Molecular distribution of zinc in biliary and pancreatic secretions

Rat bile and pancreatic fluid were examined for the presence of low molecular weight zinc complexes. Fluids were collected separately by cannulation, and zinc distribution in collected samples was analyzed by gel filtration on Sephadex G-50. Most of the zinc in bile was associated with low molecular weight zinc complexes; only a small amount of zinc was present in the high molecular weight fraction. In contrast, pancreatic secretions did not contain low molecular weight zinc complexes, but there were considerable amounts of zinc bound to high molecular weight compounds. The addition of zinc to bile resulted in an increased amount of zinc in the low molecular weight fraction, while the addition of zinc to pancreatic fluid resulted primarily in an increase in zinc bound to the high molecular weight components. Like pancreatic fluid, homogenates of pancreatic tissue had no low molecular weight zinc complex. In rats whose bile and pancreatic fluid were removed and not returned into the intestine, the amount of zinc bound to low molecular weight complexes in intestinal homogenates was reduced. This alteration of the molecular distribution of zinc in intestinal homogenates by removal of bile and pancreatic fluid suggests the potential importance of low molecular weight zinc complexes for zinc homeostasis.     

Comparative studies of zinc metabolism in cultured chinese hamster cells with differing metallothionein-induction capacities

Previous work in our laboratory led to the isolation of a cadmium (Cd)-resistant variant (Cd^r2C10) of the line CHO Chinese Hamster cell having a 10-fold greater resistance to the cytotoxic action of Cd²⁺ compared with the CHO cell. This resistance was attributed to an increased capacity of the Cd²⁺-resistant Cd^r2C10 subline to induce synthesis of the Cd²⁺- and Zn²⁺-binding protein(s), metallothionein(s) (MT). Evidence that Cd²⁺ behaves as an analog of the essential trace metal, Zn²⁺, especially as an inducer of MT synthesis, suggested that the Cd^r and CHO cell types could be employed to investigate cellular Zn²⁺ metabolism. In the present study, measurements were made to compare CHO and Cd^r cell types for (a) growth as a function of the level of ZnCl₂ added to the culture medium, (b) uptake and subcellular distribution of Zn²⁺, and (c) capacity to induce MT synthesis. The results of these measurements indicated that (a) both CHO and Cd^r cell types grew normally (T _d≊16–18 h) during exposures to Zn²⁺ at levels up to 100 μM added to the growth medium, but displayed abrupt growth inhibition at higher Zn²⁺ levels, (b) Cd^r cells incorporate fourfold more Zn²⁺ during a 24-h exposure to the maximal subtoxic level of Zn²⁺ and (c) the CHO cell lacks the capacity to induce MT synethesis while the Cd^r cell is proficient in this response during exposure to the maximal subtoxic Zn²⁺ level. These findings suggest that (a) the CHO and Cd^r cell systems will be useful in further studies of cellular Zn²⁺ metabolism, especially in comparisons of Zn²⁺ metabolism in the presence and absence of induction of the Zn²⁺-sequestering MT and (b) a relationship exists between cellular capacity to induce MT synthesis and capacity for cellular Zn²⁺ uptake.     

Interaction of molybdenum with human erythrocyte membrane proteins

This study describes the interaction of molybdenum with blood components. Molybdenum-99 was added to blood, and after four washings, 3% of the total radioactivity was found in red cells. More specifically, the radioactivity was determined to be associated with the cell membrane. Molybdenum-99 in the +VI form did not interact with the human erythrocyte membrane; however, Mo(V) forms did interact. Of five different compounds, the highes uptake was observed with a brown Mo(V)-ascorbate complex generated from Mo(VI) and ascorbic acid in the molar ratio 1∶20. A membrane suspension of Mo-ascorbate-treated human erythrocytes was prepared and the solubilized proteins were separated on a polyacrylamide gel in the presence of sodium dodecyl sulfate (SDS). Molybdenum-99 binding to spectrin was demonstrated, as well as some minor interactions with membrane hemoglobin and bands 6 and 8.     

Superoxide dismutase isoenzymes in bovine and human milk

The presence of superoxide dismutase in bovine and human milk was investigated by ultrafiltration, gel filtration, and isoelectric focusing. Conclusive evidence for the presence of this enzyme in both milks is presented. The molecular weight of the enzyme was estimated by gel filtration on Sephadex G-100 to be 30,000, which is consistent with reported values for the copper, zinc form of superoxide dismutase. In addition, enzyme activity was inhibited by cyanide, thus eliminating the possibility that the enzyme was present in the manganese form. Several isoenzymes were detected by isoelectric focusing in polyacrylamide gel, and the isoenzyme pattern in bovine milk was the same as that found for bovine plasma, suggesting that milk superoxide dismutase originates from plasma. It may be that the presence of copper, zinc superoxide dismutase in milk is important for the maintenance of its oxidative stability.     

Enzymic hydration of benzene oxide: Assay and properties

A radiometric assay for epoxide hydratase using [¹⁴C]benzene oxide as substrate has been developed. The reaction product trans-1,2-[¹⁴C]dihydroxy-1,2-dihydrobenzene (benzene dihydrodiol) was separated from the other components by simple extraction of the unreacted substrate and phenol (a rearrangement product) into a mixture of light petroleum and diethyl ether followed by extraction of the benzene dihydrodiol into ethyl acetate. The product was then estimated by scintillation counting. Using this assay the enzymic hydration of benzene oxide and the possible existence of a microsomal epoxide hydratase with a greater specificity toward benzene oxide were reinvestigated. The sequence of activities of microsomes from various organs was liver > kidney > lung > skin, the pH optimum of enzymic benzene oxide hydration was about pH 9.0, which is similar to that of styrene oxide hydration and both activities were equally stable when liver microsomal fractions were stored. The effect of low molecular weight inhibitors upon the hydration of styrene and benzene oxide by liver microsomes was similar in some cases and dissimilar in others. However, all the dissimilarities could be explained without recourse to the hypothesis of the existence of a separate benzene oxide hydratase. During enzyme purification studies the activity toward benzene oxide was inhibited by the detergent used (cutscum) but was recovered when the detergent was removed. Solubilization without significant loss of activity was successful using sodium cholate. This allowed immunoprecipitation studies, which were performed using monospecific antiserum raised against homogeneous epoxide hydratase. The dose-response curves of the extent of precipitation of activity with increasing amounts of added antiserum were indistinguishable for benzene oxide and styrene oxide as substrate. At high antiserum concentrations precipitation was complete with both substrates. The findings, taken together, indicate the presence in rat liver microsomes of a single epoxide hydratase catalyzing the hydration of both styrene and benzene oxide or the presence of enzymes so closely related that these cannot be distinguished by any of the criteria tested.     

The stacking of chloroplast thylakoids: Quantitative analysis of the balance of forces between thylakoid membranes of chloroplasts,and the role of divalent cations

An analysis is made of the van der Waals dispersion attractive forces and electrostatic repulsive forces between the grana thylakoid membranes of chloroplasts. These forces are determined for negatively charged surfaces with a pK_a value of 4.7 for a bulk pH of 7.0 with a range of mono- and divalent cation concentrations and intermembrane spacing in the range 10 to 80 Å. For equilibrium under dark conditions, it is concluded that either there is extensive electrostatic binding of divalent cations (Mg²⁺) to the negatively charged membrane groups (phospholipid, sulfolipid, and protein carboxyl), or a redistribution of these groups between stacked and unstacked regions must be invoked.     

Purification and properties of mitochondrial and peroxisomal 3-hydroxyacyl-CoA dehydrogenase from rat liver

There are two 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) in rat liver, one in mitochondria (type I enzyme), and another in peroxisomes (type II enzyme). In a series of the studies on the properties and the physiological roles of fatty acid oxidation systems in both organelles, the two enzymes were purified and compared for their properties. The final preparations obtained were judged to be homogeneous based on the results of sodium dodecyl sulfate-polyacrylamide gel electrophoresis and sedimentation velocity analysis. Type I enzyme was composed of two identical subunits of molecular weight of 32,000, whereas type II enzyme was a monomeric enzyme having a molecular weight of 70,000–77,000. These subunit structures were confirmed by the results of fluorescence studies. Both enzymes were different in amino acid compositions, especially in the contents of tryptophan and half-cystine. Antibodies against them formed single precipitin lines for the corresponding enzymes, but not for the others when subjected to an Ouchterlony double-diffusion test. The K_m values of type II enzyme for various substrates were lower than those of type I enzyme except those for acetoacetyl-CoA. As for 3-hydroxyacyl-CoA substrates, both enzymes had lower K_m's for longer-chain substrates. The V for the substrates of C₄C₁₀ were similar for each enzyme, though the value of type II enzyme for C₁₀ substrate was rather lower. The results of fluorescence studies suggested that their dissociation constants for NADH were lower and those for NAD⁺ were higher at lower pH. Both enzymes were specific to l-form of 3-hydroxyacyl-CoA substrate. The optimal pH of the forward reaction of type I and type II enzymes was 9.6 and 9.8, and of the reverse reaction, 4.5 and 6.2, respectively. From these results they were concluded to be completely different enzymes.     

Immunological properties of low molecular-weight proteins binding heavy metals in rat kidney and liver

Two homogenous fractions of hepatic metallothioneins ((Cd,Zn) MT-1 and (Cd,Zn) MT-2) and renal metal binding proteins ((Bi,Cu) BP-1 and (Bi,Cu) BP-2) were isolated from rats exposed to heavy metals and specific antisera to them were produced in rabbits.These antisera were tested by immunodiffusion and immunoelectrophoresis for their ability to bind different fractions of hepatic Cd,Zn -metallothionein and renal (Bi,Cu)-, (Hg,Cu)- and (Cd,Cu)-binding proteins. It was found that anti (Bi,Cu) BP antisera did not cross-react with hepatic (Cd,Zn) MT-1 and (Cd,Zn) MT-2. Strong immunological cross-reactions were detected between anti (Bi,Cu) BP antisera and individual forms of (Cd,Cu)-, (Hg,Cu)- and (Bi,Cu)-binding proteins isolated from rat kidneys.     

Time-dependent changes of calcium influx and efflux rates in rabbit skeletal muscle sarcoplasmic reticulum

Unfractionated and low buoyant density sarcoplasmic reticulum vesicles released calcium spontaneously after ATP- or acetyl phosphate-supported calcium uptake when internal Ca²⁺ was stabilized by the use of 50 mM phosphate as calcium-precipitating anion. This spontaneous calcium release could not be attributed to falling Ca²⁺ concentration outside the vesicles (Ca₀²⁺), substrate depletion, ADP accumulation, nonspecific membrane deterioration or the attainment of a high vesicular calcium content. Instead, spontaneous calcium release was directly proportional to Ca₀²⁺ at the time that calcium content was maximal. A causal relationship between high Ca₀²⁺ and spontaneous calcium release was suggested by the finding that elevation of Ca₀²⁺ from less than 1 μM to 3–5 μM increased the rate and extent of calcium release.The spontaneous calcium release was due both to acceleration of calcium efflux and slowing of calcium influx that was not accompanied by a significant change in the rate of ATP hydrolysis. Neither reversal of the transmembrane KCl gradient nor incubation with cation and proton ionophores abolished the spontaneous calcium release. The persistence of calcium release under conditions where the membrane was permeable to both anions and cations makes it unlikely that this phenomenon is due to a changing transmembrane potential.     

Bilayer orientation of membrane-bound NH2 groups across microsomal vesicles and their role in the function of gastric K+-stimulated adenosine triphosphatase

The transversal distribution of the free NH₂ groups associated with phosphatidyl ethanolamine and the intrinsic membrane proteins of the purified pig gastric microsomes was quantitated and their relations to the function of the gastric K⁺-stimulated ATPase was investigated. Three different chemical probes such as 2,4,6-trinitrobenzene sulfonic acid (TNBS), 1-fluoro-2,4-dinitrobenzene (FDNB), and 2-methoxy-2,4-diphenyl-3(2H)-furanone (MDPF) were used for the study. The structure-function relationship of the membrane NH₂ groups was studied after modification with the probes under various conditions and relating the inhibition of the K⁺-stimulated ATPase to the ATPase-dependent H⁺ accumulation by the gastric microsomal vesicles. TNBS (2 mm) inhibits nearly completely the K⁺-stimulated ATPase and the vesicular dye accumulation, both in presence and absence of valinomycin plus K⁺. Both the K⁺-ATPase and dye uptake were largely (about 50%) protected against TNBS inhibition if the treatment with TNBS was carried out in presence of 2 mm ATP. TNBS and FDNB labeled 70% of the total microsomal PE; the intra- and extravesicular orientation being 48 and 22%, respectively. The presence or absence of ATP did not have any effect on the TNBS labeling of microsomal PE. ATP, however, significantly (P < 0.05) reduced the labeling of protein-bound NH₂ groups of gastric microsomes by TNBS. The intra- and extravesicular orientation of the protein NH₂ groups were 60 and 40%, respectively. Eighteen percent of the total protein-NH₂ appeared to be associated with the K⁺-stimulated ATPase; the rest being associated with non-ATPase proteins of the microsomes. About half (50%) of the total free NH₂ groups of the K⁺-stimulated ATPase were exposed to the vesicle exterior and were found to play critical roles in gastric ATPase function. The generation of florescence after MDPF conjugation of gastric microsomes was largely (50%) inhibited by ATP. ATP also protected completely the MDPF inhibition of gastric K⁺-stimulated ATPase and dye uptake.