Steady-state kinetics of ADP-arsenate and ATP-synthesis in Rhodospirillum rubrum chromatophores

(1) Rates of ATP synthesis and ADP-arsenate synthesis catalyzed by Rhodospirillum rubrum chromatophores were determined with the firefly luciferase method and by a coupled enzyme assay involving hexokinase and glucose-6-phosphate dehydrogenase. (2) V_m for ADP-arsenate synthesis was about 2-times lower than V_m for ATP-synthesis. With saturating [ADP], K(As_i) was about 20% higher than K(P_i). With saturating [anion], K(ADP) was during arsenylation about 20% lower than during phosphorylation. (3) Plots of $\text{1}\text{v}$ vs. $\text{1}\text{[}\text{substrate}\text{]}$ were non-linear at low concentrations of the fixed substrate. The non-linearity was such as to suggest a positive cooperativity between sites binding the variable substrate, resulting in an increased $\text{V}{}_{\mathrm{<mtext>m</mtext>}}\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ ratio. High concentrations of the fixed substrate cause a similar increase in $\text{V}{}_{\mathrm{<mtext>m</mtext>}}\text{K}{}_{\mathrm{<mtext>m</mtext>}}$, but abolish the cooperativity of the sites binding the variable substrate. (4) Low concentrations of inorganic arsenate (As_i) stimulate ATP synthesis supported by low concentrations of P_i and ADP about 2-fold. (5) At high ADP concentrations, the apparent K_i of As_i for inhibition of ATP-synthesis was 2–3-times higher than the apparent K_m of As_i for arsenylation; the apparent K_i of P_i for inhibition of ADP-arsenate synthesis was about 40% lower than the apparent K_m of P_i for ATP synthesis. (6) The results are discussed in terms of a model in which P_i and As_i compete for binding to a catalytic as well as an allosteric site. The interaction between these sites is modulated by the ADP concentration. At high ADP concentrations, interaction between these sites occurs only when they are occupied with different species of anion.     

The effect of ammonium ions upon isolated reactions of the glycolytic scheme

In the glycolytic system derived from rat brain acetone powder, ammonium ion has been found to stimulate three different reactions: (a) the transphosphorylase reaction from phosphoenolpyruvate, (b) the phosphohexokinase reaction, and (c) the hexokinase reaction. The transphosphorylases are affected differently depending upon whether adenosine diphosphate or adenylic acid is the phosphate acceptor; in the case of the latter, the dependency upon $\text{NH}{}_4{}^{\mathrm{+}}$ is particularly marked. A highly active myokinase is present in these extracts and its activity influences the transphosphorylase reaction to a considerable extent. The phosphohexokinase reaction is stimulated to a greater extent by $\text{NH}{}_4{}^{\mathrm{+}}$ than is the hexokinase reaction. In contrast to these reactions which require the participation of the adenylic system, triose phosphate oxidase activity is uninfluenced by the presence of $\text{NH}{}_4{}^{\mathrm{+}}$.     

Asymmetric or symmetric? Cytosolic modulation of human erythrocyte hexose transfer

(1) The Michaelis-Menten parameters for hexose transfer in erythroctes, erythrocyte ghosts and inside-out vesicles at 20°C were determined using the light scattering method of Sen and Widdas ((1962) J. Physiol. 160, 392–403). (2) The external $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for infinite-cis exit of d-glucose in cells and ghosts is $\text{3.6 ± 0.5}\text{mM}$. (3) Dilution of cellular solute (up to × 90 dilution) by lysing and resealing cells in varying volumes of lysate is without effect on the $\text{V}{}_{\mathrm{<mtext>m</mtext>}}$ for net d-glucose exit. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for net exit, however, falls from $\text{32.4 ± 3.7}\text{mM}$ in intact cells to $\text{12.9 ± 2.3}\text{mM}$ in ghosts. This effect is reversible. (4) Infinite-cis net d-glucose uptake measurements in cells and ghosts reveal the presence of a low $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$, high affinity internal site of $\text{5.9 ± 0.8}\text{mM}$. The $\text{V}{}_{\mathrm{<mtext>m</mtext>}}$ for net glucose entry increases from $\text{23.2 ± 3.7}\text{mmol/l per min}$ in intact cells to $\text{55.4 ± 6.3}\text{mmol/l per min}$ in ghosts. (5) The external $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for infinite-cisd-glucose exit in inside-out vesicles is $\text{6.8 ± 2.7}\text{mM}$. The kinetics of zero-transd-glucose exit from inside-out vesicles are changed markedly when cellular solute (obtained by lysis of intact cells) is applied to either surface of inside-out vesicles. When solute is present externally, the $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ and $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ for zero-trans exit are decreased by up to 10-fold. When solute is present at the interior of inside-out vesicles, $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ for zero-trans exit is reduced; $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for exit is unaffected. In the nominal absence of cell solute, transfer is symmetric in inside-out vesicles. The orientation of transporter in the bilayer is unaffected by the vesiculation procedure. (6) External application of cellular solute to ghosts reduces $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ for d-glucose exit but is without effect on the external $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for infinite-cis exit. (7) The inhibitory potency of cell lysate on hexose transfer is lost following dialysis indicating that the factors responsible for transfer modulation are low molecular weight species. (8) We consider the hexose transfer in human erythrocytes is intrinsically symmetric and that asymmetry of transfer is conferred by interaction of the system with low molecular weight cytosolic factors.     

Regulation of cellular energy metabolism. The Crabtree effect

The Crabtree effect (inhibition of respiration by glycolysis) is observed in cells with approximately equal glycolytic and respiratory capacities for ATP synthesis. Addition of glucose to aerobic suspensions of glucose-starved cells (Sarcoma 180 ascites tumor cells) causes a burst of respiration and lactate production due to ATP utilization for glucose phosphorylation by hexokinase and phosphofructokinase. This burst of activity is followed by inhibition of both respiration and glycolysis, the former to below the value before glucose addition (Crabtree effect). Both the respiratory rate and the glycolytic flux appear to be regulated by the cytosolic $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ albeit by completely different mechanisms. Respiration is regulated by the free energy of hydrolysis of ATP, such that the rate increases as the $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ decreases and decreases as the $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ increases. The regulatory enzymes of glycolysis are activated by ADP (AMP) and P_i and inhibited by ATP. Thus both respiration and glycolysis increase or decrease as the $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ decreases or increases. The parallel regulation of both ATP-producing pathways by this common metabolite ratio is consistent with the cytoplasmic $\text{[}\text{ATP}\text{]}\text{[}\text{ADP}\text{][}\text{P}{}_{\mathrm{i}}\text{]}$ being an important determinant of homeostatic regulation of cellular energy metabolism.     

The reconstitution of a Photosystem II protein complex, P-700-chlorophyll a-protein complex and light-harvesting chlorophyll ab-protein

A Photosystem II reaction centre protein complex was extracted from spinach chloroplasts using digitonin. This complex showed (i) high rates of dichloroindophenol and ferricyanide reduction in the presence of suitable donors, (ii) low-temperature fluorescence at 685 nm with a variable shoulder at 695 nm which increased as the complex aggregated due to depletion of digitonin and (iii) four major polypeptides of 47, 39, 31 and 6 kDa on dissociating polyacrylamide gels. The Photosystem II protein complex, together woth the P-700-chlorophylla protein complex and light-harvesting chlorophyll $\text{a}\text{b-}\text{protein}$ complex (LHCP) also isolated using digitonin, were reconstituted with lipids from spinach chloroplasts to form proteoliposomes. The low-temperature (77 K) fluorescence properties of the various proteoliposomes were analysed. The $\text{F}{}_{685}\text{F}{}_{695}$ ratios of the Photosystem II reaction centre protein complex-liposomes decreased as the lipid to protein ratios were increased. The $\text{F}{}_{681}\text{F}{}_{697}$ ratios of LHCP-liposomes were found to behave similarly. Light excitation of chlorophyll b at 475 nm stimulated emission from both the Photosystem II protein complex (F₆₈₅ and F₆₉₅) and the P-700-chlorophyll a-protein complex (F₇₃₅) when LHCP was reconstituted with either of these complexes, demonstrating energy transfer between LHCP and PS I or II complexes in liposomes. No evidence was found for energy transfer from the PS II complex to the P-700-chlorophyll a-protein complex reconstituted in the same proteoliposome preparation. Proteoliposome preparations containing all three chlorophyll-protein complexes showed fluorescence emission at 685, 700 and 735 nm.     

The covariance of relatives derived from a random mating population.

Attention is drawn to errors common in the derivation of forms for the genotypic covariance of noninbred relatives from a Hardy-Weinberg population of diploids. A synthesis of Fisher's least-squares method of partitioning the genotypic variance and Malécot's probability method of expressing kinship, yields a general form. For one locus, the form is $\text{(P}{}_{\mathrm{ss}}\text{+ P}{}_{\mathrm{sd}}\text{+ P}{}_{\mathrm{ds}}\text{+ P}{}_{\mathrm{dd}}\text{)}\text{1}\text{2}\text{σ}{}_{\mathrm{a}}{}^2\text{+ (P}{}_{\mathrm{ss}}\text{P}{}_{\mathrm{dd}}\text{+ P}{}_{\mathrm{sd}}\text{P}{}_{\mathrm{ds}}\text{) σa}{}_{\mathrm{d}}{}^2$, where σ_a² is the additive genetic variance, α_d² is the variance of dominance deviations, p_ij is the probability that parental gamete i is identical by descent to parental gamete j, i = s, d indexes the parents of one relative, and j = s, d indexes those of the other. The form provides a framework for obtaining the covariance of relatives from an equilibrium population with linkage.     

A study of the sulfhydryl groups of rat brain hexokinase

Rat brain hexokinase (ATP: d-hexose-6-phosphotransferase, EC 2.7.1.1) is rapidly inactivated by reaction with 5,5′-dithiobis-(2-nitrobenzoate). The inactivation follows monophasic first-order kinetics in either the absence of ligands (k = 0.641 min^?1 at 25 °C) or in the presence of saturating levels of ATP (free or complexed with Mg²⁺) or P₁; the inactivation rate is slightly increased ($\text{k ? 0.7}$ min ^?1) in the presence of ATP or P₁. In contrast, glucose and glucose-6-P markedly decrease the inactivation rate; inactivation in the presence of these ligands is biphasic, with two first-order rates ($\text{k ? 0.5}$ min^?1 and 0.01 min^?1) being distinguishable.The enzyme contains 14 sulfhydryl groups which react with 5,5′-dithiobis-(2-nitrobenzoate); reaction of these groups in the native enzyme is complete after 2 hr at 25 °C, or in approx 5 min with the urea or guanidine-denatured enzyme. In the native enzyme, three classes of sulfhydryl groups are distinguishable and are designated as F-, I-, or S-type based on their fast (k ? 0.7 min^?1), intermediate ($\text{k ? 0.5-0.7}$ min^?1), or slow ($\text{k ? 0.02}$ min^?1 rates of reaction with 5,5′-dithiobis-(2-nitrobenzoate). The correlation of inactivation rates with the rates for reaction of the I-type sulfhydryls indicates that the I-type sulfhydryls include residues necessary for catalytic activity. The F-type residues are clearly not required for activity.The effects of ATP, P₁, glucose, and glucose-6-P on the reactivity of the sulfhydryls have been determined. As in the absence of ligands, S-, I-, and F-type sulfhydryls could be distinguished. In the presence of saturating concentrations of these ligands, the F, I, and S classes of sulfhydryls contained respectively: with ATP, 1, 4, and 7 residues; with P₁, 1, 3, and 7 residues; with glucose, 1, 2, and 5 residues; with glucose-6-P, 1, 2, and 1 residues. Comparison with rate constants for inactivation in the presence of these ligands again indicated that I-type sulfhydryls were particularly important in maintenance of enzyme activity. The present results indicate considerable similarity between the reactivity of the sulfhydryl residues in rat brain hexokinase and the sulfhydryls of the bovine brain enzyme [V. D. Redkar and U. W. Kenkare (1972), J. Biol. Chem., 247, 7576–7584].     

Oxidation of anionic nitroalkanes by flavoenzymes,and participation of superoxide anion in the catalysis

The reactivities of anionic nitroalkanes with 2-nitropropane dioxygenase of Hansenula mrakii, glucose oxidase of Aspergillus niger, and mammalian d-amino acid oxidase have been compared kinetically. 2-Nitropropane dioxygenase is 1200 and 4800 times more active with anionic 2-nitropropane than d-amino acid oxidase and glucose oxidase, respectively. The apparent K_m values for anionic 2-nitropropane are as follows: 2-nitropropane dioxygenase, 1.61 mm; glucose oxidase, 16.7 mm; and d-amino acid oxidase, 11.1 mm. Anionic 2-nitropropane undergoes an oxygenase reaction with 2-nitropropane dioxygenase and glucose oxidase, and an oxidase reaction with d-amino acid oxidase. In contrast, anionic nitroethane is oxidized through an oxygenase reaction by 2-nitropropane dioxygenase, and through an oxidase reaction by glucose oxidase. All nitroalkane oxidations by these three flavoenzymes are inhibited by Cu and Zn-superoxide dismutase of bovine blood, Mn-superoxide dismutases of bacilli, Fe-superoxide dismutase of Serratia marcescens, and other $\text{O}{}_2{}^{\mathrm{?}}$ scavengers such as cytochrome c and NADH, but are not affected by hydroxyl radical scavengers such as mannitol. None of the $\text{O}{}_2{}^{\mathrm{?}}$ scavengers tested affected the inherent substrate oxidation by glucose oxidase and d-amino acid oxidase. Furthermore, the generation of $\text{O}{}_2{}^{\mathrm{?}}$ in the oxidation of anionic 2-nitropropane by 2-nitropropane dioxygenase was revealed by ESR spectroscoy. The ESR spectrum of anionic 2-nitropropane plus 2-nitropropane dioxygenase shows signals at g₁ = 2.007 and g₁₁ = 2.051, which are characteristic of $\text{O}{}_2{}^{\mathrm{?}}$. The $\text{O}{}_2{}^{\mathrm{?}}$ generated is a catalytically essential intermediate in the oxidation of anionic nitroalkanes by the enzymes.     

Studies of the nucleotide-binding sites on the mitochondrial F1-ATPase through the use of a photoactivable derivative of adenylyl imidodiphosphate

(1) $\text{N-4-}\text{Azido}\text{-2-}\text{nitrophenyl}\text{-γ-[}{}^3\text{H]aminobutyryl-Ado}\text{PP[}\text{NH}\text{]P(}\text{NAP}{}_4\text{-}\text{Ado}\text{PP[}\text{NH}\text{]P)}$ a photoactivable derivative of 5-adenylyl imidodiphosphate (AdoPP[NH]P), was synthesized. (2) Binding of ³H]NAP₄-AdoPP[NH]P to soluble ATPase from beef heart mitochrondria (F₁) was studied in the absence of photoirradiation, and compared to that of $\text{[}{}^3\text{H]Ado}\text{PP[}\text{NH}\text{]P}$. The photoactivable derivative of AdoPP[NH]P was found to bind to F₁ with high affinity, like AdoPP[NH]P. Once $\text{[}{}^3\text{H]NAP}{}_4\text{-}\text{Ado}\text{PP[}\text{NH}\text{]P}$ had bound to F₁ in the dark, it could be released by AdoPP[NH]P, ADP and ATP, but not at all by NAP₄ or AMP. Furthermore, preincubation of F₁ with unlabeled AdoPP[NH]P, ADP, or ATP prevented the covalent labeling of the enzyme by $\text{[}{}^3\text{H]NAP}{}_4\text{-}\text{Ado}\text{PP[}\text{NH}\text{]P}$ upon photoirradiation. (3) Photoirradiation of F₁ by $\text{[}{}^3\text{H]NAP}{}_4\text{-}\text{Ado}\text{PP[}\text{NH}\text{]P}$ resulted in covalent photolabeling and concomitant inactivation of the enzyme. Full inactivation corresponded to the binding of about 2 mol $\text{[}{}^3\text{H]NAP}{}_4\text{-}\text{Ado}\text{PP[}\text{NH}\text{]P}\text{mol F}{}_1$. Photolabeling by NAP₄-AdoPP[NH]P was much more efficient in the presence than in the absence of MgCl₂. (4) Bound $\text{[}{}^3\text{H]NAP}{}_4\text{-}\text{Ado}\text{PP[}\text{NH}\text{]P}$ was localized on the α- and β-subunits of F₁. At low concentrations (less than 10 μM), bound $\text{[}{}^3\text{H]NAP}{}_4\text{-}\text{Ado}\text{PP[}\text{NH}\text{]P}$ was predominantly localized on the α-subunit; at concentrations equal to, or greater than 75 μM, both α- and β-subunits were equally labeled. (5) The extent of inactivation was independent of the nature of the photolabeled subunit (α or β), suggesting that each of the two subunits, α and β, is required for the activity of F₁. (6) The covalently photolabeled F₁ was able to form a complex with aurovertin, as does native F₁. The ADP-induced fluorescence enhancement was more severely inhibited than the fluorescence quenching caused by ATP. The percentage of inactivation of F₁ was virtually the same as the percentage of inhibition of the ATP-induced fluorescence quenching, suggesting that fluorescence quenching is related to the binding of ATP to the catalytic site of F₁.     

Kinetics of histone hyperacetylation and deacetylation in human diploid fibroblasts

We have utilized sodium butyrate-treated normal human diploid fibroblasts to study core histone hyperacetylation kinetics. We report a small, distinct population of core histone characterized by a very rapid rate of hyperacetylation ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈10–15}\text{min}$ for monoacetylated histone H4) compared to the slower rate ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{≈140–200}\text{min}$ for monoacetylated H4) observed for bulk histone. Two rates of core histone deacetylation were also detected and we demonstrated that the rapidly hypermodified histone H4 population was also rapidly deacetylated. The kinetics of histone H4 hyperacetylation and deacetylation in these cells were not significantly altered, regardless of whether cultures were exponentially growing, confluent or arrested in an essentially non-mitotic state.     

Stereoselective preparation of deuterated reduced nicotinamide adenine nucleotides and substrates by enzymatic synthesis.

A-Side (4-R)-(4-²H)-reduced nicotinamide adenine dinucleotide (NADD) was prepared by a stepwise oxidation of ethanol-d₆ to acetate in the presence of NAD, alcohol dehydrogenase, and aldehyde dehydrogenase. The B-side (4-S) isomer of NADD was prepared using the glucose dehydrogenase activity of glucose-6-phosphate dehydrogenase to oxidize to oxidize glucose-1-d in 40% dimethyl aulfoxide. Subsequent purifieation of the reduced nucleotides was achieved using a column of strongly basic polystyrene macroporous resin (AG MP-1) eluted with 0.2 m LiCl, pH 10, and applying the pooled NADD peak to a polyacrylamide gel (Bio-Gel P-2) column. The final $\text{A}{}_{260}\text{A}{}_{340}$ ratio obtained for these preparations was below 2.3. Preparation of the deuterated reduced nucleotides in this manner allows production of specifieally deuterated substrates by coupled enzymatic synthesis. L-Malate-2-d was prepared by coupled synthesis of A-side NADD to the reduction of oxaloacetate by the A-side enzyme malate dehydrogenase.     

Effect of para-chloromercuribenzenesulfonic acid and temperature on cell water osmotic permeability of proximal straight tubules

The apparent Arrhenius energy of activation ($\text{E}{}_{\mathrm{<mtext>a</mtext>}}$) of the water osmotic permeability ($\text{P}{}_{\mathrm{<mtext>os</mtext>}}{}^{\mathrm{<mtext>c</mtext>}}$) of the basolateral plasma cell membrane of isolated rabbit proximal straight tubules has been measured under control conditions and after addition of 2.5 mM of the sulfhydryl reagent, para-chloromercuribenzenesulfonic acid (pCMBS), of mersalyl and of dithiothreitol. $\text{E}{}_{\mathrm{<mtext>a</mtext>}}$ (kcal/mol) was $\text{3.2 ± 1.4}$ (controls) and $\text{9.2 ± 2.2}$ (pCMBS), while $\text{P}{}_{\mathrm{<mtext>os</mtext>}}{}^{\mathrm{<mtext>c</mtext>}}$ decreased with pCMBS to $\text{0.26 ± 0.17}$ of its control value. Mersalyl also decreased $\text{P}{}_{\mathrm{<mtext>os</mtext>}}{}^{\mathrm{<mtext>c</mtext>}}$ both in vitro and in vivo (using therapeutical doses). These actions of pCMBS and mersalyl were quickly reverted with 5 mM dithiothreitol and prevented by 0.1 M thiourea. $\text{E}{}_{\mathrm{<mtext>a</mtext>}}$ for free viscous flow is 4.2 and greater than 10 for non-pore-containing lipid membranes. By analogy with these membranes and with red blood cells, where similar effects of pCMBS on $\text{P}{}_{\mathrm{<mtext>os</mtext>}}$ are observed, it is concluded that cell membranes of the proximal tubule are pierced by aqueous pores which are reversibly shut by pCMBS. Part of the action of mercurial diuretics can be explained by their action on $\text{P}{}_{\mathrm{<mtext>os</mtext>}}{}^{\mathrm{<mtext>c</mtext>}}$.     

Plasmid vectors for high-efficiency expression controlled by the PL promoter of coliphage lambda

A family of plasmid cloning vectors has been constructed that make use of the leftward promoter ($\text{P}{}_{\mathrm{<mtext>L</mtext>}}$) of phage λ to provide for efficient expression of cloned genes in Escherichia coli. The promoter activity of $\text{P}{}_{\mathrm{<mtext>L</mtext>}}$ is fully repressed at low temperature by a thermolabile repressor product of the $\text{λc}\text{I}\text{1857}$ gene, and can be activated by heat induction. Examples are given (β-lactamse, tryptophan synthetase A) where, under optimal conditions, between 30 and 40% of the total protein synthesis is directed by the cloned gene under $\text{P}{}_{\mathrm{<mtext>L</mtext>}}$ control.     

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.     

Ligand-dependent tryptic inactivation of the ouabain sensitivity of ADP-ATP exchange catalyzed by canine renal Na+, K+-ATPase.

Phosphate transporter of bovine heart mitochondria was purified by solubilization of submitochondrial particles with octylglucoside and fractionation of the extract with ammonium sulfate. After reconstitution into liposomes the purified protein catalyzed phosphate transport which was sensitive to mersalyl and other SH reagents. Transport measured either as $\text{P}{}_{\mathrm{i}}\text{OH}$ or $\text{P}{}_{\mathrm{i}}\text{P}{}_{\mathrm{i}}$ exchange was proportional to protein concentration and time. The $\text{P}{}_{\mathrm{i}}\text{OH}$ but not the $\text{P}{}_{\mathrm{i}}\text{P}{}_{\mathrm{i}}$ exchange was stimulated several fold by valinomycin plus nigericin in the presence of K⁺. The reconstituted system provides a suitable assay during purification of the mitochondrial phosphate transporter.     

Turnover of dihydrofolate reductase in rapidly dividing cells

The turnover of dihydrofolate reductase has been studied in rapidly dividing cells of mouse lymphoma L1210 and Lactobacillus casei. Cells in culture were exposed to [¹⁴C]leucine for 24 hr and the subsequent decrease in radioactivity of the enzyme was followed as a function of time. The L1210 enzyme was isolated in pure form by subjecting the cell sonicate to affinity chromatography on amethopterin-Sepharose. The L. casei cells were processed by a multistep procedure which yielded the pure enzyme in both of its principal forms: (I), without TPNH; and (II), containing an equimolal amount of noncovalently bound TPNH. The half-lives ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2></mtext>}}$) of dihydrofolate reductase in the amethopterin-sensitive L1210 cells (L1210/S) and in the cells of a partially resistant subline (L1210/R2), characterized by an 8-fold increase in enzyme level, were 18 and 19 hr. When these cells were grown in the presence of sublethal concentrations of amethopterin, $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ values were increased to 39 and 90 hr. These results suggest that the transient increase in dihydrofolate reductase activity, observed when cells are exposed to amethopterin, is due largely to a decreased susceptibility of the enzyme-inhibitor complex to degradation. Bound TPNH also increases the half-life of dihydrofolate reductase as shown by the fact that forms (I) and (II) of the L. casei enzyme had $\text{t}{}_{\mathrm{<mtext>x1</mtext><mtext>2</mtext>}}$ values of approximately 3 and 9 hr.     

Glucose transport in nongrowing yeast

The inducible, nonenergy-requiring glucose transport system of the yeast Kluyveromyces lactis is inactivated upon starving cells of glucose by (1) transferring logarithmic phase glucose-grown cells to synthetic medium containing a nonglycolytic carbon source, and (2) upon transition of logarithmic phase glucose-grown cells to stationary phase. The steady-state accumulation of nonmetabolizeable 6-deoxyglucose and the apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ of transport of 6-deoxyglucose is the same in stationary phase cells and in logarithmic phase cells. The rate of transport is lower in the nongrowing cells. Restoration of activity requires energy and protein synthesis as well as inducer.     

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.     

A convective mass transfer model for determining intestinal wall permeabilities: laminar flow in a circular tube

A convective mass transfer model as analyzed and developed for use in determining intestinal wall permeabilities from external perfusion experiments. Analysis of the model indicates that the ratio of the exit to inlet concentration $\text{C}{}_{\mathrm{m}}\text{C}{}_0$ is a function of only two dimensionless independent variables, the wall permeability, $\text{P}{}_{\mathrm{w}}{}^1$ and Graetz number, Gz = πDL/2Q. The Graetz number contains the independent variables of interest, length, diflusivity, and flow rate. The radius of the intestine is included implicitly in the flow rate. Since $\text{C}{}_{\mathrm{m}}\text{C}{}_0$ and Gz are the experimental quantities, and the solution to the model system contains P_ω¹ implicitly, a convenient approximate method is developed which allows a direct calculation of P_ω¹. This method is in error by 10–20% in the worst cases. The approach is illustrated by application to the determination of the wall permeabilities for two non polar compounds.