Biomedical subjects
Publications and source records attributed to P Narayanan.
Laryngeal foreign bodies, especially in children, mostly present as an acute emergency. Few cases of long-standing laryngeal foreign bodies have been reported in the literature. This case illustrates one of the sequelae of a neglected laryngeal foreign body, resulting in significant granulation tissue formation in the larynx, and its management.
A recognized cause of incomplete or cancelled MRI examinations is anxiety and claustrophobic symptoms in patients undergoing MR scanning. This appears to be a problem in many MRI centres in Western Europe and North America, where it is said to be costly in terms of loss of valuable scan time, and has led to researchers suggesting several anxiety-reducing approaches for MRI. To determine the incidence of failed MRI examination among our patients and if there are any associations with a patient's sex, age and education level, we studied claustrophobia that led to premature termination of the MRI examination in the University Malaya Medical Centre (UMMC) in 3324 patients over 28 months. The incidence of failed MRI examinations due to claustrophobia in the UMMC was found to be only 0.54%. There are associations between claustrophobia in MRI with the patients' sex, age and level of education. The majority of those affected were male patients and young patients in the 25-45-years age group. The patients' education level appears to be the strongest association with failed MRI examinations due to claustrophobia, where the majority of the affected were highly educated individuals. Claustrophobia in MRI is more of a problem among the educated individuals or patients from a higher socio-economic group, which may explain the higher incidence in Western European and North American patients.
Explore the source record for details and available documents.
PURPOSE: Epidemiologic studies have shown that the onset of cortical cataract occurs primarily in the inferonasal human lens and that the incidence of cortical cataract is correlated with ultraviolet light. Ray tracing analysis has suggested that the peripheral cornea concentrates light on the opposite peripheral lens and that the nose and orbit block peripheral light, except temporally, resulting in a relative concentration of light on the inferonasal lens. Studies were performed to test these theories. METHODS: A model cornea and anterior chamber, set on a disc of light-sensitive paper, was placed in the orbit of a human skull coated with wax to simulate soft tissue. The "eye" was exposed to summer sunlight at various times of day, with and without sunglasses. Discs from the different experimental groups were scanned, and the digitized images were analyzed densitometrically using image analysis software. RESULTS: With the head upright, the inferonasal section of the disc exhibited the most intense exposure under all lighting conditions. Sunglasses decreased the intensity of overall light exposure but did not eliminate the inferonasal bias. Only blocking the temple eliminated this effect. CONCLUSIONS: This model supports the idea that the peripheral cornea focuses light on the inferonasal portion of the human lens. These results may explain the correlation between light and the location of cortical cataract.
A toxic protein component (Bt-II) was isolated from the venom of the Indian scorpion, Buthus tamulus, by ion-exchange chromatography on Biorex-70 and reverse phase high pressure liquid chromatography (RP-HPLC). This 7800 mol. wt protein was found to have an LD50 value of 2.25 mg/kg body weight in mice by subcutaneous injection. Amino acid composition of the protein revealed the presence of aromatic and basic amino acids; the N-terminal amino acid sequence was also determined. Structural characterization of Bt-II, based on circular dichroic and fluorescent quenching studies, has also been reported.
In the three-dimensional architecture of macromolecules, the structural stability and proper folding manifest due to cooperative packing interaction of various segments. Hydrophobicity is the major factor stabilizing protein-protein associations. In the disulfide-containing proteins, S-S bonds are integral part of structural motifs and large part of the protein-folding problem can be reduced to identifying and understanding motifs and subdomains of these proteins. Identifying such a motif with S-S bonds in 'scorpion-toxin' type proteins, and from model-building studies, five tertiary structural models for these type of proteins can be proposed. These canonical structural models can be refined by regular minimum energy and computer simulation methods to arrive at the final tertiary structures. Such 'models' can be of considerable use i) in understanding the biochemical reaction mechanisms in the structure-function relationships, ii) structure determination by X-ray methods (molecular replacement method), iii) drug design etc.
The soluble portion (F1 ATPase) of the mitochondrial ATP-synthesizing system is a multisubunit enzyme of molecular weight 380,000. It is composed of five different subunits, alpha, beta, gamma, and epsilon. The subunit stoichiometry is not known but there are strong suggestions that it is alpha 3 beta 3 gamma delta epsilon. We have determined the three-dimensional structure of the F1 ATPase of rat liver mitochondria to 9-A resolution by using x-ray diffraction techniques. The molecule appears to be formed by two equivalent halves, each formed by three regions of approximately equal size. These regions form a distorted hexagonal or octahedral arrangement. None of the regions form closed symmetrical trimers in the complex. It is proposed that, if the subunit stoichiometry is alpha 3 beta 3 gamma delta epsilon, the major subunits exist in at least two different environments in the complex. In this arrangement, the different copies of the major subunits are functionally not equivalent. This observation appears to offer a natural explanation of the complicated binding and labeling data of F1 ATPases.
Explore the source record for details and available documents.
To ascertain the sites that are available for strong binding between magnesium ions and phosphate groups in yeast phenylalanine transfer RNA, all distances below 5.5 A separating the phosphoryl oxygens (Op) of the 76 nucleotide residues have been computed from the latest atomic coordinates for the monoclinic form of the tRNA crystallized in the presence of magnesium chloride. The 5.5 A distance is chosen as the upper limit expected for Op....Op distances involved in strong magnesium-phosphate binding, on the basis of studies on a model magnesium phosphodiester hydrate, taking into account the quoted standard deviation in the tRNA atomic coordinates. It is concluded that there are four possible sites for strong magnesium binding in the tRNA molecule, in addition to the three sites previously reported. One of the hypothetical sites: m2G10-OL, U47-OR, could be involved in the first stage of melting of the tRNA molecule, and may be relevant to tertiary structure stabilization, since it links the dihydrouridine arm with the extra (V) loop.
The crystal structures of two nucleosides, 5-carbamoylmethyluridine (1) and 5-carboxymethyluridine (2), were determined from three-dimensional x-ray diffraction data, and refined to R = 0.036 and R = 0.047, respectively. Compound 1 is in the C3'-endo conformation with chi +5.2 degrees (anti), psiinfinity = +63.4 degrees and psialpha = +180.0 degrees (tt); 2 is in the C2'endo conformation with chi +49.4 degrees (anti), psiinfinity -60.5 degrees and psialpha +60.0 degrees (gg). For each derivative, the plane of the side chain substituent is skewed with respect to the plane of the nucleobase; for 1, the carboxamide group is on the same side of the uracil plane vis a vis the ribose ring; for 2, the carboxyl group is on the opposite side of this plane. No base pairing is observed for either structure. Incorporation of structure 1 into a 3'-stacked tRNA anticodon appears to place 08 within hydrogen bonding distance of the 02' hydroxyl of ribose 33, which may limit the ability of such a molecule of tRNA to "wobble".
Explore the source record for details and available documents.
The crystal and molecular structure of 6-amino-10-(beta-D-ribofuranosylamino)-pyrimido[5,4-d]pyrimidine has been determined by single crystal X-ray diffraction methods. The crystals are triclinic, of noncentric space group Pl, with cell dimensions a equals 5.434 (5), b equals 12.269 (19), c equals 4.574 (4) A, alpha equals 92.3 (1), beta equals 94.0 (1), gamma equals 95.3 degrees (1) and Z equals 1. The structure has been refined to an R value of 0.049 (Rw equals 0.063), by use of counter measured intensity data for 1063 observed reflections. The pyrimidopyrimidine ring is planar. The sugar moiety is in the envelope conformation with O-1'-endo (0E), and there is an intramolecular hydrogen bond (2.58 A) (O-3'-H-O3'...O-2'). All oxygen atoms except O-1' ring oxygen-atom are involved in hydrogen bonding. The pyrimidopyrimidine rings lie in planes 3.4 A apart.
Explore the source record for details and available documents.
Explore the source record for details and available documents.