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Biomedical subjects

B Johnson

Publications and source records attributed to B Johnson.

At least 379 records · Page 21Linked to original sources
PubMed1976

Accumulation of replicative DNA intermediates in Tetrahymena after excision-repairable damage to DNA.

Inhibition of DNA synthesis in the eukaryotic organism Tetrahymena pyriformis by thymine starvation results in the formation and accumulation of a distinct class of DNA during the first hour after re-initiation of the synthesis. This DNA, when synthesized in the presence of 5-bromodeoxyuridine (BrdUrd), bands as a distinct peak at an intermediate density of (see article) in a neutral CsCl buoyant density gradient. Following short pulses of BrdUrd, the DNA of intermediate density accounts for up to 90% of the newly synthesized DNA and binding assays on nitrocellulose filters show the presence of single-stranded regions. Alkaline buoyant density and alkaline velocity gradients clearly demonstrate the presence of non-covalently linked newly synthesized fragments with an average length shorter than that of the parental strands. Pulse experiments show that the DNA of intermediate density is progressively converted to fully hybrid (light - heavy) DNA molecules. It is therefore suggested that in response to damage caused by thymine starvation, a replicative DNA intermediate accumulates at the growing point. A similar phenomenon has also been observed after irridation with ultraviolet light. The effect of the protein synthesis inhibitor cycloheximide on the formation on the intermediate and the fully hybrid material is also described.

Animals↗
PubMed1975

The amino acid pool of Hansenula holstil: characterisation, and changes mediated by environment.

Amino acid pools extracted from Hansenula holstii grown in continuous culture with either ammonia or nitrate as sole source of nitrogen, under a variety of substrate limitations, were characterised and quantified. Pools from corresponding cultures were shown to be similar in size and composition, regardless of whether ammonia or nitrate was the nitrogen source. Large changes in pools (both quantitative and qualitative) occurred when cultures were grown under different substrate limitations. Such changes were particularly large in glutamate, glutamine, alanine, lysine and arginine; the possible significance of such environment-mediated changes is discussed.

Amino Acids↗
PubMed1975

Clinical testing of the hypothalamic-pituitary-adrenocortical system in states of hypo- and hypercortisolism.

Cortisol production is appropriately maintained by a complex control system which involves primarily the hypothalamus, the pituitary, and the adrenal cortices. Very small quantities of ACTH stimulate cortisol production, and maximum stimulation occurs with serum concentrations of only 3 mU/100 ML. Under normal circumstances, cortisol is secreted in bursts about ten times each day and circulates predominately bound to a specific binding protein which is rather completely saturated. Radioimmunoassay of plasma cortisol is now generally available and is the assay method of choice, but because of the episodic nature of its secretion, random values of plasma cortisol must be interpreted with great reservation, and even the comparison of morning and evening values in assessing circadian rhythmicity is not often helpful. Urinary free cortisol determinations provide excellent discrimination between normal function and all forms of hypercortisolism. Although the response of the adrenal cortices to ACTH may be evaluated in a number of different ways, the simplest but most definitive procedure involves continuous intravenous administration over a 48-hr period. Of the various tests which indirectly assess the potential for ACTH secretion, the use of metyrapone is most helpful. In the test of greatest utility, plasma cortisol and 11-desoxycortisol are determined the morning after a single midnight oral dose of 30 mg/kg. The detection of all forms of pathologic hypercortisolism is still best accomplished by the oral administration of dexamethasone. Plasma cortisol can be determined the morning after a single midnight dose of 1 mg, or urinary 17-hydroxycorticosteroids can be determined after 2 days in which 0.5 mg is given at 6-hr intervals. Patients with hypercortisolism of hypothalamic-pituitary origin usually evidence appropriate suppression of urinary steroids if the dose is increased to 2.0 mg every 6 hr for another 48 hr. In patients who do not suppress on this or even higher doses of dexamethasone, the distinction between those with adrenal tumor and those with the ectopic ACTH syndrome can be accomplished most definitively by the assay of plasma ACTH where this determination is available.

Adrenal Cortex↗