Cytoplasmic poly(A) polymerase from sea urchin eggs, merogons, and embryos

The presence of cytoplasmic poly(A) polymerase has been established in sea urchin eggs and four-cell embryos by subcellular fractionation and use of enucleate egg halves. ATP is the only ribonucleoside triphosphate incorporated. This incorporation is time dependent, contingent on input protein concentration, and immune to a variety of antimetabolites known to inhibit DNA-directed RNA synthesis. Both the unfertilized egg and the four-cell embryo cytoplasmic poly(A) polymerase activities display a preference for Mn²⁺. While oligo(A)₄ is inactive as a primer, addition of $\text{oligo(A)}{}_{\mathrm{<mtext>16</mtext>}}\text{,}\text{poly(A)}{}_{\mathrm{<mtext>45</mtext>}}$ and $\text{poly(A)}{}_{\mathrm{<mtext>90</mtext>}}$ stimulates ATP incorporation. On a unit per milligram protein basis, the endogenous activity associated with cytoplasmic fractions obtained from nucleate and enucleate egg halves is 36 and 83% that obtained with the cytoplasmic fraction prepared from the unfertilized egg. In the presence of $\text{oligo(A)}{}_{\mathrm{<mtext>16</mtext>}}$, both the nucleate and enucleate egg halves exhibit 81% of the activity associated with the unfertilized egg cytoplasmic fraction. The level of Mn²⁺ cytoplasmic poly(A) polymerase activity from the four-cell embryo is approximately 50% that of the unfertilized egg. This decrease does not appear to be due to either a postfertilization alteration in the subcellular localization of poly(A) polymerase or an increase in RNase activity. Supplementation with $\text{oligo(A)}{}_{\mathrm{<mtext>16</mtext>}}$ failed to restore the four-cell embryo cytoplasmic poly(A) polymerase potential to a level comparable to that of the unfertilized egg. Suppression of postfertilization protein synthesis by emetine, however, prevents this developmental decline in ATP incorporation thereby suggesting that postfertilization cytoplasmic poly(A) polymerase activity is subject to negative translational control.     

Cellular aspects of tolerance. VII. Inheritance of the resistance to tolerance induction

The half-lives of elimination ($\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$) of ¹³¹I-RGG from the body of normal A or Balb/c animals was much longer than the $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of SJL mice. At all ages, the $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of normal hybrids (A × SJL, SJL × A, Balb/c × SJL) was similar to or longer than that of the A or Balb/c parents. Thus, in terms of the $\text{T}\text{1}\text{2}$ of normal animals, the SJL responsiveness to ¹³¹I-RGG appeared to be a recessive trait. Tolerance could be induced in newborn animals and, in terms of $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, the degree of unresponsiveness at the age of 6 weeks, was the same in A, Balb/c, A × SJL, and Balb/c × SJL animals but was much shorter in SJL mice. Thus, in neonatally induced tolerance, the duration of tolerance was recessive for the SJL type. The average $\text{T}{}_{\mathrm{<mtext>built1</mtext><mtext>2</mtext>}}$ after tolerance induction in 3-week-old hybrids (A × SJL, SJL × A, Balb/c × SJL) was similar to that of the A or Balb/c parent, but by the 8th and 12th week it approached the average $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of the SJL parent. Comparing 8-week-old hybrids, the average $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ was longest in A × SJL hybrids and identical in SJL × A and Balb/c × SJL mice. An examination of $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ distribution in various 8- and 12-week-old crosses and backcrosses revealed a fairly large proportion of individuals with a $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ which was intermediate between SJL and the other parent. There was a tendency for this number to decrease in 12 weeks as compared to 8-week-old mice. In 8-week-old mice, the number of animals with intermediate $\text{T}{}_{\mathrm{<mtext>built1</mtext><mtext>2</mtext>}}$ was smallest when SJL was the maternal animal [(SJL × A); SJL × (A × SJL); SJL × (SJL × A)]. There was no link between $\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of tolerant animals and either the immunoglobulin allotype (MuAl/MuA2) or the C5 eniotype (MuB1 positive/MuB1 negative).     

Effect of calcium ionophore A23187 on electrogenic acid-base transport in turtle bladder Inhibition of acidification and stimulation of alkalinization

Turtle bladders bathed on both surfaces with identical HCO^?₃/CO₂-rich, Cl^?-free Na⁺ media and treated with ouabain and amiloride exhibit a transepithelial potential serosa electronegative to mucosa and a short-circuit current ($\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$) which is a measure of the net luminal acidification rate. Addition of calcium ionophore A23187 (10 μM) to the mucosal side of the epithelium rapidly reverses the direction of the potential difference and $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ and decreases tissue resistance. The resulting positive $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ resembles that previously observed in response to isobutylmethylxanthine (IBMX) and cAMP analogs. Reversal of the $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ is enhanced in bladders from severely alkalotic turtles. In contrast, in severely acidotic turtles, ionophore A23187 decreases, but does not reverse, the $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$. The data suggest that, like IBMX and cAMP analogs, the Ca ionophore stimulates an electrogenic alkalinization mechanism, but, unlike the former agents inhibits the concurrent acidification process as well.     

CNDO/2 calculations of the relative stability of the goniomers in poly(dl-alanine)

Goniomers of single-stranded poly(dl-alanine) $\text{π}{}_{\mathrm{<mtext>dl</mtext>}}$ helices were treated by the semiempircal CNDO/2 MO procedure. The α_R and $\text{α}{}_{\mathrm{<mtext>dl</mtext>}}$ forms were also calculated. From the calculated total energies and partitioned energies we discuss the conformational stablity of goniomers of poly(dl-alanines). The conformational stability of the α and π forms is also compared.     

The explicit analysis of consecutive pseudo-first-order reactions: Application to kinetics of thiolysis of dithiasuccinoyl (Dts) amino acids

An explicit set of general methods for the experimental determination of the rates k₁ and k₂ of consecutive pseudo-first-order reactions is described and discussed. These rely on the direct simultaneous analytical quantitation of the starting material, intermediate, and product of the reaction, and thus differ from present techniques based on measurement of coreactant consumption or coproduct appearance. The quantity $\text{k}{}^{\mathrm{<mtext>env</mtext>}}\text{=}\text{k}{}_1\text{k}{}_2\text{(k}{}_1\text{+ k}{}_2\text{)}$ is shown to define a good “envelope” approximation to product formation according to the simple law 100% [1 ? exp(?k^envt)]. The theory of envelopes is useful for comparing overall rates of reactions with widely differing values of $\text{κ =}\text{k}{}_2\text{k}{}_1$. The kinetic pattern of thiolysis of dithiasuccinoyl amino acids to carbamoyl disulfide intermediates to product free amino acids is analyzed and shown to agree quantitatively with theory.     

A Cl−/HCO3−-ATPase in the gils of Carassius Auratus. Its inhibition by thiocyanate

An ATPase is demonstrated in plasma membrane fractions of goldfish gills. This enzyme is stimulated by Cl^? and HCO₃^?, inhibited by SCN^?.Biochemical characterization shows that HCO₃^? stimulation ($\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{= 2.5}\text{mequiv./l}$) is specifically inhibited in a competitive fashion by SCN^? ($\text{K}{}_{\mathrm{<mtext>i</mtext>}}\text{= 0.25}\text{mequiv./l}$). The residual Mg²⁺-dependent activity is weakly is weakly affected by SCN^?.In the microsomal fraction chloride stimulation of the enzyme occurs in the presence of HCO₃^? ($\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{for chloride = 1 mequiv./l}$); no stimulation is observed in the absence of HCO₃^?. Thiocyanate exhibits a mixed type of inhibition ($\text{K}{}_{\mathrm{<mtext>i</mtext>}}\text{= 0.06}\text{mequiv./l}$) towards the Cl^? stimulation of the enzyme.Bicarbonate-dependent ATPase from the mitochondrial fraction is stimulated by Cl^?, but this enzyme has a relatively weak affinity for this substrate ($\text{K}{}_{\mathrm{<mtext>m</mtext>}}\text{= 14}\text{mequiv./l}$).     

Proton nuclear magnetic resonance studies of the manganese (II) binding site of concanavalin A. Effect of saccharides on the solvent relaxation rates

The longitudinal relaxation rate ($\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$) of water protons was studied in solutions of Mn(II)-concanavalin A at a number of frequencies. These relaxation rates were lowered in the presence of a variety of saccharides which have affinities for concanavalin A which range over two orders of magnitude. A good correlation was found in which saccharides which bind tightly have the greatest effect and saccharides which bind weakly or not at all have little effect on the $\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ values. The temperature dependence of the proton relaxation rates showed that the lowering of these rates in the presence of saccharides was most likely due to a change in the exchange rate of solvent interacting with protein-bound Mn(II), $\text{1}\text{T}{}_{\mathrm{<mtext>m</mtext>}}$.An analysis of the temperature and frequency dependence of the $\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ and $\text{1}\text{T}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext>}}$ (transverse) solvent proton relaxation rates resulted in evaluation of a number of parameters for solvent water molecules interacting in the first coordination sphere of Mn(II) bound to concanavalin A. The ratio of the number of water molecules (q) to the Mn(II)-proton distance (r) obtained from a computer fit of the data over a limited temperature range is in accord with the findings of Koenig et al. ((1973) Proc. Nat. Acad. Sci.70, 475) and Meirovitch and Kalb ((1973) Biochim. Biophys. Acta303, 258). However, our studies of $\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ and $\text{1}\text{T}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext>}}$ of water over a more extensive temperature range are best fit with the following conclusions: at low temperatures (<20 °C), the data are consistent with an outer-sphere relaxation process. At higher temperatures (> 30 °C), the water molecule in the inner coordination sphere of the bound Mn(II) begins exchanging more rapidly and contributes to the relaxation processes ($\text{1}\text{T}{}_{\mathrm{<mtext>1</mtext><mtext>p</mtext>}}$ and $\text{1}\text{T}{}_{\mathrm{<mtext>2</mtext><mtext>p</mtext>}}$). The relaxation time of protons in the inner coordination shell, T_1M, contributes over the entire temperature range and produces a frequency dependence in the relaxivity data from 6 to 100 MH_z since the contributions to the correlation times are in the range 10^?9-10^?8 sec.     

Effects of oligomycin and quercetin on the hydrolytic activities of the (Na+ + K+)-dependent ATPase

Quercetin inhibited a dog kidney ($\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}\text{)-}\text{ATPase}$ preparation without affecting $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for ATP or $\text{K}{}_{0.5}$ for cation activators, attributable to the slowly-reversible nature of its inhibition. Dimethyl sulfoxide, a selector of E₂ enzyme conformations, blocked this inhibition, while the K⁺-phosphatase activity was at least as sensitive to quercetin as the ($\text{Na}{}^{\mathrm{+}}\text{+}\text{K}{}^{\mathrm{+}}\text{)-}\text{ATPase}$ activity, all consistent with quercetin favoring E₁ conformations of the enzyme. Oligomycin, a rapidly-reversible inhibitor, decreased the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for ATP and the $\text{K}{}_{0.5}$ for cation activators, and its inhibition was also diminished by dimethyl sulfoxide. Although oligomycin did not inhibit the K⁺-phosphatase activity under standard assay conditions, a reaction presumably catalyzed by E₂ conformations, its effects are nevertheless accommodated by a quantitative model for that reaction depicting oligomycin as favoring E₁ conformations. The model also accounts quantitatively for effects of both dimethyl sulfoxide and oligomycin on $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$, $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for substrate, and $\text{K}{}_{0.5}$ for K⁺, as well as for stimulation of phosphatase activity by both these reagents at low K⁺ but high Na⁺ concentrations.     

A fluctuation analysis study of the development of amiloride-sensitive Na+ transport in the skin of larval bullfrogs (Rana catesbeiana)

In the presence of the Na⁺-channel blocker amiloride, the short-circuit current across the skins of bullfrog tadpoles in metamorphic stages XIX–XXIV was subjected to fluctuation analysis. The resulting power spectra contained a Lorentzian component of which the plateau value ($\text{S}{}_0$) decreased while the corner frequency ($\text{f}{}_{\mathrm{<mtext>c</mtext>}}$) increased as the mucosal amiloride concentration was increased from 0.5 to 24 μM. From the linear relationship between the $\text{f}{}_{\mathrm{<mtext>c</mtext>}}$ values and the amiloride concentrations it was possible to determine the binding ($\text{k′}{}_{01}$) and unbinding ($\text{k}{}_{10}$) constants for amiloride to its receptor on the Na⁺ channel. With these parameters as well as short-circuit current and $\text{S}{}_0$ values, the current through the individual Na⁺ channels ($\text{i}$) was calculated (average 0.58 pA). It did not increase significantly during late metamorphosis. The density of Na⁺ channels ($\text{M}$) in the apical membrane, on the other hand, increased significantly. It would appear that the increase in short-circuit current which occurs at this time is due primarily to an increase in amiloride-blockable Na⁺ channels. Unexpectedly, a Lorentzian component could be fitted to power spectra in amiloride-treated skins (stages XIX–XXI) which showed no amiloride-sensitive short-circuit current. Moreover, the typical increase in $\text{f}{}_{\mathrm{<mtext>c</mtext>}}$ with the amiloride concentration did not occur in these animals.     

Phase transition characteristics of diphosphatidylglycerol (cardiolipin) and stereoisomeric phosphatidyldiacylglycerol bilayers: Mono- and divalent metal ion effects

Synthesis and phase transition characteristics of aqueous dispersions of the homologous (12 : 0, 14 : 0, 16 : 0) diphosphatidylglycerols (cardiolipins) and phosphatidyldiacylglycerols are reported. Electron microscopy of the negatively stained aqueous dispersions reveals a characteristic lamellar structure suggesting that these phospholipid molecules are organized as bilayers in the aqueous dispersions. The phase transition temperature ($\text{T}{}_{\mathrm{<mtext>m</mtext>}}$) and the enthalpy of transition ($\text{ΔH}$) increase monotonically with chain length in the cardiolipin and phosphatidyldiacylglycerol series; $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ for phosphatidyldiacylglycerol is higher than that for cardiolipin of the same chain-length. The transition temperatures for the enantiomeric $\text{sn-3,3-}\text{and}\text{sn-1,1-}\text{phosphatidyldiacylglycerol}$ and for the diastereomeric, $\text{meso}\text{-sn-1,3-}\text{phosphatidyldiacylglycerol}$ are approximately the same. The molar enthalpy for the transition of cardiolipin-NH₄⁺ bilayers is approximately twice the value for the phosphatidylcholines of the same chain length, i.e., the molar enthalpy per acyl chain is approximately the same in the two systems. The transition temperatures for metal ion salts of C_{1 6}-cardiolipin exhibit a biphasic dependence upon the unhydrated ionic radii, i.e. the highest $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ is observed for Ca²⁺- cardiolipin and decreases for the salts of ions with smaller and larger ionic radii than that of Ca²⁺. The lowest $\text{T}{}_{\mathrm{<mtext>m</mtext>}}$ is observed for Rb⁺-cardiolipin. Monovalent metal salts of cardiolipin exhibit two phase transitions. This effect may result from different conformational packing of the four acyl chains due to differences in metal-phosphate binding.     

In vitro reaction of radioactive 7beta, 8alpha, --dihydroxy--9alpha, 10alpha-epoxy--7,8,9,10--tetrahydrobenzo (A) pyrene and 7beta,8alpha--dihydroxy--9beta, 10beta--epoxy--7,8,9,10--tetrahydrobenzo (A) pyrene with DNA.

The $\text{in vitro}$ reaction of bacteriophage T7-DNA with the radioactive diastereomeric benzo(a)pyrene-diol-epoxides, (±) [${}^3\text{H}{}_9$, ${}^3\text{H}{}_{10}$]-7β,8α-dihydroxy-9α,10β-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene, and (±) [${}^3\text{H}{}_9$, ${}^3\text{H}{}_{10}$]-7β,8α-dihydroxy-9β,19β-epoxy-7,8,9,10-tetrahydrobenzo(1)pyrene, was investigated. Chromatographic analysis of digests of the DNA allowed the distinction of characteristic deoxynucleoside adduct peaks for the two benzo(a)pyrene-diol-epoxides. Our results, together with data from the literature, allow the identification of these adducts as mostly N₂-(10-7β,8α,9α-trihydroxy-7,8,9,10-tetrahydrobenzo(a)pyreney1)deoxyguanosine and N₂-(10-7β,8α,9β-trihydroxy-7,8,9,10-tetrahydrobenzo(a)pyreney1)deoxyguanosine, respectively. DNA-benzo(a)pyrene adducts with the same chromatographic properties were formed in mouse embryo fibroblasts upon treatment with benzo(a)pyrene.     

A chloride requirement for Na+-dependent amino-acid transport by brush border membrane vesicles isolated from the intestine of a mediterranean teleost (Boops salpa)

The uptake of d-glucose, 2-aminoisobutyric acid and glycine was studied with intestinal brush border membrane vesicles of a marine herbivorous fish: Boops salpa. The uptake of these three substances is stimulated by an Na⁺ electrochemical gradient ($\text{C}{}_{\mathrm{<mtext>out</mtext>}}\text{C}{}_{\mathrm{<mtext>in</mtext>}}$). For glucose, an increase of the electrical membrane potential generated by a concentration gradient of the liposoluble anion, SCN^?, increases the Na⁺-dependent transport. This responsiveness to the membrane potential was confirmed by valinomycin. Differently from glucose, uptake of glycine and 2-aminoisobutyric acid requires, besides the Na⁺ gradient, the presence of Cl^? on the external side of the vesicles. In the absence of Cl^?, amino acid uptake is not stimulated by the Na⁺ gradient and is not influenced by an electrical membrane potential generated by SCN^? gradient ($\text{C}{}_{\mathrm{<mtext>out</mtext>}}\text{>C}{}_{\mathrm{<mtext>in</mtext>}}$) or by a K⁺ diffusion potential ($\text{C}{}_{\mathrm{<mtext>in</mtext>}}\text{>C}{}_{\mathrm{<mtext>out</mtext>}}$). This Cl^? requirement differs from the Na⁺ requirement, since a Cl^? gradient ($\text{C}{}_{\mathrm{<mtext>out</mtext>}}\text{>C}{}_{\mathrm{<mtext>in</mtext>}}$) does not result in an accumulation of glycine or 2-aminoisobutyric acid similar to that produced by an Na⁺ gradient.     

On the mechanism of the reaction of tris(hydroxymethyl)aminomethane with activated carbonyl compounds: A model for the serine proteinases

The pH dependence of the reaction of tris(hydroxymethyl)aminomethane (Tris) with the activated carbonyl compound 4-trans-benzylidene-2-phenyloxazolin-5-one (I) is given by the equation $\text{k′}{}_2\text{=}\text{k}{}_{\mathrm{<mtext>b</mtext>}}\text{K}{}_{\mathrm{<mtext>a</mtext>}}\text{(K}{}_{\mathrm{<mtext>a</mtext>}}\text{+ [}\text{H}{}^{\mathrm{+}}\text{])}\text{+}\text{k}{}_{\mathrm{<mtext>a</mtext>}}\text{[}\text{OH}{}^{\mathrm{?}}\text{]K}{}_{\mathrm{<mtext>a</mtext>}}\text{(K}{}_{\mathrm{<mtext>a</mtext>}}\text{+ [}\text{H}{}^{\mathrm{+}}\text{])}$, where K_a is the dissociation constant of TrisH⁺. Spectrophotometric experiments show that the Tris ester of α-benzamido-trans-cinnamic acid is formed quantitatively over a range of pH values, regardless of the relative contribution of k_b and k_a terms to k′₂. Hence, both terms refer to alcoholysis. While the mechanism of the reaction is not determined unequivocally in the present work, the magnitude of the k_b term, together with its dependence on the basic form of Tris, suggests that ester formation is occurring by nucleophilic attack of a Tris hydroxyl group on the carbonyl carbon of the oxazolinone, with intramolecular catalysis by the Tris amino group. The rate enhancement due to this group is at least 10² and possibly of the order 10⁶. This system is compared with other model systems for the acylation step of catalysis by serine esterases and proteinases.     

Thiophosphorylation of (Na + K+)-ATPase yields an ADP-sensitive phosphointermediate

(1) Treatment of $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ from rabbit kidney outer medulla with the $\text{γ-}{}^{35}\text{S}$ labeled thio-analogue of ATP in the presence of $\text{Na}{}^{\mathrm{+}}\text{+ Mg}{}^{\mathrm{2+}}$ and the absence of K⁺ leads to thiophosphorylation of the enzyme. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ value for [γ-S]ATP is 2.2 μM and for Na⁺ 4.2 mM at 22°C. Thiophosphorylation is a sigmoidal function of the Na⁺ concentration, yielding a Hill coefficient $\text{n}\text{H}\text{= 2.6}$. (2) The thio-analogue ($\text{K}{}_{\mathrm{<mtext>m</mtext><mtext>\; =\; 35\; \mu M</mtext>}}$) can also support overall $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ activity, but $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ at 37°C is only $\text{1.3 γ}\text{mol}\text{· (mg protein)}{}^{\mathrm{?}}\text{·}$ h^?1 or 0.09% of the specific activity for ATP ($\text{K}{}_{\mathrm{<mtext>m</mtext><mtext>\; =\; 0.43\; mM</mtext>}}$). (3) The thiophosphoenzyme intermediate, like the natural phosphoenzyme, is sensitive to hydroxylamine, indicating that it also is an acylphosphate. However, the thiophosphoenzyme, unlike the phosphoenzyme, is acid labile at temperatures as low as 0°C. The acid-denatured thiophosphoenzyme has optimal stability at pH 5–6. (4) The thiophosphorylation capacity of the enzyme is equal to its phosphorylation capacity, indicating the same number of sites. Phosphorylation by ATP excludes thiophosphorylation, suggesting that the two substrates compete for the same phosphorylation site. (5) The (apparent) rate constants of thiophosphorylation (0.4 s^?1 vs. 180 s^?1), spontaneous dethiophosphorylation (0.04 s^?1 vs. 0.5 s^?1) and K⁺-stimulated dethiophosphorylation (0.54 s^?1 vs. 230 s^?1) are much lower than those for the corresponding reactions based on ATP. (6) In contrast to the phosphoenzyme, the thiophosphoenzyme is ADP-sensitive (with an apparent rate constant in ADP-induced dethiophosphorylation of 0.35 s^?1, $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$$\text{ADP = 48 μM at 0.1 mM ATP}$) and is relatively K⁺-insensitve. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for K⁺ in dethiophosphorylation is 0.9 mM and in dephosphorylation 0.09 mM. The thiophosphoenzyme appears to be for 75–90% in the ADP-sensitive E₁-conformation.     

Amiloride stimulation of sodium transport in the presence of calcium and a divalent cation chelator

Amiloride in nM to μM concentrations stimulates the short circuit current ($\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$) of the toad urinary bladder by as much as 120% when applied in conjunction with apical Ca²⁺ and a divalent cation chelator. A significant decrease in transepithelial resistance ($\text{R}{}_{\mathrm{<mtext>t</mtext>}}$) is observed simultaneously. This response is spontaneously reversible and its amplitude is dependent upon apical sodium concentrations. The stimulated $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ persisted when acetazolamide (1 mM) was introduced, HPO₄^2? substituted for HCO₃^? or SO₄^2? replaced Cl^?. Consequently, the increase in $\text{I}{}_{\mathrm{<mtext>sc</mtext>}}$ is not due to the change of Cl^?, H⁺ or HCO₃^? flux. This behavior in a ‘tight’ epithelium may be related to the mechanism controlling apical sodium permeability.     

Phlorizin as a probe of the small-intestinal Na+,d-glucose cotransporter. A model

(1)‘Uptake’ of phlorizin by intestinal brush border membrane vesicles is stimulated, much as that of d-glucose, by the simultaneous presence of Na_out⁺ and $\text{Δψ?0}$. However, phlorizin contrary to d-glucose, fulfills all criteria of a non-translocated ligand (i.e., of a fully competitive inhibitor) of the Na⁺,d-glucose cotransporter. (2) The stoicheiometry of Na⁺/phlorizin binding is 1, as shown by a Hill coefficient of approx. 1 in the Na_out⁺-dependence of phlorizin binding. (3) The preferred order of binding at $\text{Δψ?0}$ is Na⁺ first, phlorizin second (4) The velocity of association of phlorizin to the cotransporter, but not the velocity of its dissociation therefrom, responds to Δψ. These observations while agreeing with the effect of $\text{Δψ?0}$ on the $\text{K}{}_{\mathrm{<mtext>d</mtext>}}$ of phlorizin binding in the steady-state time range, also confirm that the mobile part of the cotransporter bears a negative charge of 1. (5) A model is proposed describing the Na⁺,Δψ-dependent interaction of phlorizin with the cotransporter and agreeing with a more general model of Na⁺,d-glucose cotransport. (6) The $\text{k}{}_{\mathrm{<mtext>on</mtext>}}$, $\text{k}{}_{\mathrm{<mtext>off</mtext>}}$ and $\text{K}{}_{\mathrm{<mtext>d</mtext>}}$ constants of phlorizin interaction with the Na⁺,d-glucose cotransporter are smaller in the kidney than in the small-intestinal brush border membrane, which results in a number of quantitative differences in the overall behaviour of the two systems.     

H/2H isotope effect in redox reactions of cytochrome c

The rate of reaction of ferro- and ferricytochrome c (C(II) and C(III)) with ferri- and ferrocyanide and of C(III) with O₂^? and CO₂^? was determined in H₂O and in ²H₂O in the temperature range 5–35 °C. No isotope effect was evident in any of the reductions of C(III); the apparent energy of activation was identical in H₂O and ²H₂O. An isotope effect with , depending on pH for instance was observed in the oxidation of C(II), in the slow phase of oxidation which involves conformational changes. An interpretation (supported by evidence from previous work) involving water molecules in the close vicinity of the reaction site on the protein is discussed.