Biomedical subjects
Publications and source records attributed to L Stevens.
A two component system of pure gold and an alloy of silver, copper, palladium and zinc has been shown to have similar mechanical properties to those of Type C casting golds. The system may offer some cost advantages in dental gold alloy restorations.
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A method is described for the measurement of enzyme activity under xeric conditions. The reaction mixtures had water contents ranging between 0.1 and 0.6g/g of reaction mixture. For glucose 6-phosphate dehydrogenase, hexokinase and fumarase, enzyme activity became detectable (about 0.05% of the fully hydrated rate) when the water content was about 0.2g/g of reaction mixture, and for phosphoglucose isomerase, around 0.15g/g of reaction mixture. With the water content raised to 0.3g/g of reaction mixture the reaction rates were only increased to 0.1-3% of the fully hydrated rate. When the combined rates for phosphoglucose isomerase and glucose 6-phosphate dehydrogenase were measured, reasonable agreement was found between the experimental data and those calculated from the individual experimentally determined rates on the assumption that diffusion was not further limiting. A method was devised for measuring the diffusion coefficients of low-molecular-weight substances in solutions having low water contents. The diffusion coefficients of riboflavin in sorbitol solution decreased by about 100-fold when the water content of the latter was reduced from 3 to 0.25g/g of sorbitol. It is concluded that to detect enzyme activity a certain minimal amount of water is required and that above this minimum the rate is still restricted by diffusion limitation. The relevance of the results to the physical state of water in reaction mixtures and to metabolism in seeds and spores in xeric conditions is discussed.
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The tensile strength of a dental gypsum was measured indirectly using both the diametral and the double-punch tests. Observed differences between the effects of specimens size on the diametral and double-punch strengths could be consistently interpreted in terms of the statistical theory of brittle fracture. The double-punch test, when compared with the diametral test, has several important advantages. These advantages include less experimental scatter, a weaker dependence of the measured strength on the specimen size and a simpler testing procedure.
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1,4-Diaminobutanone, a competitive inhibitor of ornithine decarboxylase in Aspergillus nidulans, is able to increase the half-life of this enzyme and thus stimulate an increase in its activity in vivo. It also protects ornithine decarboxylase against proteolysis by chymotrypsin in vitro.
Nineteen families were interviewed after their loss of an infant by sudden infant death syndrome. Their bereavement responses are described. The outcome after 13 months is studied in terms of resumption of normal living, and health changes. The parents' perceptions of intervention are discussed, and recommendations are made about how best to help such families.
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The alloys tested varied with respect to their castability. This variability is related to density. Problems caused by low density may be solved by increasing the casting force. For all parameters measured in the laboratory, Thermocraft, a precious alloy, proved to be the most satisfactory- Wiron S, a nonprecious alloy, was comparable to Degudent Universal, a gold alloy. The application of Wiron S as a successful gold substitute should, therefore, be further investigated clinically. Equipment, investment, and casting techniques, all designed for the casting of gold alloys, were also used for the nonprecious alloys. These are standard facilities readily available in clinical practice. Modifications or technical improvement to these factors may result in more successful application to the other nonprecious alloys.
Comment is made regarding the cost, related to working properties, of non-precious metal alloys compared to precious metal alloys for ceramics and their possible potential for the production of allergic response.
A comparison between conventional and live television methods of demonstrating an exercise in dental technology was carried out and indicated that they were equally effective as judged by grading of students' work. Students' comments suggested that the television method was received less favourably. Their comments form a basis for discussion regarding the use of material recorded on cassette or cartridge with immediate replay facilities rather than television demonstrations in real time.
A description of educational technology is given and the importance of establishing clear teaching-learning objectives is related to curriculum, evaluation and teaching methods.
1. The activities of ornithine decarboxylase, S-adenosylmethionine decarboxylase and ornithine-2-oxoglutarate aminotransferase were studied during the first 24 h of conidial germination in Aspergillus nidulans. 2. Increases (over 100-fold) in the activities of ornithine decarboxylase and S-adenosylmethionine decarboxylase occurred during the emergence of the germ-tube and before the doubling of DNA and this was followed by a sharp fall in the activities of both enzymes by 16h. 3. The increase in ornithine decarboxylase could be largely suppressed if 0.6 mM-putrescine was added to the growth medium. 4. Low concentrations of cycloheximide, which delayed germination by 2h, caused a corresponding delay in the changes in ornithine decarboxylase activity. 5. Ornithine-2-oxoglutarate aminotransferase activity increased steadily during the first 24h of germination. 6. Ornithine or arginine in the growth medium induced higher activity of ornithine-2-oxoglutarate aminotransferase, but did not affect ornithine decarboxylase activity. 7. The significance of these enzyme changes during germination is discussed.
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