Circular dichroism study of modified nucleosides

CD study of four modified nucleosides, constituents of tRNA molecules, revealed that 2-thio-5-methyluridine and 5-methyluridine in aqueous solution, 0.1N HCl, and organic solvents essentially occur in an anti-conformation. 5-Methylcytidine also occurs in an anti-conformation similar to cytidine in aqueous solution and organic solvents, while 2-thiocytidine dihydrate appears to occur in an anti-conformation. It is stressed that the CD data of thionucleosides might be applied to the successfully conformational analysis of tRNA molecules.     

l-tyrosine-binding proteins on melanoma cells

Summary Crosslinking of [¹⁴C]l-tyrosine to at least five hamster melanoma cell surface proteins is reported. This effect was abolished by addition of nonradioactivel-tyrosine,l-phenylalanine, orl-dopa, but not byd-tyrosine, tyramine, dopamine, norepinephrine, or epinephrine. The above proteins can be purified by tyrosine-affinity chromatography. They have molecular weights different from proteins staining for dopa oxidase and proteins that bind anti-tyrosinase antibody in Western blots. It is suggested that they may be a hithergo unrecognized part of the cellular apparatus governing melanogenesis.     

Glycerol permeability of human spermatozoa and its activation energy

Glycerol has commonly been employed as a cryoprotectant in cryopreservation of human spermatozoa. However, the addition of glycerol into the sperm before freezing and the removal of glycerol from the sperm after freezing and thawing result in anisotonic environments to the cells, which can cause cell injury. To define optimal procedures for the addition/removal of glycerol and to minimize the cell injury, one needs to know the kinetics of glycerol permeation across the sperm plasma membrane at different temperatures. For this, one has to determine the permeability coefficient of glycerol (Pg) and its activation energy (Ea). Values of Pg at different temperatures and at different glycerol concentrations were determined by measuring the time required for 50% spermolysis in hyperosmotic glycerol solutions which were hypotonic with respect to electrolytes. Value of the Ea was determined assuming an Arrhenius type temperature dependence of Pg. A dual fluorescent staining technique (propidium iodide and 6-carboxyfluoroscein diacetate) and flow cytometry were used to measure the spermolysis. The values of Pg in 0.5, 1.0, 1.5, and 2.0 M glycerol at 22 degrees C are 1.62, 1.88, 1.68, and 1.54 x 10(-3) cm/min, respectively. The values of Pg in 1 M glycerol at 0, 8, 22, and 30 degrees C are 0.33, 0.54, 1.88, and 2.60 x 10(-3) cm/min, respectively. The value of Ea is 11.76 kcal/mol.     

Disruptive medical patients. Forensically informed decision making.

Patients who disrupt medical care create problems for physicians. The risks are not entirely clinical. Although these patients may compromise sound clinical judgment, some are also litigious and express their dissatisfaction in legal or other forums. It then becomes necessary for treating physicians to be aware of the legal and ethical boundaries of their patient care responsibilities. Some disruptive patients are treated by setting limits, which is usually affirmed by health care agreements. A hospital review board may advise clinicians on these agreements and on the management of disruptive patients. If termination of the physician-patient relationship is considered, physicians must follow proper protocol. We examine these forensic considerations and place them in the context of malpractice. Communication, consultation, and documentation are the key elements in reducing liability.     

Color regulation in the archaebacterial phototaxis receptor phoborhodopsin (sensory rhodopsin II)

Phoborhodopsin, a repellent phototaxis receptor in Halobacterium halobium, exhibits vibrational fine structure, a feature that has not been identified for any other rhodopsin pigment at physiological temperatures. This conclusion follows form analysis of the absorption properties of the pigment in H. halobium membranes containing native retinal and an array of retinal analogues. The absorption spectrum of the native pigment has a maximum at 487 nm with a pronounced shoulder at 460 nm; however, the bandwidth is that expected for a single retinylidene species. Gaussian band-shape simulation with a spacing corresponding to the vibrational frequencies of polyene stretching modes reproduces the structured absorption spectra of native pigment as well as of analogue phoborhodopsin. Absorption shifts produced by a series of dihydroretinal and other retinal analogues strongly indicate that the dominant factor regulating the color of the pigment is planarization of the retinal ring with respect to the polyene chain.     

A competing salt-bridge suppresses helix formation by the isolated C-peptide carboxylate of ribonuclease A

The C-peptide of ribonuclease A (residues 1 to 13) is obtained by cyanogen bromide cleavage at Met13, which converts methionine to a mixture of homoserine lactone (giving C-peptide lactone) and homoserine carboxylate (giving C-peptide carboxylate). The helix-forming properties of C-peptide lactone have been reported. The helix is formed intramolecularly in aqueous solution, is stabilized at low temperatures (0 to 20 °C) and also by a pH-dependent interaction between sidechains. The C-peptide lactone helix is about 1000-fold more stable than expected from “host-guest” data for helix formation in synthetic polypeptides.Here we report the failure of C-peptide carboxylate to form an α-helix in comparable conditions. Formation of a salt-bridge between the α-COO^? group and the imidazolium ring of His12⁺ appears to be responsible for the suppression of helix formation. The presence of the Hse13-COO^? … His12⁺ salt-bridge in C-peptide carboxylate is shown by ¹H nuclear magnetic resonance titration of the amide proton resonances of His12 and Hse13, and is expected from model peptide studies. The most probable reason why C-peptide carboxylate does not form an α-helix is that the Hse13-COO^? … His12⁺ salt-bridge competes successfully with a helix stabilizing salt-bridge (Glu9^? … His12⁺).S-peptide (residues 1 to 20 of ribonuclease A) does form an α-helix with properties similar to those of the C-peptide (lactone) helix, which shows that the lactone ring of C-peptide lactone is not needed for helix formation.These results support the hypothesis that a Glu9^? … His12⁺ salt-bridge stabilizes the C-peptide (lactone) helix, and they show that specific interactions between side-chains can be important in preventing as well as in promoting α-helix formation.     

HineI is an isoschizomer of HinfIII restriction endonuclease

HineI is a restriction enzyme isolated from Haemophilus influenzae strain Re. Like other type III restriction endonucleases it requires ATP for cleavage and S-adenosyl-methionine for methylation of DNA. This enzyme recognises the same sequence as HinfIII (Piekarowicz et al., 1981) and cleaves and methylates DNA in a manner similar to all type III restriction enzymes.