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Biomedical subjects

F Funk

Publications and source records attributed to F Funk.

At least 19 recordsLinked to original sources
PubMed2000

Characterization of an integral protein of the brush border membrane mediating the transport of divalent metal ions.

The transport of Fe(2+) and other divalent transition metal ions across the intestinal brush border membrane (BBM) was investigated using brush border membrane vesicles (BBMVs) as a model. This transport is an energy-independent, protein-mediated process. The divalent metal ion transporter of the BBM is a spanning protein, very likely a protein channel, that senses the phase transition of the BBM, as indicated by a break in the Arrhenius plot. The transporter has a broad substrate range that includes Mn(2+), Fe(2+), Co(2+), Ni(2+), Cu(2+), and Zn(2+). Under physiological conditions the transport of divalent metal ions is proton-coupled, leading to the acidification of the internal cavity of BBMVs. The divalent metal ion transporter can be solubilized in excess detergent (30 mM diheptanoylphosphatidylcholine or 1% Triton X-100) and reconstituted into an artificial membrane system by detergent removal. The reconstituted membrane system showed metal ion transport characteristics similar to those of the original BBMVs. The properties of the protein described here closely resemble those of the proton-coupled divalent cation transporter (DCT1, Nramp2) described by, Nature. 388:482-488). We may conclude that a protein of the Nramp family is present in the BBM, facilitating the transport of Fe(2+) and other divalent transition metal ions.

Animals↗
PubMed1999

Heavy metal effects on beta-glucosidase activity influenced by pH and buffer systems.

Inhibition of beta-glucosidase activity by Cu(II), Zn(II) and Ni(II) was investigated as a function of pH and buffer type. Both factors were found to exert a strong effect on the activity of the enzyme. All three of the investigated heavy metals inhibited the enzyme activity in acetate buffer. At metal concentrations of 0.6 mM, Zn and Ni reduced the enzyme activity by 25-30% under optimal pH conditions (pH 5-5.2). Under the same conditions, Cu showed an even more pronounced inhibitory effect than Zn and Ni. In presence of 0.6 mM Cu, the enzyme activity was lowered by more than 90% in comparison to metal free systems. In contrast to these results, no enzyme inhibition was observed in citrate buffer, even in the presence of 1 mM Cu. The inhibition of beta-glucosidase activity by Cu increased with increasing pH. Inhibition by Zn and Ni was less pH-dependent in the observed pH range (pH 4-5.5). Copper caused a distinct shift in the pH optimum of enzyme activity, whereas this was not the case for Zn or Ni. The effects of buffer and pH on enzyme inhibition by Cu, Zn and Ni were successfully described using a chemical speciation model, based on the assumption that enzyme activity depends on the protonation of the amino acids at the reactive site and that enzyme activity is inhibited by complexation of the reactive sites by the heavy metal cations. The results show the importance of taking chemical conditions and speciation into account when investigating the effect of heavy metal cations on biological systems.

Buffers↗
PubMed1997

Iron uptake by rabbit intestinal brush border membrane vesicles involves movement through the outer surface, membrane interior, inner surface and aqueous interior.

Iron uptake in rabbit brush border membrane vesicles was measured in the presence of nitrilotriacetate. The complexes formed ranged from stable mononuclear species to hydrolyzed polynuclear complexes and are considered as a good model for nutritional iron compounds with respect to their chemical reactivity. Uptake includes both binding to and penetration through the membrane. A strategy was developed to localize iron in the following four compartments: outer membrane surface, membrane interior, inner membrane surface and aqueous phase within the vesicles. Both surfaces as well as the membrane interior revealed a high metal binding capacity. After an incubation for 10 min with 182 mumol/L iron and 364 mumol/L nitrilotriacetate, 35% of total vesicle iron was found to be bound to the outer membrane surface, 34% to the inner membrane surface, and 23% was not accessible to EDTA. Thus, by adsorption of polynuclear iron complexes to the outer surface, the residence time of iron may be prolonged. The remaining 8% of total iron was in the aqueous phase within the vesicles. Nitrilotriacetate enters the rabbit vesicles in a concentration-dependent manner. As a consequence, iron concentration in the aqueous phase within the vesicles will be driven to the medium equilibrium concentration.

Animals↗
PubMed1986

A comparative study on iron sources for mitochondrial haem synthesis including ferritin and models of transit pool species.

The rates of reaction of various exogenic iron(III) complexes with deuteroporphyrin IX in isolated mitochondria to form deuterohaem were measured. Ferritin was shown to supply iron readily for haem synthesis if the ferritin iron was reductively mobilised by the mitochondrial respiratory chain with succinate as substrate and FMN as mediator. In contrast, polynuclear complexes of iron(III) were able to form deuterohaem without added FMN. Rates of haem formation are about five times higher for the lowest polynuclear units than for ferritin. Sorbitol, gluconate, and bovine serum albumin were used as scavengers for polynuclear complexes with restricted size. Strong chelators of iron(II) compete favourably for deuterohaem formation, which supports the multistep mechanism for haem formation suggested by a priori arguments. Rates of deuterohaem formation were measured in homologous and heterologous systems of ferritins and mitochondria. Slightly differing rates of haem formation were shown to originate in different rates of iron mobilisation from the ferritins. The lack of species specificity in the interaction of ferritin with mitochondria also shows up in the linear dependence of ferritin binding on its bulk concentration as measured using 3H-labeled ferritin. Rates of haem formation are virtually the same in mitoplasts and mitochondria which indicates insignificant influences of the outer membrane. The hypothesis of low polynuclears as major components of the intracellular transit iron pool implies that both ferritin and transit iron pool species are largely equivalent sources of iron for mitochondrial haem synthesis.

Animals↗
PubMed1985

Reductive mobilisation of ferritin iron.

The reductive mobilisation of iron from ferritin, the principal protein of iron storage, was studied. The kinetic characteristics of iron release by dithionite, thioglycollate, and dihydroriboflavin 5'-phosphate (FMNH2) were found to differ widely. The dependence on pH is most pronounced for the dithionite reduction which proceeds 100 times faster at pH 4 than at pH 7. The experimental data can be consistently explained in terms of specific interactions of products or educts with interfacial iron(III) hydroxide of the ferritin core. Surface complexes with the product sulfite are postulated in the dithionite reaction, and with the educt in the thioglycollate reaction. Iron(II) complexes with the radical anion FMN-. are suggested to be involved in the iron release by FMNH2. The mobilisation of iron by a series of thiols of different size and coordinative properties confirmed the importance of surface complex formation. No evidence was found for predominant effects of hindered shell penetration.

Dithionite↗
PubMed1970

Process of infection with bacteriophage phiX174. 33. Templates for the synthesis of single-stranded deoxyribonucleic acid.

The origin of the templates for the synthesis of phiX174 progeny single-stranded deoxyribonucleic acid was studied by means of the mutagenic activity associated with the decay of incorporated (3)H-labeled 5-cytosine. The results indicate that the single-strand synthesis occurs in an asymmetric semiconservative manner using as template the complementary strands of the pool of replicative from molecules accumulated during the eclipse period. These complementary strands are repeatedly used as templates, and there is no detectable preferential use of complementary strand templates made early in the eclipse versus those made late.

Bromodeoxyuridine↗
PubMed1969

Cytosine to thymine transitions from decay of cytosine-5-3H in bacteriophage S13.

Decay of cytosine-5-(3)H incorporated into bacteriophage S13 DNA causes a molectular rearrangement of the cytosine molecutle undergoing the decay. The molecular rearrangement produces a cytosine to thyimine coding change with at efficiency approaching one. Decay of either thymidine-(methyl)-(3)H or cytosine-6-(3)H is less than 1 percent as effective in calusing either cytosine to thymine or thymine to cytosine transitions.

Centrifugation, Density Gradient↗
PubMed1968

Calculated energy deposits from the decay of tritium and other radioisotopes incorporated into bacteria.

Transmutation of the radioisotope tritium occurs with the production of a low energy electron, having a range in biological material similar to the dimensions of a bacterium. A computer program was written to determine the radiation dose distributions which may be expected within a bacterium as a result of tritium decay, when the isotope has been incorporated into specific regions of the bacterium. A nonspherical model bacterium was used, represented by a cylinder with hemispherical ends. The energy distributions resulting from a wide variety of simulated labeled regions were determined; the results suggested that the nuclear region of a bacterium receives on the average significantly different per decay doses, if the labeled regions were those conceivably produced by the incorporation of thymidine-(3)H, uracil-(3)H, or (3)H-amino acids. Energy distributions in the model bacterium were also calculated for the decay of incorporated (14)carbon, (35)sulfur, and (32)phosphorous.

Amino Acids↗