum) are incubated in the light with [32P]-orthophosphate, isotope is incorporated into several polypeptides. Among the most conspicuous phosphoproteins are two which form a very closely spaced doublet on dodecyl sulphate/polyacrylamide gels and co-electrophorese with the major polypeptide component of the light-harvesting chlorophyll a/b binding complex. Like the light-harvesting polypeptide, the phosphoprotein doublet is bound to thylakoids, sediments with the heavy particles released from thylakoids after digitonin treatment, is soluble in chloroform/methanol and has an apparent molecular weight of about 26 000. The doublet also appears in the highly purified light-harvesting chlorophyll a/b binding complex isolated from thylakoids by hydrosylapatite chromatography. I conclude that two polypeptide components of the complex are phosphorylated. One of these components may be the major light-harvesting chlorophyll a/b binding protein.

Chlorophyll↗
PubMed1979

The cytochemistry of Limulus eggs.

Cytochemical studies on uninseminated mature eggs of Limulus demonstrate the presence of carbohydrates, lipids and proteins in the egg envelopes and yolk. The vitelline envelope, cortical region and yolk are rich in 1,2-glycols, with the vitelline envelope, containing fewer reactive 1,2-glycol groups than other components of the egg. Neutral mucopolysaccharides are found in the cortical region and yolk, but only the cortical region of the eggs demonstrate the presence of sulfated mucosubstances (which are in part glycoprotein in nature) and glucose-6-phosphatase. Protein is evident in all egg components. Biochemical analysis demonstrate the protein in the egg envelopes of uninseminated eggs is composed of sixteen amino acids while that of developing eggs contain seventeen amino acid residues. Electrovalent linkages and non-S-S- covalent linkages between protein chains are shown to be instrumental in maintaining the stuctural integrity of Limulus egg envelopes. Neutral lipids, unsaturated lipids, phospholipids and fatty acids are demonstrated in yolk bodies and lipoproteins, unsaturated lipids and fatty acids constitute part of the egg envelopes. DNA is concentrated in the cortical region and the yolk bodies

Animals↗
PubMed1978

Late endocrine effects of L-dopa, 5-HTP, and 6-OH-dopa administered to neonatal rats.

Neonatal rats were injected with L-dopa, 5-hydroxytryptophan (5-HTP) or 6-hydroxy-dopa (6-OH-dopa) and allowed to mature. Eye opening, vaginal opening, endocrine organ weights and pituitary hormone concentration in the serum were measured. Treatment with either the dopamine (DA) precursor or the serotonin (5-HT) precursor caused an acceleration in eye opening, a significant decrease in serum TSH (males) an elevation of serum prolactin (Prl; males), and a lowering of serum growth hormone (females). After chemical sympathectomy (6-OH-dopa) vaginal opening was delayed, thyroid weights were increased and serum TSH decreased (males). It is concluded that during the critical neonatal period overloading either of 2 of the principal monaminergic systems results in similar and persistent alterations in pituitary function. Chemical sympathectomy produced a different pattern of abnormalities.

5-Hydroxytryptophan↗
PubMed1978

Late endocrine effects of administering monosodium glutamate to neonatal rats.

Rats were injected with monosodium 1-glutamate (MSG) daily for the 1st 5 days of life and allowed to mature. This is known to cause selective destruction of neurons in the retina and in the arcuate nucleus of the hypothalamus. The adult animals had a significant increase in body fat without an increase in weight, a marked reduction in pituitary, thyroid, adrenal, gonadal and prostate weights. Pituitary, hypothalamic and serum thyrotropin (TSH) were significantly reduced in the males. Serum growth hormone (GH) was markedly reduced in both sexes and the serum prolactin (Prl) was increased significantly in females. FSH did not appear to be abnormal and the LH may have been increased in the males. Serum T4 was significantly reduced in females. The fertility of the females was normal, but treated males mated with normal females showed a marked reduction in fertility and, although the litter sizes of the offspring were normal, the birth weights of the pups of both sexes were significantly reduced. These persistent alterations in neuroendocrine function indicate that lesions produced by neonatal MSG treatment provide a convenient model for studying hypothalamic function.

Animals↗