Measurements of cytoplasmic and vacuolar pH in Neurospora using nitrogen-15 nuclear magnetic resonance spectroscopy

The nitrogen-15 chemical shift of the N1 (tau)-nitrogen of 15N-labeled histidine and the half-height line widths of proton-coupled resonances of the delta- and omega,omega'-nitrogens of 15N-labeled arginine and of the alpha-nitrogens of 15N-labeled alanine and proline were measured in intact mycelia of Neurospora crassa to obtain to estimates of intracellular pH. For intracellular 15N-labeled histidine, the N1 (tau)-nitrogen chemical shift was 200.2 ppm. In vitro measurements showed that the chemical shift was slightly affected by the presence of phosphate, with which the basic amino acids may be associated in vivo. These considerations indicate a pH of 5.7-6.0 for the environment of intracellular histidine. The half-height line widths of the delta- and omega,omega'-nitrogens of [15N]arginine were 15 and 26 Hz, respectively. In vitro studies showed that these line widths also are influenced by the presence of phosphate, and, after suitable allowance for this, the line widths indicate pH 6.1-6.5 for intracellular arginine. The half-height line widths for intracellular alanine and proline were 17 and 12 Hz, respectively, which are consistent with an intracellular pH of 7.1-7.2. Pools of histidine and arginine are found principally in the vacuole of Neurospora, most likely in association with polyphosphates. Proline and alanine are cytoplasmic. The results reported here are consistent with these localizations and indicate that the vacuolar pH is 6.1 +/- 0.4 while the cytoplasmic pH is 7.15 +/- 0.10. Comparisons of these estimates with those obtained by other techniques and their implications for vacuolar function are discussed.     

Three-dimensional structure of unstained, frozen-hydrated extended tails of bacteriophage T4

Unsupported, unstained frozen-hydrated extended tails of bacteriophage T4 have been studied by cryo-electron microscopy. Their three-dimensional structure has been reconstructed after correlation and averaging of the information from different particles. While the reconstructions of hydrated tails show all the features found by conventional electron microscopy, they are characterized by an open structure. Individual subunits constituting the axial repeat cannot be outlined unambiguously, as the density connectivity is sensitive to the phase-contrast transfer function effects. In order to minimize these effects, we found that the best data set for three-dimensional reconstruction is composed of layer-lines corrected for the phase-contrast transfer function and an uncorrected equator.     

Thermodynamic linkages in rabbit muscle pyruvate kinase: kinetic, equilibrium, and structural studies

The mechanism of allosteric regulation of rabbit muscle pyruvate kinase (PK) was examined in the presence of the allosteric inhibitor phenylalanine (Phe). Steady-state kinetic, equilibrium binding, and structural studies were conducted to provide a broad data base to establish a reasonable model for the interactions. Phe was shown to induce apparent cooperativity in the steady-state kinetic measurements at pH 7.5 and 23 degrees C. The apparent Km for phosphoenolpyruvate was shown to increase with increasing Phe concentrations. These results imply that Phe reduces the affinity of PK for phosphoenolpyruvate. This conclusion was substantiated by equilibrium binding studies which yielded association constants of phosphoenolpyruvate as a function of Phe concentration. The binding constant of Phe was also determined at pH 7.0 and 23 degrees C. The effect of ligands on the hydrodynamic properties of PK was monitored by difference sedimentation velocity, sedimentation velocity, and equilibrium experiments. The results showed that PK remains tetrameric both in the presence and in the absence of Phe. However, Phe induces a small decrease in the sedimentation coefficient of the enzyme; hence, it suggests a loosening of the protein structure. The accessibility of the sulfhydryl residues of the enzyme also increases in the presence of Phe. Furthermore, the Phe-induced conformational change was approximately 90% complete when only 25% of the binding sites were saturated. This result suggested that the regulatory behavior of PK might satisfactorily be described by the two-state model of Monod-Wyman-Changeux [Monod, J., Wyman, J., & Changeux, J.-P. (1965) J. Mol. Biol. 12, 88-118].     

A new tubulin-binding protein

A new brain protein is described which forms an insoluble complex with tubulin, with concomitant stoichiometric hydrolysis of GTP. The complex contains a maximum of one tubulin-binding protein (MW 52,500) per two tubulin dimers. The tubulin-binding protein (TBP) does not compete with colchicine, but in the presence of microtubule-associated proteins tubulin appeared less accessible to it. Proteins such as TBP might sequester tubulin and thereby function either to inhibit indiscriminate polymerization, or to promote ordered nucleation by maintaining high local concentrations.     

Cytochrome P-450-catalyzed desaturation of valproic acid in vitro. Species differences, induction effects, and mechanistic studies

The cytochrome P-450-mediated desaturation of valproic acid (VPA) to its hepatotoxic metabolite, 2-n-propyl-4-pentenoic acid (4-ene-VPA), was examined in liver microsomes from rats, mice, rabbits and humans. The highest substrate turnover was found with microsomes from rabbits (44.2 +/- 2.7 pmol of product/nmol P-450/15 min), while lower activities were observed in preparations from human, mouse, and rat liver, in that order. Pretreatment of animals with phenobarbital led to enhanced rates of formation of 4-ene-VPA in vitro and yielded induction ratios for desaturation ranging from 2.5 to 8.4, depending upon the species. Comparative studies in the rat showed that phenobarbital is a more potent inducer of olefin formation than either phenytoin or carbamazepine. The mechanism of the desaturation reaction was studied by inter- and intramolecular deuterium isotope effect experiments, which demonstrated that removal of a hydrogen atom from the subterminal C-4 position of VPA is rate limiting in the formation of both 4-ene- and 4-hydroxy-VPA. Hydroxylation at the neighboring C-5 position, on the other hand, was highly sensitive to deuterium substitution at that site, but not to deuteration at C-4. Based on these findings, it is proposed that 4-ene- and 4-hydroxy-VPA are products of a common P-450-dependent metabolic pathway, in which a carbon-centered free radical at C-4 serves as the key intermediate. 5-Hydroxy-VPA, in contrast, derives from an independent hydroxylation reaction.     

Dityrosine is a prominent component of the yeast ascospore wall. A proof of its structure

The yeast ascospore wall consists of four morphologically distinct layers. The hydrophobic surface layers are biogenically derived from the prospore wall and appear dark after OsO4 staining. They seem to be responsible for the stability of the spores against attack by lytic enzymes. By amino acid analysis of acid hydrolysates of ascospore walls, two new peaks were detected, which were shown to be the racemic and meso form, respectively, of dityrosine. The identity of this hitherto unknown component of the yeast ascospore wall with standard dityrosine was proven by 1H NMR and by mass spectrometry. A 13C NMR spectroscopic investigation of the structure of dityrosine confirmed that, in natural dityrosine, the biphenyl linkage is located ortho, ortho to the hydroxyl groups. Following digestion of the inner layers of isolated ascospore walls it was shown that dityrosine is very probably located only in the surface layers. The same conclusion was reached independently by an investigation of spores of a strain homozygous for the mutation gcn1, which lack the outermost layers of the spore wall and were practically devoid of dityrosine. In sporulating yeast, L-tyrosine was readily incorporated into the dityrosine of the ascospore wall. Control experiments involving vegetative a/alpha cells and nonsporulating alpha/alpha cells under sporulation conditions showed that dityrosine is indeed sporulation-specific.     

Cytochrome b561 spectral changes associated with electron transfer in chromaffin-vesicle ghosts

The involvement of cytochrome b561, an integral membrane protein, in electron transfer across chromaffin-vesicle membranes is confirmed by changes in its redox state observed as changes in the absorption spectrum occurring during electron transfer. In ascorbate-loaded chromaffin-vesicle ghosts, cytochrome b561 is nearly completely reduced and exhibits an absorption maximum at 561 nm. When ferricyanide is added to a suspension of these ghosts, the cytochrome becomes oxidized as indicated by the disappearance of the 561 nm absorption. If a small amount of ferricyanide is added, it becomes completely reduced by electron transfer from intravesicular ascorbate. When this happens, cytochrome b561 returns to its reduced state. If an excess of ferricyanide is added, the intravesicular ascorbate becomes exhausted and the cytochrome b561 remains oxidized. The spectrum of these absorbance changes correlates with the difference spectrum (reduced-oxidized) of cytochrome b561. Cytochrome b561 becomes transiently oxidized when ascorbate oxidase is added to a suspension of ascorbate-loaded ghosts. Since dehydroascorbate does not oxidize cytochrome b561, it is likely that oxidation is caused by semidehydroascorbate generated by ascorbate oxidase acting on free ascorbate. This suggests that cytochrome b561 can reduce semidehydroascorbate and supports the hypothesis that the function of cytochrome b561 in vivo is to transfer electrons into chromaffin vesicles to reduce internal semidehydroascorbate to ascorbate.