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Biomedical subjects

C H Fung

Publications and source records attributed to C H Fung.

33 records · Page 2Linked to original sources
PubMed1977

Cystadenoma of the pancreas. An ultrastructural study.

A case of cystadenoma of the pancreas was studied at the light (LM) and the ultrastructural (EM) levels. The observation made under LM that the flattened epithelial cells lining the cystic spaces were similar to the centroacinar cells of a normal pancreas was supported by EM findings. Numerous glycogen granules were demonstrated in the cells, a feature characteristic of fetal but not adult centroacinar cells. A few epithelial cells contained occasional membrane bound dense granules of unknown nature. This observation and the finding of normally appearing islets of Langerhans raise the possibility that this may be an example of a mixed insular-duct cystadenoma. However, if the islets of Langerhans were considered entrapped structures, this tumor should be labelled as a simple serous cystadenoma.

Aged↗
PubMed1976

Chromium(III)-adenosine triphosphate as a paramagnetic probe to determine intersubstrate distances on pyruvate kinase. Detection of an active enzyme-metal-ATP-metal complex.

The interaction of CrATP, a stable, substitution-inert, paramagnetic tridentate complex of ATP, with muscle pyruvate kinase has been studied by measuring the effects of CrATP on the kinetics of pyruvate enolization and on the longitudinal nuclear magnetic relaxation rate (1/T1) of the protons of water and the protons and carbon atoms of pyruvate to investigate the existence and activity of bimetallic enzyme-M(II)-CrATP complexes and to determine intersubstrate distances on a kinase. The paramagnetic effect of CrATP on 1/T1 of water protons is enhanced upon complexation with the enzyme. Titrations of the enzyme with CrATP yielded characteristic enhancements of 1/T1 for the binary enzyme-CrATP, ternary enzyme-Mg(II)-crATP, and quaternary enzyme-Mg(II)-crATP-pyruvate complexes of 3.5, 1.7, and 1.2 and dissociation constants of CrATP of 400, 200, and 200 muM, respectively. From the frequency dependence of 1/T1, the number of fast exchanging water protons in the coordination spheres of Cr(III) is approximately 6 in CrATP and in both the ternary enzyme-Mg(II)-CrATP complex and the quaternary enzyme-Mg(II)-CrATP-pyruvate complex. The paramagnetic effect of enzyme-bound Mn(II) on 1/T1 of water protons decreases upon the addition of CrATP. Titration of the binary enzyme-Mn(II) complex with CrATP decreases the characteristic enhancement due to Mn(II) from 24 +/- 3 to 6 +/- 1. Titration of the ternary eznyme-Mn(II)-pyruvate complex with CrATP decreases the enhancement from 6 +/- 1 to 0.5 +/- 0.1. The affinity of the enzyme for Mn(II) is increased 2-fold upon binding of CrATP as indicated by decreases in the amplitude of the EPR spectrum of free Mn(II). The dissociation constants of CrATP from the enzyme-Mn(II)-CrATP complex, the enzyme-CrATP-pyruvate complex, and the enzyme-Mn(II)-CrATP-pyruvate complex are all 200 muM. The observed titration behavior, the characteristic enhancement values, the tightening by Mg(II) of the binding of CrATP to the enzyme, and the tightening of the binding of Mn(II) to the enzyme by CrATP establish the existence of enzyme-M(II)-CrATP and enzyme-M(II)-CrATP-pyruvate complexes containing two cations, Mg(II) or Mn(II) and Cr(III), at the active site.

Adenosine Triphosphate↗
PubMed1976

Arrangement and conformations of substrates at the active site of pyruvate kinase from model building studies based on magnetic resonance data.

Seventeen distances from two paramagnetic reference points, as determined by nuclear relaxation studies of six active complexes of rabbit muscle pyruvate kinase, have been used to construct molecular models of two composite enzyme complexes. In the model of the hypothetical pyruvate kinase-M(I)-M(II)-ATP-Cr(III)-P-enolpyruvate complex, overlap of the transferred phosphoryl groups of the two substrates, which is required to explain the observed competition, is incomplete, allowing greater than or equal to 1 A for the transition state to form. In the active enzyme-M(I)-M(II)-ATP-Cr(III)-pyruvate complex, the gamma-phosphoryl phosphorus of ATP is in molecular contact (3.0 +/- 0.5 A) with the carbonyl oxygen of pyruvate, consistent with direct phosphoryl transfer, indicating no need for intermediate phosphorylation of the enzyme. The enzyme-bound divalent cation, which forms second sphere complexes with the phosphoryl groups of P-enolpyruvate and ATP, may activate the transferred phosphoryl group indirectly, through a water ligand. By analogy with the position of Cr(III), a second divalent cation may participate more directly by coordination of the triphosphate chain of ATP.

Adenosine Triphosphate↗
PubMed1976

1H and 31P Fourier transform magnetic resonance studies of the conformation of enzyme-bound propionyl coenzyme A on the transcarboxylase.

The relaxation rates of the carbon-bound protons and of the three assigned phosphorus resonances of propionyl-CoA were measured in solutions of free propionyl-CoA and of the transcarboxylase-propionyl-CoA complex. In free propionyl-CoA, analysis of the 1/T1 values of 15 protons at 100 and 220 MHz and of 1/T1 and 1/T2 of the three phosphorus atoms at 40.5 MHz indicated free rotation of the propionyl region (taur approximately 3 x 10(-11) sec) but hindered motion of the remainder of the molecule with correlation times of 1-3. 5 x 10(-10) sec, approaching the tumbling time of the entire molecule (taur - 6 x 10(-10) sec. The correlation times of the three phosphorus atoms were indistinguishable from those of their nearest neighbor protons. The effects of three homogeneous enzyme preparations with varying contents of Zn(II), Co(II), and Cu(II) on 1/T1 of 12 protons and 3 phosphorus atoms of prionyl-CoA were analyzed with the help of simultaneous equations to yield the individual contributions at the three metal sites. Only diamagnetic effects were detected on the relaxation rates of the three phosphorus atoms. From the diamagnetic effects it was calculated that the motions of the prionyl side chain and of the terminal pantetheine methylene protons were hindered on the enzyme by an order of magnitude (taur approximately 6 x 10(-10) sec) and that the phosphorus atoms were hindered by two orders of magnitude (taur approximately 1 x 10(-8) sec) over the taur values found in free propionyl-CoA, but that these taur values remained well below that of the entire protein molecule (taur =6 x 10(-7) sec)...

Binding Sites↗
PubMed1976

Magnetic resonance studies of the proximity and spatial arrangement of propionyl coenzyme A and pyruvate on a biotin-metalloenzyme, transcarboxylase.

A spin-labeled ester of CoA, R-CoA (3-carboxy-2,2,5,5-tetramethyl-1-pyrolidinyl-1-oxy CoA thioester), has been shown by competition studies using electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) to bind specifically to the propionyl-CoA binding sites of transcarboxylase. Titrations indicate 0.7 +/- 0.2 binding site for R-CoA per enzyme-bound biotin with a dissociation constant of 0.33 +/- 0.12 mM. Propionyl-CoA binds to this site with a 1.3-fold lower disonable agreement with kinetically determined inhibitor constants of CoA and propionyl-CoA and propionyl-CoA (D. B. Northrop (1969), J. Biol. Chem. 244, 5808). The bit of this spin-label on 1/T1 of water protons. The formation of a ternary transcarboxylase-R-CoA-pyruvate complex is suggested by the failure of pyruvate to displace R-CoA from the tight site and is established by the paramagnetic effects of enzyme-bound R-CoA on the relaxation rates of the protons and 13C atoms of enzyme-bound pyruvate. From the paramagnetic effects of R-CoA on the relaxation rates of the methyl protons of pyruvate at 40.5 and 100 MHz, and on the 13C-enriched carbonyl and carboxyl carbon atoms of pyruvate at 25.1 MHz, a correlation time of 7 nsec and distances from the bound nitroxide radical to the methyl protons, the carbonyl, and carboxyl carbon atoms of bound pyruvate of 7.9 +/- 0.7, 10.3 +/- 0.8, and 12.1 +/- 0.9 A, respectively, are calculated. These distances establish the close proximity of the CoA ester and keto acid sites on transcarboxylase. Together with the previously determined distances from the enzyme-bound (Co(II) to the methyl protons and 2 carbon atoms of bound pyruvate and to 12 protons and 3 phosphorus atoms of bound propionyl-CoA, the present distances are used to derive a composite model of the bound substrates in the overall transcarboxylation reaction. In this model the distance from the methyl carbon of pyruvate and the methylene carbon of propionyl-CoA, between which the carboxyl transfer takes place is only approximately 7 A. Depending on the detailed mechanism of the carboxyl transfer, the distance through which the carboxybiotin must migrate is therefore between 0 and 7 A. Hence the major role of the 14-A arm of carboxybiotin is not to permit a large carboxyl migration but, rather to permit carboxybiotin to traverse the gap which occurs at the interface of three subunits and to insinuate itself between the CoA and keto acid sites.

Binding Sites↗
PubMed1976

Laboratory suggestion. Interpretation of plasma protamine paracoagulation test.

The reliability of the plasma protamine paracoagulation test has been questioned because of the controversy surrounding its interpretation and because of false positives reported with high frequency in some series. To minimize the frequency of false positives, it is recommended that when the test is done at room temperature, clot or fibrin strand formation be considered a positive result and precipitates be considered negative. Use of the wire hook to distinguish fibrin strands from precipitates is suggested. Nascent clotting and blood drawn from indwelling catheters should be avoided.

Blood Coagulation Tests↗
PubMed1975

Stereochemistry of propionyl-coenzyme A and pyruvate carboxylations catalyzed by transcarboxylase.

The stereochemistry of the two half-reactions catalyzed by the biotin-containing enzyme, transcarboxy-lase from Propionobacteria shermanii, has been determined. The pro-R hydrogen at C-2 of propionyl-coenzyme A is replaced by CO2 in formation of the S isomer of methylmalonyl-CoA, defining the process as retention of configuration. This C-2 hydrogen is abstracted at a rate identical with product formation. For the other half-reaction, pyruvate to oxalacetate, the chiral methyl group methodology of Rose (I. A. Rose (1970), J. Biol. Chem. 245, 6052) was employed. First, it was determined with [3-2-He]pyruvate that a kinetic deuterium isotope effect of 2.1 occurs at Vmax in this carboxyl transfer, indicating that the necessary requirement for discrimination against heavy isotopes of hydrogen existed. Then, 3(S)-[3-2-H,3-H]pyruvate, generated from 3(S)-]E-2-H,3-H]phosphoglycerate, was carboxylated and the oxalacetate trapped as [3030H]malate using malate dehydrogenase. Exhaustive incubation of the tritiated malate (3-H/14-C = 1.95) with fumarase to labilize the pro-R hydrogen at C-3 resulted in release of 65% of the tritium into water. Reisolation of the malate after fumarase action yielded a 30H/14-C ration of 0.67, indicating 34% retention as expected. The theoretical enantiotopic distribution for the observed k1H/k2H of 2.1 is 68:32. Selective enrichment of tritium in the pro-R position at C-3 of malate indicates enzymatic carboxylation of pyruvate with retention of configuration in this half-reaction also.

Carboxy-Lyases↗
PubMedDate not supplied

Simple BASIC program for calculating the cervicovaginal FNP and for estimating the sample size of the number of cervicovaginal smears to be rescreened.

OBJECTIVE: To guide cytotechnologists and pathologists in calculating the false negative proportion, or rate, and the number of Papanicolaou smears to be reevaluated for a meaningful assessment of screening performance, a computer program written in BASIC was prepared, based on several recent publications in the field of cytopathology. RESULTS: A complete program listing and sample runs to help users be cognizant of the necessary inputs to run the program. The output from the program gives the results of the various calculations. CONCLUSION: Since the tedious manual calculations are handled by the computer program, it is more likely for those involved in the interpretation of Papanicolaou smears to follow the approaches suggested by experts in these two areas.

Female↗