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Biomedical subjects

R Kahn

Publications and source records attributed to R Kahn.

At least 109 records · Page 6Linked to original sources
PubMed1985

Crystal structure study of Opsanus tau parvalbumin by multiwavelength anomalous diffraction.

The crystal structure of a small calcium-binding protein, the parvalbumin IIIf from Opsanus tau in which Tb was substituted for Ca, has been analysed by multiwavelength anomalous diffraction. Data at a resolution of 2.3 A were collected at three wavelengths near the L3 absorption edge of Tb (1.645-1.650 A), using the synchrotron radiation emitted by a storage ring and a multiwire proportional counter. The phases of the reflections were determined from this single derivative, without native data. Prior to any refinement, the resulting electron density map shows a good agreement with the model of the homologous carp parvalbumin in regions of identical amino-acid sequence.

Animals↗
PubMed1984

Separation of the presynaptic and synaptic phases of homologous pairing promoted by recA protein.

Homologous pairing of single strands with duplex DNA promoted by recA protein occurred without a lag only when the protein was preincubated with ATP and single-stranded DNA. The rate-limiting presynaptic interaction of recA protein and single strands showed a high temperature coefficient: it proceeded 30 times more slowly at 30 degrees C than at 37 degrees C, whereas synapsis showed a normal temperature coefficient. Thus, the presynaptic phase could be separated experimentally from the rest of the reaction by preincubation of single strands with recA protein and ATP at 37 degrees C, followed by a shift to 30 degrees C before double-stranded DNA was added. The presynaptic phase was an order of magnitude more sensitive to inhibition by ADP than was subsequent strand exchange. Presynaptic complexes that were formed at 37 degrees C decayed only slowly at 30 degrees C, but Escherichia coli single strand binding protein caused complexes to form rapidly at 30 degrees C which indicates that single strand binding protein accelerated the rate of formation of complexes. Preincubation synchronized the initial pairing reaction, and further revealed the rapid formation of nascent heteroduplex DNA 250-300 base pairs in length.

Adenosine Triphosphate↗
PubMed1984

Coronary artery dissection secondary to coronary arteriography: case report and review.

A case of catheter-induced proximal dissection of an angiographically normal left coronary artery is reported. Dissection was not associated with pressure damping and myocardial ischemia was delayed until 1 hour after dissection occurred. Prompt recognition of this entity is essential as emergency revascularization is the treatment of choice when significant amounts of myocardium are threatened.

Cardiac Catheterization↗
PubMed1984

Splitting of the circadian rhythm of body temperature in the golden hamster.

The circadian rhythm of hamster locomotor activity "splits" into two distinct circadian components during conditions of constant illumination. To determine if the circadian rhythm of body temperature also splits under these conditions, body temperature and locomotor activity were monitored concurrently in animals housed in constant illumination. Splitting of the body temperature rhythm into two circadian components was observed in animals manifesting split activity rhythms. Concurrent splitting of both rhythms suggests a common mechanism either for the generation or coupling of these rhythms.

Animals↗
PubMed1984

Hepatitis B virus antibody in blood donors and the occurrence of non-A, non-B hepatitis in transfusion recipients. An analysis of the Transfusion-Transmitted Viruses Study.

Patients who received transfusions and nontransfused control patients were followed to assess the incidence and cause of post-transfusion hepatitis and to identify donor factors that might relate to risk of hepatitis. We evaluated as risk factors in donors the presence of antibody to hepatitis B virus compared with elevated alanine aminotransferase (ALT) level. Units of blood that were positive for antibody to hepatitis B core antigen (anti-HBc) were associated with a twofold to threefold greater risk of non-A, non-B hepatitis in the recipients than were units without anti-HBc. In the absence of specific serologic tests for non-A, non-B agents, screening of donors for anti-HBc might be considered. Our data suggest that the incidence of non-A, non-B hepatitis might have been reduced by about one third by such screening. However, elevated ALT levels in donors had a similar association with non-A, non-B hepatitis in recipients but would have resulted in fewer units of blood being discarded than would screening for anti-HBc.

Alanine Transaminase↗
PubMed1983

Interactions between insulin and the cyclic AMP system of Cloudman S91 mouse melanoma cells.

Insulin inhibits the proliferation of wild-type Cloudman S91 mouse melanoma cells. The effects, which are mediated through specific, high-affinity receptors for insulin, appear to involve interactions with the cAMP system. Our evidence is as follows: 1) Cloudman cells have a cAMP requirement for proliferation and pigmentation. Exposure of cells to insulin results in a lowering of intracellular cAMP levels and inhibition of both cell division and pigment formation. 2) The effects of insulin are reversed by agents which raise cAMP levels, or by the cAMP analogue dibutyryl cAMP. 3) A mutant cell line with a temperature-dependent requirement for cAMP is most sensitive to the growth inhibitory effects of insulin when its requirements for cAMP are maximal. 4) Mutants selected only for alterations in their response to insulin frequently have concomitant alterations in their cAMP systems. 5) The melanotropin-responsive adenylate cyclase system is stimulated following prolonged exposure of cells in culture to insulin. Although we do not know the mechanism(s) for the interactions between the insulin and the cAMP system, our initial findings suggest that protein phosphorylation/dephosphorylation reactions are involved.

Adenylyl Cyclases↗
PubMed1982

Formation of nascent heteroduplex structures by RecA protein and DNA.

E. coli RecA protein promotes homologous pairing in two distinguishable phases: synapsis and strand exchange. With circular single strands (plus strand only) and linear duplex DNA, polarized or unidirectional strand exchange appeared to cause heteroduplex joints to form and grow from a unique end of the duplex DNA. However, a variety of other pairs of substrates appeared to form joint molecules without regard to the polarity of the strands involved. This paradox has been resolved by observations that show that synapsis is fast, nonpolar and sensitive to inhibition by ADP, whereas strand exchange is slow, directional and relatively insensitive to inhibition by ADP. Thus a heteroduplex joint initiated at one end of the duplex DNA grows by continued strand exchange, whereas a joint initiated at the other end dissociates and is unable to start again because accumulating ADP inhibits synapsis. RecA protein appears to form a nascent protein-DNA structure, the RecA synaptic structure, in which at least 100-300 bp in the duplex molecule are held in an unwound configuration and in which the incoming strand is aligned with its complement.

Adenosine Diphosphate↗
PubMed1981

Concerted strand exchange and formation of Holliday structures by E. coli RecA protein.

RecA protein makes stable joint molecules from fully duplex DNA and molecules that are partially single-stranded; the latter may be either duplex molecules with an internal gap in one strand or molecules with single-stranded ends. Stable joint molecules form only when the end of at least one strand is in a homologous region. When RecA protein pairs linear duplex molecules and tailed molecules that share the same sequence end to end, the joints, which are located away from the single-stranded tails in most instances, have the electron microscopic appearance associated with the Holliday structure resulting from the reciprocal exchange of strands. The reaction leading to reciprocal strand exchange involves the concerted displacement of a strand from the end of the duplex molecule. These observations support the view that RecA protein makes stable joint molecules only by transferring strands and not by the side-by-side pairing of duplex regions.

Bacterial Proteins↗
PubMed1981

Polarity of heteroduplex formation promoted by Escherichia coli recA protein.

When recA protein pairs circular single strands with linear duplex DNA, the circular strand displaces its homolog from only one end of the duplex molecule and rapidly creates heteroduplex joints that are thousands of base pairs long [DasGupta, C., Shibata, T., Cunningham, R. P. & Radding, C. M. (1980) Cell 22, 437-446]. To examine this apparently polar reaction, we prepared chimeric duplex fragments of DNA that had M13 nucleotide sequences at one end and G4 sequences at the other. Circular single strands homologous to M13 DNA paired with a chimeric fragment when M13 sequences were located at the 3' end of the complementary strand but did not pair when the M13 sequences were located at the 5' end. Likewise circular single-stranded G4 DNA paired with chimeric fragments only when G4 sequences were located at the 3' end of the complementary strand. To confirm these observations, we prepared fd DNA labeled only at the 5' or 3' end of the plus strand, and we examined the susceptibility of these labeled ends to digestion by exonucleases when joint molecules were formed. Eighty percent of the 5' label in joint molecules became sensitive to exonuclease VII. Displacement of that 5' end by recA protein was concerted because it did not occur in the absence of single-stranded DNA or in the presence of heterologous single strands. By contrast, only a small fraction of the 3' label became sensitive to exonuclease VII or exonuclease I. These observations show that recA protein forms heteroduplex joints in a concerted and polarized way.

Bacterial Proteins↗
PubMed1980

Inhibition of proliferation of cloudman S91 melanoma cells by insulin and characterization of some insulin-resistant variants.

Insulin is a potent, reversible inhibitor of proliferation in Cloudman S91 mouse melanoma cells. The inhibition seems to be unique to the Cloudman line since five other cell lines, including the mouse B16 and hamster Greene melanomas, were unaffected by insulin under the same culture conditions. Variants of Cloudman S91 cells were isolated which differed from wild-type (WT) cells in their response to insulin. Most of these variants were resistant to insulin (INSres) and had the same generation time independent of the presence of the hormone. One line (INSdep) was found to require insulin for growth. This line was about 15 times more sensitive to the proliferative effects of insulin than the WT. Revertants of the INSdep line were selected for their ability to proliferate in the absence of insulin. Five out of five such revertants were insulin resistant, suggesting that the INSdep line arose as a results of at least two separate mutations. Both WT and INSdep cells showed enhanced uptake of 14C-alpha-aminoisobutyric acid (AIB) when exposed to insulin. Dose-response curves of the stimulation of AIB uptake in WT and INSdep cells were superimposable. Stimulation of AIB uptake and stimulation of proliferation by insulin were not under coordinate control since AIB uptake was increased equally in the wild-type cells when proliferation was inhibited and in INSdep cells when proliferation was enhanced. Binding of 125I-insulin was used to demonstrate the presence of specific, high affinity insulin receptors on the cells. INSres variants generally had fewer receptors than WT, but in no case did the magnitude of this effect appear to be sufficient to explain the insensitivity to insulin. The INSdep variant showed a greater than two-fold increase in the number of insulin receptors per cell, compared to WT. Revertants of the INSdep line had the same number of receptors as WT. The specificity for both binding and for the effects on proliferation were the same in WT and INSdep cells. Since the effects of insulin on proliferation were opposite in the two lines, we propose at least two distinct sites of insulin action on the cells. Further isolation and analyses of Cloudman lines with unusual responses to insulin should be useful for understanding the molecular basis of action of this hormone.

Aminoisobutyric Acids↗
PubMed1978

Proximal versus peripheral hepatic artery embolization experimental study in monkeys.

A case is described in which extensive embolization of the hepatic artery to reduce insulin production by a metastatic islet-cell tumor resulted in liver failure, intrahepatic abscesses, and death. The difference between proximal and peripheral hepatic artery obstruction was investigated in monkeys by embolizing hepatic arterial beds with Gelfoam and silicone. Gelfoam obstructed proximally, arterial collaterals developed rapidly, and liver function remained normal. Peripheral hepatic artery embolization with silicone produced liver infarction and severe functional abnormalities. Peripheral hepatic artery occlusion was more effective in preventing the development of collateral circulation but involved a significant risk of hepatic failure or abscess.

Adenoma, Islet Cell↗