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Z Leifer

Publications and source records attributed to Z Leifer.

6 recordsLinked to original sources
PubMed1992

Independent studies program in the basic sciences. A curricular innovation.

The Independent Studies Program was initiated at the New York College of Podiatric Medicine during the early months of 1987, and it has achieved continued success and acceptance from the administration, faculty, and students. The program emphasizes competence as the constant within a relatively time-independent framework. This paper presents, in a longitudinal fashion, the Independent Studies Program as a program model that combines criterion-referencing concepts, mastery-based learning combinations, and self-learning strategies. As a result, students have taken the initiative for diagnosing their learning needs, formulating their learning goals, identifying their human and material resources, and evaluating their learning outcomes.

Curriculum↗
PubMed1983

Putrescine and spermidine sensitivity of lysine decarboxylase in Escherichia coli: evidence for a constitutive enzyme and its mode of regulation.

Cells of Escherichia coli grown under physiological (noninducing) conditions have a low level of lysine decarboxylase activity. This activity differs from the enzyme found in induced cells in its sensitivity to putrescine (33% of control in the presence of 20 mM putrescine). It is also sensitive to spermidine (20% of control in the presence of 6 mM spermidine). A mixture of putrescine and spermidine completely eliminated lysine decarboxylase activity. This provides evidence for the existence of a biosynthetic enzyme and suggests a mechanism to explain the appearance of cadaverine in polyamine-depleted cells.

Carboxy-Lyases↗
PubMed1981

An evaluation of tests using DNA repair-deficient bacteria for predicting genotoxicity and carcinogenicity. A report of the U.S. EPA's Gene-TOX Program.

The detection of DNA-damaging agents by repair-deficient bacterial assays is based on the differential inhibition of growth of repair-proficient and repair-deficient bacterial pairs. The various methodologies used are described and recommendations are made for their improved use. In a survey of the literature through April 1979, 91 of 276 papers evaluated contained usable data, resulting in an analysis of 611 compounds that had been assayed in 1 or more of 55 pairs of repair-proficient and repair-deficient strains. The results indicate that (1) a liquid suspension assay is more sensitive than a spot (diffusion) test. In a review of the Escherichia coli polA assay, 45 compounds that gave "No Test" in the spot test were clearly positive or negative in the liquid suspension assay. (2) Of the 21 compounds analyzed by the E. coli polA assay and by other E. coli repair-deficient strains (e.g., rec, uvr, hcr, and exr derivatives of WP2 and AB1157), 10 were in complete agreement in all strains except uvrA strains. This indicates that strains other than polA+/polA- are useful for detecting DNA-damaging agents. However, in selecting strains for use in these assays, care should be taken to consider repair pathway specificity for particular compounds. (3) There was a 78% correspondence between results obtained with E. coli polA and Bacillus subtilis (H17/M45, 17A/45T) rec assay and between E. coli polA and Proteus mirabilis. (4) In a comparison of test results with carcinogenicity data, 44 of 71 (62%) carcinogenic compounds assayed by the polA system were positive, 10 (14%) were negative, and 17 (24%) gave No Test or doubtful results. 7 carcinogens were assayed by other E. coli strains and all were positive. 56 carcinogens were assayed in B. subtilis: 24 (43%) were positive, 9 (16%) were negative, and 23 (41%) gave No Test or doubtful results. Of the 7 carcinogens assayed in P. mirabilis, 6 (86%) were positive and 1 (14%) was negative. (5) The results were analyzed with respect to chemical classes. E. coli polA detected the highest percentage of hydroxylamines and alkyl epoxides. The B. subtilis rec assay detected the highest percentage of nitrosamines and sulfur and nitrogen oxides. It is concluded that some of these test systems are effective tools for the detection of DNA-damaging and potentially carcinogenic compounds, especially if the assay is done in liquid suspension and if more than 1 pair of tester strains is used. Advantages and disadvantages of the assay are discussed and suggestions are made for improvements in the system.

Animals↗
PubMed1980

The E. coli Pol A-1 assay: a quantitative procedure for diffusible and non-diffusible chemicals.

A procedure is described for determining the preferential killing of DNA repair-deficient E. coli. Dilute suspensions of DNA repair-proficient and -deficient bacteria are exposed to a graded series of test chemical concentrations. Survivors are determined by enumeration on agar plates. An expression used to express preferential killing (or lack thereof) is shown to be dose-dependent and potentially useful for determining genotoxic potencies.

DNA Repair↗
PubMed1970

Isolation and characterization of a mutant of Escherichia coli blocked in the synthesis of putrescine.

A mutant of Escherichia coli is described which is defective in the conversion of arginine to putrescine. The activity of the enzyme agmatine ureohydrolase is greatly reduced, whereas the activity of the other two enzymes of the pathway, the constitutive arginine decarboxylase and the inducible arginine decarboxylase, are within the normal range. The growth behavior of the mutant reflects the enzymatic block. It grows well in the absence of arginine, but only poorly in the presence of arginine. Under the former conditions, putrescine can be formed from ornithine as well as arginine, whereas under the latter conditions, because of feedback control, it can be formed only from arginine.

Arginine↗