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Biomedical subjects

E J Johnson

Publications and source records attributed to E J Johnson.

At least 55 records · Page 3Linked to original sources
PubMed1977

Threonyl-transfer ribonucleic acid synthetase and the regulation of the threonine operon in Escherichia coli.

Two threonine-requiring mutants with derepressed expression of the threonine operon were isolated from an Escherichia coli K-12 strain containing two copies of the thr operon. One of them carries a leaky mutation in ilvA (the structural gene for threonine deaminase), which creates an isoleucine limitation and therefore derepression of the thr operon. In the second mutant, the enzymes of the thr operon were not repressed by threonine plus isoleucine; the threonyl-transfer ribonucleic acid(tRNA) synthetase from this mutant shows an apparent Km for threonine 200-fold higher than that of the parental strain. The gene, called thrS, coding for threonyl-tRNA synthetase was located around 30 min on the E. coli map. The regulatory properties of this mutant imply the involvement of charged threonyl-tRNA or threonyl-tRNA synthetase in the regulation of the thr operon.

Amino Acyl-tRNA Synthetases↗
PubMed1977

Benin City mothers: their beliefs concerning infant feeding and child care.

One hundred and forty-three mothers residing in a representative area of Benin City were interviewed concerning their beliefs about child care, nutrition, and sources of information related to child care. Although all the mothers believed that breast-feeding was a natural procedure which should start from birth, they were not aware of the importance of breast milk to the baby. 58.7 per cent of the mothers would introduce artificial milk within the first month of life, and only 19.6 per cent would give breast milk alone for the first three months. Kwashiorkor was not widely believed to be related to protein-calorie deficiency, although some other food-related causes were given. The food taboos mentioned were not thought by the investigators to have any significant adverse effect on the sample children's health. Most of the information on health was obtained from friends and relatives and from infant welfare clinics. Recommendations are made for improvement of mothers' knowledge in total child care and use of health facilities.

Animals↗
PubMed1976

A heterophile system in human renal transplantation. V. Relationship of heterophile transplantation antigen and common antigen of Enterobacteriaceae.

Heterophile transplantation antigen and common antigen of Enterobacteriaceae appear serologically to be separate specificities. However, both antigens are common to Enterobacteriaceae, rat erythrocytes, and some human kidneys. Both antigens are obtained from various tissues by the same chemical procedure. Immunity to each antigen is frequently produced by renal transplantation. We suggest that the antigens are either separate molecules which are similar in chemical structure in the region of the antigenic determinant as well as in tissue distribution or separate reactive sites located on the same molecule. The possibility that common antigen may be a human alloantigen raises theoretical possibilities relative to susceptibility to infection and pyelonephritis, as well as to its relationship to histocompatibility.

Animals↗
PubMed1976

Anatomical locus of the common enterobacterial antigen.

Cell walls, membrane vesicles and the soluble cytoplasmic components have been isolated from Escherichia coli 0:14 and Salmonella typhimurium and examined by electron microscopy as well as hemagglutination inhibition with heterologous antisera for enterobacterial common antigen (ECA). The outer and cytoplasmic membranes also were isolated and examined for common antigen. Although common antigen was present in the isolated surface structures it was not localized exclusively in these fractions. Additional experiments revealed that, whatever the nature of the integration of ECA of Kunin into the cell envelope, it is a much more stable association in E. coli 0:14 than in S. typhimurium.

Antigens, Bacterial↗
PubMed1972

Properties of phosphoribulokinase from Thiobacillus neapolitanus.

Partially purified preparations of ribulose-5-phosphate kinase (specific activity, 50 to 125 mumoles per min per mg of protein) were employed in a series of kinetic experiments in the presence of several concentrations of H(+), Mg(2+), adenosine triphosphate (ATP), and phosphoenolpyruvate (PEP). The pH optimum of the enzyme was found to be 7.9; at this pH and above, response of the enzyme to variations in ATP concentration was hyperbolic, exhibiting a K(m) of 7 x 10(-4)m ATP. At pH values below the optimum the response to ATP was sigmoidal, as it was throughout the entire pH range in the presence of PEP at a concentration greater than 5 x 10(-4)m. In the presence of PEP the pH optimum shifted to pH 8.4. In contrast, phosphoribulokinase from spinach exhibited hyperbolic responses throughout its pH range with no inhibition caused by PEP. Thiobacillus neapolitanus phosphoribulokinase was inhibited by PEP in a sigmoidal manner; however, in the presence of suboptimal concentrations of Mg(2+) the addition of PEP caused significant stimulation of activity. It is postulated that the enzyme consists of interacting subunits with several sites on the enzyme for binding ATP and with several separate sites binding PEP. It is suggested that PEP functions as a regulator of CO(2) fixation when the organism is under conditions of unlimited concentrations of substrate and CO(2).

Adenosine Triphosphate↗
PubMed1969

Enzymes of intermediary carbohydrate metabolism in the obligate autotrophs Thiobacillus thioparus and Thiobacillus neapolitanus.

Levels of enzymes operative in the Embden-Meyerhof-Parnas (glycolytic) pathway, pentose phosphate cycle, citric acid cycle, and certain other phases of intermediary carbohydrate metabolism have been compared in Thiobacillus thioparus and T. neapolitanus. All enzymes of the glycolytic pathway except phosphofructokinase were demonstrated in both organisms. There were some striking quantitative differences between the two organisms with respect to the activities of the individual enzymes of the glycolytic pathway and the citric acid cycle. Qualitative differences were also found: the isocitrate dehydrogenase activity of T. thioparus is strictly nicotinamide adenine dinucleotide phosphate (NADP)-dependent, whereas that of T. neapolitanus is primarily nicotinamide adenine dinucleotide-dependent, activity with NADP being low; the glucose-6-phosphate dehydrogenase of T. thioparus is particulate, whereas that of T. neapolitanus is partly soluble and partly particulate; the 6-phosphogluconate dehydrogenase of T. thioparus is soluble, that of T. neapolitanus is partly soluble and partly particulate. All enzymes which function in the carbon reduction cycle were present at very high levels. In contrast, enzymes which operate exclusively in cycles other than the carbon reduction cycle were present at low levels. Of the enzymes not operative in the carbon reduction cycle that were examined, isocitric dehydrogenase had the highest specific activity. Both organisms possessed reduced nicotinamide adenine dinucleotide dehydrogenase activity. The qualitative and quantitative aspects of the data are discussed in relation to possible biochemical explanations of obligate autotrophy.

Carbohydrate Metabolism↗
PubMed1969

Assimilation and metabolism of exogenous organic compounds by the strict autotrophs Thiobacillus thioparus and Thiobacillus neapolitanus.

The assimilation and utilization of the individual carbon atoms of pyruvate and acetate by cells of Thiobacillus thioparus and T. neapolitanus, in the presence and absence of an energy source, were studied by use of radioactive substrates. Both organisms produced (14)CO(2) from (14)C-labeled pyruvate, but more came from carbon 1 than from carbons 2 or 3. The conversion of the carbons of acetate to CO(2) by both organisms was much less than that from any of the pyruvate carbons. When labeled pyruvate and acetate were incubated with these organisms, small amounts of radioactivity were found in the tricholoacetic acid-soluble material, nucleic acids, and lipids, and larger amounts were found in the protein fraction. The composition of the incubation medium affected the amount of utilization and incorporation of labeled substrates by both organisms. The presence of an exogenous energy source (Na(2)S(2)O(3)) suppressed incorporation of the labeled substrates into various cellular components by T. thioparus, but enhanced incorporation by T. neapolitanus. When (14)C-pyruvate was used as a substrate, as many as 12 radioactive compounds were found in the water-soluble fraction in the experiments with T. neapolitanus, whereas no more than three radioactive compounds were detected in this fraction in the experiments with T. thioparus. Of the total (14)C activity found in the water-soluble fractions, malic acid contained the highest percentage. These findings are discussed in light of the overall metabolism of these two sulfur-oxidizing obligate chemoautotrophs, as well as in relation to the biochemical basis of chemoautotrophy.

Acetates↗
PubMed1968

Effects of salts on the halophilic alga Dunaliella viridis.

Determinations of the salt sensitivity of enzymes extracted from the halophilic alga Dunaliella viridis revealed that pentose phosphate isomerase, ribulose diphosphate carboxylase, glucose-6-phosphate dehydrogenase, and phosphohexose isomerase were inhibited by NaCl concentrations far lower than that in the growth medium (3.75 m). The inhibition was reversible and was not prevented by preparing the extracts in the presence of salt. Potassium, lithium, and cesium chlorides were equally inhibitory. In contrast, whole cells require rather high levels of NaCl for optimal growth, whereas growth is inhibited by low levels of the other cations. The results suggest a specific mechanism for the exclusion of sodium from the interior of the cell.

Carbon Dioxide↗