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Biomedical subjects

S Clarke

Publications and source records attributed to S Clarke.

At least 361 records · Page 20Linked to original sources
PubMed1984

Bilateral transitory projection to visual areas from auditory cortex in kittens.

A transitory projection from primary and secondary auditory areas to the contralateral and ipsilateral areas 17 and 18 exists in newborn kittens. Distinct neuronal populations project to ipsilateral areas 17-18, contralateral areas 17-18 and contralateral auditory cortex; they are at different depth in layers II, III, and IV. By postnatal day 38 the auditory to visual projections have been lost, apparently by elimination of axons rather than by neuronal death. While it was previously reported that the elimination of transitory axons is responsible for focusing the origin of callosal connections to restricted portions of sensory areas it now appears that similar events play a more general role in the organization of cortico-cortical networks. Indeed, the elimination of juvenile projections is largely responsible for determining which areas will be connected in the adult.

Animals↗
PubMed1984

Mammalian brain and erythrocyte carboxyl methyltransferases are similar enzymes that recognize both D-aspartyl and L-isoaspartyl residues in structurally altered protein substrates.

Two purified isozymes of protein carboxyl methyltransferase from bovine brain catalyze the substoichiometric transfer of methyl groups in vitro from S-adenosyl-L-[methyl-3H]methionine to several erythrocyte membrane proteins, which include bands 2.1, 3, and 4.1, as well as several integral membrane polypeptides. D-Aspartic acid beta-[3H]methyl ester has been isolated from proteolytic digests of these methylated proteins, suggesting that protein D-aspartyl residues can serve as methyl-acceptor sites for the two brain enzymes. This formation of D-aspartic acid beta-[3H]methyl ester is competitively inhibited by the peptide L-Val-L-Tyr-L-Pro-L-isoAsp-Gly-L-Ala, which contains an L-aspartyl residue in an unusual beta-peptide linkage. Since this peptide is a stoichiometric substrate for the brain methyltransferases, it appears that one enzymatic activity can catalyze methyl ester formation at both D-aspartyl and L-isoaspartyl sites. In these respects, the activity of both brain isozymes closely resembles those previously described for the erythrocyte enzyme. The results are discussed in terms of a model in which derivatized aspartyl residues in proteins, arising by either racemization or isomerization, are recognized by the methyltransferase; the enzyme may function in either the metabolism or correction of the altered structures. The presence of a similar enzyme in both translationally active (brain) and inactive (erythrocyte) tissues suggests that the reactions are of general importance to cellular integrity.

Amino Acid Sequence↗
PubMed1984

Provision of leucocyte poor blood at the bedside.

The Imugard IG 500 cotton wool filter and the Cellselect cellulose acetate filter were adapted for filtration of leucocytes from packed cell transfusions at the bedside. Sixty five transfusions were given via the Imugard IG 500 filter and 54 transfusions were given via the Cellselect filter. Packed red cell concentrates from the National Blood Transfusion Service provided for routine blood transfusions were used in all cases. No patient in either group of multitransfused patients experienced a febrile blood transfusion reaction during the study. The Imugard IG 500 removed 91% +/- 9 (SEM) leucocytes; the Cellselect removed 96% +/- 7 (SEM) leucocytes. In the Imugard IG 500 group one patient received greater than 0.5 X 10(9) leucocytes. In the Imugard IG 500 group one patient received greater than 0.5 X 10(9) leucocytes, but no patient in the Cellselect group received greater than 0.5 X 10(9) leucocytes in any single transfusion. This is a safe method of providing leucocyte poor blood at the bedside.

Acute Disease↗
PubMed1984

The elderly at home: health and social status.

The health, social status and contacts with services of all people over the age of seventy-five living at home in and around Melton Mowbray was assessed by interview. Nearly half the sample (46%) lived alone, and just over a third (37%) were married. Most respondents were able to complete the basic activities of daily living without any assistance. The apparent prevalence of dementia at 1.6% was lower than had been found in previous population studies. The levels of social contact were similar to those reported previously. While service provision compared well with that found in other studies, only a relatively small proportion of the population were in receipt of nursing, paramedical and social services.

Activities of Daily Living↗
PubMed1983

Multiple sets of visual cortical neurons projecting transitorily through the corpus callosum.

In areas 17 and 18 of adult cats only a few neurons send bifurcating axons to more than one contralateral area or to areas in both hemispheres; most neurons seem to project to only one area. We now found that such a selective connectivity is already present at birth, i.e. several weeks before transitory callosal axons are eliminated. This is true even for those portions of cortex which will lose access to the corpus callosum: in particular different neurons project transitorily from medial area 17 to different contralateral areas. Thus cortical neurons may be induced to project to specific targets by a mechanism operating long before (and possibly independent of) the axon elimination. The latter may, however, be responsible for the final topographic organization of the connections.

Animals↗
PubMed1983

S-adenosylmethionine synthetase in cultured normal and oncogenically-transformed human and rat cells.

We have investigated the enzymatic formation of S-adenosylmethionine in extracts of a variety of normal and oncogenically-transformed human and rat cell lines which differ in their ability to grow in medium in which methionine is replaced by its immediate precursor homocysteine. We have localized the bulk of the S-adenosylmethionine synthetase activity to the post-mitochondrial supernatant. We show that in all cell lines a single kinetic species exists in a dialyzed extract with a Km for methionine of about 3-12 microM. In selected lines we have demonstrated a requirement for Mg2+ in addition to that needed to form the Mg X ATP complex for enzyme activity and have shown that the enzyme can be regulated by product feedback inhibition. Because we detect no differences in the enzymatic ability of these cell extracts to utilize methionine for S-adenosylmethionine formation in vitro, we suggest that the failure of oncogenically-transformed cell lines to grow in homocysteine medium may result from the decreased methionine pools in these cells or from the loss of ability of these cells to properly metabolize homocysteine, adenosine, or their cellular product S-adenosylhomocysteine.

Animals↗
PubMed1983

Methylation of erythrocyte membrane proteins at extracellular and intracellular D-aspartyl sites in vitro. Saturation of intracellular sites in vivo.

A cytosolic protein carboxyl methyltransferase (S-adenosyl-L-methionine:protein O-methyltransferase, E.C. 2.1.1.24) purified from human erythrocytes catalyzes the methylation of erythrocyte membrane proteins in vitro using S-adenosyl-L-[methyl-3H]methionine as the methyl group donor. The principal methyl-accepting proteins have been identified by sodium dodecyl sulfate-gel electrophoresis at pH 2.4 and fluorography as the anion transport protein (band 3), ankyrin (band 2.1), and integral membrane proteins with molecular weights of 45,000, 28,000, and 21,000. Many of the methylation sites associated with intrinsic membrane proteins may reside in their extracellular portions, since these same proteins are methylated when intact cells are used as the substrate. The maximal number of methyl groups transferred in these experiments is approximately 30 pmol/mg of membrane protein, a value which represents less than one methyl group/50 polypeptide chains of any methyl-accepting species. The number of methylation sites associated with the membranes is increased, but not to stoichiometric levels, by prior demethylation of the membranes. The additional sites are associated primarily with bands 2.1 and 4.1, the principal methyl acceptors in vivo, suggesting that most methylation sites are fully modified in vivo. Extracellular methylation sites are not increased by demethylation of membranes. The aspartic acid beta-methyl ester which can be isolated from carboxypeptidase Y digests of [3H]methylated membranes is in the unusual D-stereoconfiguration. Similar results have been obtained with [3H]methylated membranes isolated from intact cells (McFadden, P.N., and Clarke, S. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2460-2464). It is proposed that the methyltransferase recognizes D-aspartyl residues in proteins and is involved with the metabolism of damaged proteins in vivo.

Aspartic Acid↗
PubMed1983

Protein carboxyl methyltransferase from cow eye lens.

Protein carboxyl methyltransferase activity (S-adenosyl-L-methionine: protein carboxyl-0-methyltransferase; E.C. 2.1.1.24) has been detected in crude soluble extracts of cow eye lens. The activity incorporates methyl groups from S-adenosyl-L-methionine into endogenous lens proteins in vitro, and several of these species co-migrate electrophoretically with lens crystallins. A 2600-fold purification of the enzyme free of endogenous substrates was achieved by gel filtration and affinity chromatography. The lens methyltransferase has a native molecular weight of approximately 27,000, and catalyzes the substoichiometric incorporation of highly alkali-labile methyl ester groups into a broad range of protein substrates. The lens enzyme appears to be similar to that found in human erythrocytes, which specifically recognizes and modifies D-aspartic acid residues in aged proteins in a postulated degradative or racemization-repair pathway (McFadden, P.N., and Clarke, S. (1982) Proc. Natl. Acad. Sci. U.S.A. 79, 2460-2464).

Animals↗
PubMed1983

S-adenosyl-L-methionine synthetase from human erythrocytes: role in the regulation of cellular S-adenosylmethionine levels.

The properties of human erythrocyte S-adenosyl-L-methionine synthetase (ATP:L-methionine S-adenosyltransferase, EC 2.5.1.6) were studied with respect to the role of S-adenosylmethionine in transmethylation reactions. Kinetic values obtained with both a cytosolic and a 350-fold purified preparation of enzyme were compared with measured intracellular concentrations of substrates and products. This analysis revealed that effective regulation of enzyme activity and product concentration can occur through feedback inhibition by S-adenosylmethionine (Ki = 2.0-2.9 microM; the endogenous concentration is 3.5 microM). This enzyme can be distinguished from S-adenosylmethionine synthetases found in other tissues and appears to be specialized for its role in erythrocyte methyl group metabolism, especially with regard to protein carboxyl methyl-transfer reactions.

Cytosol↗
PubMed1983

Membrane protein carboxyl methylation increases with human erythrocyte age. Evidence for an increase in the number of methylatable sites.

The level of carboxyl methylation of membrane proteins has been measured in intact human erythrocyte populations of different ages separated by density gradient centrifugation. Age separation was confirmed by measurement of cytosolic pyruvate kinase specific activity in each fraction. When cells of different ages were incubated with L-[methyl-3H]methionine, the steady state level of 3H radioactivity covalently bound to membrane proteins is observed to be at least 3-fold higher in older erythrocytes. Because the specific radioactivity of the methyl group donor S-adenosyl-L-[methyl-3H]methionine was identical in all age fractions, this represents an increase in the extent of modification of membrane proteins by carboxyl methylation. Of the three major methylated erythrocyte membrane proteins, this increase in carboxyl methylation with age is 4 to 7-fold for bands 2.1 and 3, while the increase in band 4.1 is 3 to 4-fold. This increase in the steady state level of methylation with age cannot be explained by changes in either the intrinsic rate of methyl transfer or by changes in the rate constant of methyl turnover. We, therefore, propose that the age-dependent change in carboxyl methylation is due to an increase in the number of available acceptor sites as the erythrocyte ages in vivo. Since methylation of acidic residues on erythrocyte membrane proteins has been detected exclusively on D-aspartic acid residues (McFadden, P. N., and Clarke, S. (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 2460-2464), these results are consistent with an accumulation of D-aspartic acid in membrane protein due to spontaneous racemization a the cell ages. The relationship of these observations to possible functions of erythrocyte membrane protein carboxyl methylation is discussed.

Anion Exchange Protein 1, Erythrocyte↗
PubMed1983

Transitory macrophages in the white matter of the developing visual cortex. I. Light and electron microscopic characteristics and distribution.

Injections of horseradish peroxidase (HRP) into the occipital cortex of the kitten and diffusing to the white matter label a widely distributed microglial population and in addition, cells with light and electron microscopic features of 'gitter cells'. The latter are concentrated in a complex and highly consistent system of interconnected clusters in the white matter of the lateral, postlateral, middle suprasylvian and posterior ectosylvian gyri, as well as on the roof of the lateral ventricle. The 'gitter cells' have the ultrastructural and (as described by others) chemical characteristics of macrophages, and may be involved in the elimination of transitory axons.

Aging↗
PubMed1983

Transitory macrophages in the white matter of the developing visual cortex. II. Development and relations with axonal pathways.

Clusters of 'gitter cells' develop in the white matter of the occipital cortex of the cat at the end of the first postnatal week. These clusters, and others already present at birth, disappear by the end of the first postnatal month. The life span of the clusters in the occipital white matter corresponds to the period when transitory callosal axons are eliminated. The clusters have close contact with callosal axons and can be labeled by HRP injected in the contralateral hemisphere and transported through the corpus callosum. One of the clusters clearly forms in a part of the white matter crossed by transitory callosal axons. The 'gitter cells' might be involved in the elimination of these axons. Consistent with this hypothesis, ultrastructural observations show groups of axons completely surrounded by 'gitter cell' cytoplasm as if they were being phagocytosed.

Aging↗
PubMed1983

The effects of oral digoxin therapy in primary mitral leaflet prolapse.

The effects of oral digoxin on symptoms, arrhythmias, exercise tolerance and echocardiographic function in primary mitral leaflet prolapse were studied in 23 patients using a double-blind crossover protocol. Digoxin reduced the incidence and severity of chest pain compared with both the control (P = 0.0002) and placebo (P = 0.0005) periods. We found a high (83%) incidence of predominantly minor arrhythmias on continuous ambulatory monitoring. Digoxin favourably affected the incidence of frequent supraventricular ectopic beats and supraventricular tachycardia but was associated with a significant number (P less than 0.0025) of asymptomatic bradyarrhythmias. In patients with frequent ventricular ectopics, digoxin had no consistent effect. No difference in exercise tolerance between treatment periods was found on maximal treadmill stress testing, but digoxin administration resulted in an increase in echocardiographic mean circumferential fibre shortening velocity (P less than 0.01) and fractional shortening percent (P less than 0.01). This study demonstrates the efficacy of oral digoxin therapy in ameliorating chest pain in patients with primary mitral leaflet prolapse and suggests a favourable effect on supraventricular arrhythmias in such patients.

Administration, Oral↗