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Biomedical subjects

K Ramakrishnan

Publications and source records attributed to K Ramakrishnan.

36 records · Page 2Linked to original sources
PubMed1983

Stereoselectivity of chloroperoxidase-dependent halogenation.

The stereoselectivity of chloroperoxidase halogenation of four substrates has been examined. Chloroperoxidase catalyzes the bromination, but not chlorination, of racemic 2-exo-methylbicyclo[2.2.1]hept-5-ene-2-endo-carboxylic acid (to the delta-lactone) and racemic bicyclo-[3.2.0]hept-2-en-6-one (to the 2-exo-bromo-3-endo-hydroxy-bromohydrin). These products are obtained in near quantitative yield and are racemic. The circumstances of the bromination strongly suggest that halogenation does not occur at the active site but rather by chloroperoxidase-catalyzed formation of Br2 and its release into solution. The inability of chloroperoxidase to halogenate these two alkenes at its active site most probably derives from a steric exclusion from the active site. The stereoselectivity of two additional substrates that undergo active site chlorination was determined. Methionine is quantitatively converted to a 50:50 ratio of the two methionine sulfoxide diastereomers. 2-Methyl-4-propylcyclopentane-1,3-dione is quantitatively chlorinated to 2-chloro-2-methyl-4-propylcyclopentane-1,3-dione. On the basis of optical rotation and proton nuclear magnetic resonance, this product is present as a 40:60 ratio of the racemic diastereomers. It is concluded that active site chlorination by chloroperoxidase proceeds without appreciable stereoselectivity.

Bromine↗
PubMed1983

Orchiopexy: trans-septal ipsilateral positioning.

Orchiopexy based on a method of testicular fixation by ipsilateral placement with a valvular passage through the septum is described. By passing the testis through the soft septum twice, a low scrotal position is assured without tension or risk of compression of the cord.

Adolescent↗
PubMed1982

Complexation of anthracycline antibiotics by the apo egg white riboflavin binding protein.

The apo chicken egg white riboflavin binding protein complexes several anthracycline antitumor antibiotics and their metabolites. The Kd value for three important anthracycline glycosides (adriamycin, daunomycin, aclacinomycin A) is approximately identical at 0.5 micro M. The anthracycline occupies the flavin binding site in this complex, having its D-B rings overlaying the region normally occupied by the riboflavin A-C rings, respectively. The glycoside of the anthracycline, attached to C-7 of the A ring, is exposed to the solvent; consequently, the binding protein discriminates poorly between anthracycline A ring geometric isomers. Anthracyclinones, metabolites lacking the C-7 glycoside, are bound about 10-fold more tightly. The basis for the occupancy by anthracyclines of this flavin binding site is a steric homology (both ligands contain planar, linear conjugated rings) permitting the energetically favorable displacement of water from the hydrophobic pocket. The binding protein-anthracycline complex has been used for anaerobic aqueous redox titrations. Dithionite reduction of the anthracycline glycoside provides, in a two-electron process, the 7-deoxyanthracyclinone, via reductive elimination. The bound 7-deoxyanthracyclinone is then further two-electron reduced to the bound hydroquinone. Semiquinone radical intermediates are observed transiently; these are neither stable in solution nor complexed to the binding protein. Oxidation of the hydroquinone is accomplished by several reagents (oxygen, hydrogen peroxide, ferricyanide, cytochrome c). In the case of 7-deoxydaunomycinone hydroquinone, a mixture of two products is produced upon oxidation; these are the chromo-7-deoxydaunomycinone (identical with the first 2-e- reduction intermediate) and its leuco isomer, 8-acetyl-7,8,9,10-tetrahydro-5,8,12-trihydroxy-1-methoxy-6,11-naphthacenedione (iso-7-deoxydaunomycinone). These studies provide useful information concerning the redox properties of the anthracyclines and suggest that these antitumor antibiotics may be capable of functioning as riboflavin antagonists in vivo.

Animals↗
PubMed1982

Synthesis and biological evaluation of a metazocine-containing enkephalinamide. Evidence for nonidentical roles of the tyramine moiety in opiates and opioid peptides.

In an effort to test the hypothesis that the tyramine moiety present in opiates and in opioid peptides plays an identical functional role at opioid receptors, a hybrid enkephalinamide (3) that contains (-)-metazocine (4a) in place of Tyr was synthesized. It was found that 3 and its congeners are inactive or feebly active in the electrically stimulated guinea pig ileum and mouse vas deferens preparations. The results of these studies suggest that the tyramine moiety in opiates and related structures does not play the same functional role as that in the opioid peptides. It is suggested that the different functional roles of the tyramine moiety in opiates and opioid peptides is a consequence of different modes of interaction with common receptors.

Animals↗
PubMed1980

Tongue-flaps--turntable and a two-stage forked flap in primary reconstruction after excision of oral carcinomas.

The tongue provides adequate material for lining in repair after excisional surgery for oral carcinoma, involving primarily the cheek and the floor of the mouth. Two flaps are described that utilize about one-third of the tongue. One of the flaps is based on a turntable principle and is attached only at its base to the mucosa. The other is a two-stage procedure, in which the tongue is forked at the time of admission when the patient is being prepared. The smaller prong on the same side is opened on the external aspect like an oyster shell. This procedure has the advantage of needing no twisting of the flap to bridge the defect.

Adult↗