A co-twin study of the relative effect of birth weight and gestational age on retinopathy of prematurity

PURPOSE

To determine the relative effect of birth weight and gestational age on retinopathy of prematurity (ROP) using preterm twin pairs discordant for birth weight.

METHODS

This study was a retrospective cohort study including 55 consecutive twin pairs of 110 preterm infants (gestational age ≤33 weeks). The outcomes of ROP including occurrence (any stage), severe ROP (stage 3 or more), and clinically significant ROP requiring laser treatment were compared between twins with the lower birth weight from each pair and their co-twins with the higher birth weight. Using twin pairs having different birth weight and identical gestational age, the independent effects of prematurity and intrauterine growth on ROP could be evaluated. Other perinatal morbidities related to prematurity were also compared between twin pairs.

RESULTS

No significant differences in ROP between larger and smaller infants were observed in the twin-paired analysis while analysis on individual infants showed strong association between small birth weight and ROP outcomes. However, in both the larger and smaller infant groups, gestational age of <28 weeks was significantly associated with ROP outcomes. No differences were found between twin pairs regarding other perinatal morbidities including bronchopulmonary dysplasia, respiratory distress syndrome, patent ductus arteriosus, intraventricular hemorrhage, and periventricular leukomalacia.

CONCLUSIONS

Birth weight is not associated with ROP, while gestational age is in the twin-paired study, suggesting that gestational age is a better predictor of ROP than birth weight. This indicates that maturity is more important in the pathogenesis of ROP than intrauterine growth.

Predictive value of intra-amniotic and serum markers for inflammatory lesions of preterm placenta

OBJECTIVE

To compare the relative predictive values of amniotic fluid (AF) matrix metalloproteinase-9 (MMP-9), interleukin-6 (IL-6), and serum C-reactive protein (CRP) for histologic chorioamnionitis and intra-amniotic infection in women with preterm labor or preterm premature rupture of membranes (PROM).

STUDY DESIGN

This retrospective cohort study included 99 consecutive women with preterm labor or preterm PROM (21-35 weeks' gestation) who delivered within 72 h of transabdominal amniocentesis. The AF was cultured for aerobic and anaerobic bacteria and for genital mycoplasmas and was assayed for MMP-9 and IL-6 levels. Maternal serum CRP was measured immediately after amniocentesis. The placentas were examined histologically.

MAIN OUTCOME MEASURES

histologic chorioamnionitis and intra-amniotic infection.

RESULTS

The prevalence of histologic chorioamnionitis and a positive AF culture was 44% (44/99) and 28% (28/99), respectively. In predicting intra-amniotic infection, AF MMP-9 had a significantly higher area under the curve (AUC: 0.94 [95% CI, 0.87-0.98]) than AF IL-6 (0.87 [95% CI, 0.78-0.84]; P < 0.05) and serum CRP (0.76 [95% CI, 0.66-0.84]; P < 0.001) and a higher sensitivity and specificity than serum CRP (P < 0.01, respectively). However, in predicting histologic chorioamnionitis, there were no significant differences in AUCs among the three tests (AF MMP-9: 0.78 [95% CI, 0.68-0.85]; AF IL-6: 0.76 [95% CI, 0.66-0.84]; serum CRP: 0.76 [95% CI, 0.66-0.84]). In a sub-analysis of 71 women without intra-amniotic infection, histologic chorioamnionitis was associated with an elevated serum CRP level (P < 0.05), but not with the level of AF IL-6 or MMP-9 (P = 0.232 and P = 0.402, respectively).

CONCLUSIONS

The AF MMP-9 has a better overall diagnostic performance than the AF IL-6 and maternal serum CRP in predicting intra-amniotic infection. However, the serum CRP level obtained up to 72 h before delivery appears to be an important marker for early identification of histologic chorioamnionitis in women without intra-amniotic infection.

Comparison between sonographic cervical length and Bishop score in preinduction cervical assessment: a randomized trial

OBJECTIVE

To compare sonographically measured cervical length with the Bishop score in determining the requirement for prostaglandin administration for preinduction cervical ripening in nulliparae at term.

METHODS

One hundred and fifty-four women with singleton pregnancies at term who were scheduled for induction of labor were randomly assigned to receive prostaglandin for preinduction cervical ripening based on the Bishop score or sonographic cervical length. A cervix unfavorable for treatment with prostaglandin for preinduction cervical ripening was defined as having either a Bishop score of ≤ 4 or a cervical length of ≥ 28 mm. The primary outcome measures were induction success (defined as an ability to achieve the active phase of labor) and the percentage of patients treated with prostaglandin for preinduction cervical ripening.

RESULTS

The two groups were similar with respect to maternal demographics, gestational age, cervical length, and Bishop score. The rates of induction success and Cesarean delivery, the interval to active phase of labor, and the interval to delivery were also similar in the two groups. However, in the transvaginal ultrasound group (n = 77), prostaglandin was administered to only 36% of the nulliparae compared with 75% of those in the Bishop score group (n = 77) (P < 0.0001).

CONCLUSION

In comparison with the Bishop score, the use of sonographic cervical length for assessing the cervix prior to induction of labor can reduce the need for prostaglandin administration by approximately 50% without adversely affecting the outcome of induction in nulliparae at term if the cut-off values used are a Bishop score of ≤ 4 and a cervical length of ≥ 28 mm.

Non-invasive prediction of intra-amniotic inflammation in women with preterm labor

OBJECTIVE

To develop a model based on non-invasive variables to predict the probability of intra-amniotic inflammation in women with preterm labor and intact membranes.

METHODS

Transvaginal ultrasonography and digital examination for the assessment of cervical length and cervical dilatation were performed, and maternal blood was collected for the determination of C-reactive protein and white blood cell (WBC) count immediately after amniocentesis in 153 consecutive women with preterm labor. Amniotic fluid obtained by amniocentesis was cultured for aerobic and anaerobic bacteria and mycoplasmas, and the WBC was determined. Intra-amniotic inflammation was defined as an elevated amniotic fluid interleukin-6 concentration (> 2.6 ng/mL). Receiver-operating characteristics (ROC) curves and logistic regression analysis were used for statistical analysis.

RESULTS

The prevalence of a positive amniotic fluid culture was 7.2% (11/153) and the prevalence of intra-amniotic inflammation was 19.6% (30/153). The final logistic regression model was based on non-invasive clinical variables, including gestational age at assessment, cervical length and maternal blood WBC count, which were the best predictors of intra-amniotic inflammation. The model was shown to have an adequate goodness of fit (P = 0.754), and the area under the ROC curve was 0.724, indicating reasonably good discrimination.

CONCLUSION

In women with preterm labor and intact membranes, the risk for intra-amniotic inflammation can be predicted non-invasively with a risk score based on gestational age, cervical length and maternal blood WBC count.

Expression of extracellular signal-regulated kinase1/2 and p38 mitogen-activated protein kinase in the invasive trophoblasts at the human placental bed

BACKGROUND

Mitogen-activated protein kinases (MAP kinases) participate in signal transduction pathways that control embryogenesis, cell differentiation, cell proliferation and cell death. The roles of extracellular signal-regulated kinase1/2 (ERK1/2) and p38 MAP kinase in the differentiation and invasion of human trophoblasts have been studied. However, the in vivo expression and activation of ERK1/2 and p38 at the placental bed have not been elucidated.

METHODS

The study group consisted of placental bed biopsy tissues obtained from the pregnancies without preeclampsia (n=24) and with preeclampsia (n=8) between 31 and 40 weeks of gestation. We evaluated the expressions and phosphorylations of ERK1/2 and p38 MAP kinase in the invasive trophoblasts in the placental bed tissues using immunohistochemistry.

RESULTS

p38 and phospho-p38 MAP kinase were not detected in invasive trophoblasts in cases or controls. ERK1/2 and phospho-ERK1/2 were positive in invasive trophoblasts albeit with variable staining. Phosphorylation of ERK1/2 was significantly less frequent in invasive trophoblasts in placental bed biopsies from women with preeclampsia compared with normotensive controls.

CONCLUSION

These findings suggest that preeclampsia is associated with decreased activation of ERK1/2 in invasive trophoblasts in vivo.

Arthroscopic management of acute painful hip following occult subluxation: evidence-based case report

Arthroscopy of the hip joint has gained popularity in the recent past leading to an explosive increase in our knowledge of intra-articular hip pathologies. However, a spectrum of intra-articular hip lesions still needs to be explored to further advance the understanding, diagnosis and treatment of hip pathologies. The orthopedic surgeon occasionally affronts a situation when etiology of traumatic painful hip joint is not vivid and lack of definitive diagnosis prolongs the patient's suffering; however, an elaborate history taking and pragmatic apt arthroscopic intervention can curtail the illness span. Radiological examination generally fails to provide complete diagnosis in hip joints due to compact anatomy of the joint, and a negative report should not be considered as a deterrent for arthroscopic intervention. We report two evidence-based cases to highlight the significance of arthroscopic evaluation and management for occult subluxation of the hip. In both the cases, there was significant and prompt relief of symptoms after arthroscopic debridement.

Effects of local infusion of OP-1 on particle-induced and NSAID-induced inhibition of bone ingrowth in vivo

Excessive polyethylene wear particles from joint replacements may lead to periprosthetic osteolysis and loosening. Nonsteroidal anti-inflammatory drugs (NSAIDs) decrease fracture healing and bone ingrowth. We hypothesized that continuous local infusion of OP-1 (BMP-7) would increase local bone formation in the presence of two different adverse stimuli, polyethylene particles, and an oral NSAID. The Drug Test Chamber (DTC) was implanted in the proximal tibia of mature rabbits. The tissue growing into the chamber was exposed to OP-1 solution (110 ng/day), which was infused via an osmotic pump. Infusion of OP-1 alone for 6 weeks enhanced local bone formation in the chamber by 80% (p < 0.05) over infusion of carrier alone. In the presence of polyethylene particles, infusion of OP-1 increased local bone formation by 38% (p < 0.05) over treatment with particles and carrier. Oral administration of NSAID reduced local bone formation by 58% (p < 0.05); this suppressive effect caused by NSAIDS was completely reversed by the infusion of OP-1 (p < 0.05). These findings underline a potential role for local treatment with OP-1 to increase bone formation in the presence of potentially adverse stimuli such as polyethylene wear particles or NSAID use.

Reversible renal vasoconstriction in a patient with acute renal failure after exercise

Acute renal failure (ARF) with severe loin pain induced by anaerobic exercise is a rare condition that is accompanied by wedge-shaped contrast enhancement seen on computerized tomographic (CT) scan without evidence of rhabdomyolysis. An 18-year-old Korean male was transferred to our hospital for evaluation of mild azotemia, that developed after anaerobic exercise. The laboratory tests revealed that the serum creatinine was 2.1 mg/dl and the serum uric acid level was 1.6 mg/dl without any elevation of the serum myoglobin or creatine phosphokinase. Under the impression of exercise-induced ARF, we tried to determine the relationship between the occurrence of clinical symptoms, renal dysfunction and the characteristic CT findings by observing those changes prospectively before and after anaerobic exercise. After obtaining a written consent, the patient underwent a strenuous period of anaerobic exercise to induce the clinical symptoms. Before exercise, he was completely asymptomatic; his serum creatinine level was 0.9 mg/dl and CT scan of the kidneys showed no abnormalities. Loin pain developed 2 hours after exercise, and the serum creatinine level increased to 1.2 mg/dl 18 hours after the exercise. CT scan 18 hours after exercise showed multiple perfusion defects, and a 24-hour delayed CT scan showed multiple areas of wedge-shaped enhancement on both kidneys. These changes were completely resolved on the follow-up CT scan obtained 13 days after exercise with the return of a normal serum creatinine level. We conclude that reversible renal vasoconstriction is probably the main pathophysiologic mechanism of acute renal failure induced by anaerobic exercise.

Estimation of inter-residue distances in spin labeled proteins at physiological temperatures: experimental strategies and practical limitations

Magnetic dipolar interactions between pairs of solvent-exposed nitroxide side chains separated by approximately one to four turns along an alpha-helix in T4 lysozyme are investigated. The interactions are analyzed both in frozen solution (rigid lattice conditions) and at room temperature as a function of solvent viscosity. At room temperature, a novel side chain with hindered internal motion is used, along with a more commonly employed nitroxide side chain. The results suggest that methods developed for rigid lattice conditions can be used to analyze dipolar interactions between nitroxides even in the presence of motion of the individual spins, provided the rotational correlation time of the interspin vector is sufficiently long. The distribution of distances observed for the various spin pairs is consistent with rotameric equilibria in the nitroxide side chain, as observed in crystal structures. The existence of such distance distributions places important constraints on the interpretation of internitroxide distances in terms of protein structure and structural changes.

Nucleation of water and methanol droplets on cations and anions: the sign preference

The barrier height to cation- and anion-induced nucleation to produce water and methanol droplets are calculated by means of an umbrella-sampling Monte Carlo method. The computer simulation corroborates the century-old finding of Wilson that the anion is a better nucleator to produce water droplets than the cation having the same magnitude of charge, even without the presence of external electric field. The simulation also shows that the cation is a better nucleator to produce methanol droplets than the anion.

Roles of histidine residues in tobacco acetolactate synthase

Acetolactate synthase (ALS) catalyzes the first common step in the biosynthesis of valine, leucine, and isoleucine in plants and microorganisms. ALS is the target of several structurally diverse classes of herbicides, including sulfonylureas, imidazolinones, and triazolopyrimidines. The roles of three well-conserved histidine residues (H351, H392, and H487) in tobacco ALS were determined using site-directed mutagenesis. Both H487F and H487L mutations abolished the enzymatic activity as well as the binding affinity for the cofactor FAD. Nevertheless, the mutation of H487F did not affect the secondary structure of the ALS. The K(m) values of H351M, H351Q, and H351F are approximately 18-, 60-, and fivefold higher than that of the wild-type ALS, respectively. Moreover, the K(c) value of H351Q for FAD is about 137-fold higher than that of wALS. Mutants H351M and H351Q showed very strong resistance to Londax (a sulfonylurea) and Cadre (an imidazolinone), whereas mutant H351F was weakly resistant to them. However, the secondary structures of mutants H351M and H351Q appeared to be different from that of wALS. The mutation of H392M did not have any significant effect on the kinetic parameters nor the resistance to ALS-inhibiting herbicides. These results suggest that the His487 residue is located at the active site of the enzyme and is likely involved in the binding of cofactor FAD in tobacco ALS. Mutational analyses of the His351 residue imply that the active site of the ALS is probably close to its binding site of the herbicides, Londax and Cadre.

Pro-apoptotic cascade activates BID, which oligomerizes BAK or BAX into pores that result in the release of cytochrome c

We review data supporting a model in which activated tBID results in an allosteric activation of BAK, inducing its intramembranous oligomerization into a proposed pore for cytochrome c efflux. The BH3 domain of tBID is not required for targeting but remains on the mitochondrial surface where it is required to trigger BAK to release cytochrome c. tBID functions not as a pore-forming protein but as a membrane targeted and concentrated death ligand. tBID induces oligomerization of BAK, and both Bid and Bak knockout mice indicate the importance of this event in the release of cytochrome c. In parallel, the full pro-apoptotic member BAX, which is highly homologous to BAK, rapidly forms pores in liposomes that release intravesicular FITC-cytochrome c approximately 20A. A definable pore progressed from approximately 11A consisting of two BAX molecules to a approximately 22A pore comprised of four BAX molecules, which transported cytochrome c. Thus, an activation cascade of pro-apoptotic proteins from BID to BAK or BAX integrates the pathway from surface death receptors to the irreversible efflux of cytochrome c. Cell Death and Differentiation (2000) 7, 1166 - 1173

Posttranslational N-myristoylation of BID as a molecular switch for targeting mitochondria and apoptosis

Many apoptotic molecules relocate subcellularly in cells undergoing apoptosis. The pro-apoptotic protein BID underwent posttranslational (rather than classic cotranslational) N-myristoylation when cleavage by caspase 8 caused exposure of a glycine residue. N-myristoylation enabled the targeting of a complex of p7 and myristoylated p15 fragments of BID to artificial membranes bearing the lipid composition of mitochondria, as well as to intact mitochondria. This post-proteolytic N-myristoylation serves as an activating switch, enhancing BID-induced release of cytochrome c and cell death.

The diphtheria toxin channel-forming T-domain translocates its own NH2-terminal region and the catalytic domain across planar phospholipid bilayers

The T-domain of diphtheria toxin, which extends from residue 202 to 378, causes the translocation of the catalytic A fragment (residues 1-201) across endosomal membranes and also forms ion-conducting channels in planar phospholipid bilayers. The carboxy-terminal 57-amino acid segment (residues 322-378) in the T-domain is all that is required to form these channels, but its ability to do so is greatly augmented by the portion of the T-domain upstream from this. Here we show that in association with channel formation by the T-domain, its hydrophilic 63-amino acid NH2-terminal region (residues 202-264) as well as the entire catalytic A fragment (residues 1-201) cross the lipid bilayer. The phenomenon that enabled us to demonstrate this was the rapid closure of channels at cis negative voltages when a histidine tag was placed at various positions in the NH2-terminal region of the T-domain or in the A fragment; the inhibition of this effect by trans nickel established that the histidine tag was present on the trans side of the membrane. Thus, all of the machinery necessary to translocate the A fragment across membranes is built into the 114 residues at the carboxy-terminal end of the T-domain (residues 265-378), without the requirement of any proteins in the plasma membrane (e.g., toxin receptor) or of any other cellular components.

Crystal structures of spin labeled T4 lysozyme mutants: implications for the interpretation of EPR spectra in terms of structure

High resolution (1.43-1.8 A) crystal structures and the corresponding electron paramagnetic resonance (EPR) spectra were determined for T4 lysozyme derivatives with a disulfide-linked nitroxide side chain [-CH(2)-S-S-CH(2)-(3-[2,2,5,5-tetramethyl pyrroline-1-oxyl]) identical with R1] substituted at solvent-exposed helix surface sites (Lys65, Arg80, Arg119) or a tertiary contact site (Val75). In each case, electron density is clearly resolved for the disulfide group, revealing distinct rotamers of the side chain, defined by the dihedral angles X(1) and X(2). The electron density associated with the nitroxide ring in the different mutants is inversely correlated with its mobility determined from the EPR spectrum. Residue 80R1 assumes a single g(+)()g(+)() conformation (Chi(1) = 286, X(2) = 294). Residue 119R1 has two EPR spectral components, apparently corresponding to two rotamers, one similar to that for 80R1 and the other in a tg(-)() conformation (Chi(1) = 175, X(2) = 54). The latter state is apparently stabilized by interaction of the disulfide with a Gln at i + 4, a situation also observed at 65R1. R1 residues at helix surface site 65 and tertiary contact site 75 make intra- as well as intermolecular contacts in the crystal and serve to identify the kind of molecular interactions possible for the R1 side chain. A single conformation of the entire 75R1 side chain is stabilized by a variety of interactions with the nitroxide ring, including hydrophobic contacts and two unconventional C-H.O hydrogen bonds, one in which the nitroxide acts as a donor (with tyrosine) and the other in which it acts as an acceptor (with phenylalanine). The interactions revealed in these structures provide an important link between the dynamics of the R1 side chain, reflected in the EPR spectrum, and local protein structure. A library of such interactions will provide a basis for the quantitative interpretation of EPR spectra in terms of protein structure and dynamics.

Conformation of the diphtheria toxin T domain in membranes: a site-directed spin-labeling study of the TH8 helix and TL5 loop

The isolated T domain of diphtheria toxin was mutated by cysteine-scanning mutagenesis at 28 consecutive sites (residues 328-355) that comprise the TH8 helix and the TL5 interhelical loop in the native toxin. After derivatizing the mutant proteins with a sulfhydryl-selective nitroxide reagent, we examined the mobility of each nitroxide and its accessibility to polar and nonpolar paramagnetic reagents, before and after insertion into phospholipid bilayers. The data obtained with the proteins in solution at pH 8 are generally consistent with predictions from the crystal structure of the toxin. Upon membrane binding at pH 4.6, a major structural reorganization of the domain was seen, which dramatically reduced the accessibility of most residues in this region to the polar reagent nickel(II)-ethylenediaminediacetate complex (NiEDDA). Many of these residues also showed reduced accessibility to the nonpolar reagent O(2). Periodic accessibility of the nitroxide side chains along the sequence to these reagents shows that TH8 remains largely helical in the membrane-bound state, with one surface associated with protein and the other facing the hydrophobic interior of the bilayer. In addition, the TL5 loop also appears to become alpha-helical in the membrane, with one surface in contact with protein and the other in contact with the bilayer interior. These findings provide a structural framework for understanding how the T domain forms a transmembrane channel and mediates translocation of diphtheria toxin's enzymic moiety across a membrane.

Translocation of the catalytic domain of diphtheria toxin across planar phospholipid bilayers by its own T domain

The T domain of diphtheria toxin is known to participate in the pH-dependent translocation of the catalytic C domain of the toxin across the endosomal membrane, but how it does so, and whether cellular proteins are also required for this process, remain unknown. Here, we report results showing that the T domain alone is capable of translocating the entire C domain across model, planar phospholipid bilayers in the absence of other proteins. The T domain therefore contains the entire molecular machinery for mediating transfer of the catalytic domain of diphtheria toxin across membranes.

Metastatic Hepatocellular Carcinoma of the Distal Phalanx of the Thumb

A case of metastatic hepatocellular carcinoma of the distal phalanx of the thumb is presented. This case illustrated a rare example of acrometastasis of hepatocellular carcinoma.

Development of effective hydrogen sulphide removing equipment using Thiobacillus sp.IW

Hydrogen sulphide is one of the commonest odours emitted by chemical plants. To remove the hydrogen sulphide biologically, a three phase fluidised bed bioreactor was used in which Thiobacillus sp.IW was immobilised on activated carbon. The optimum operating conditions of the bioreactor were 30 degrees C, pH7, aspect ratio (L/D) = 1 and at these conditions, the system removed over 94% of the hydrogen sulphide in the concentration range of 100-200 ppm and flow rate of 1.0-2.0 litre min(-1). From the upset and recovery test, the system proved stable within the moderate inlet concentration changes investigated.

Probing the structure of the diphtheria toxin channel. Reactivity in planar lipid bilayer membranes of cysteine-substituted mutant channels with methanethiosulfonate derivatives

Previous work has established that the 61 amino acid stretch from residue 322 to 382 in the T-domain of diphtheria toxin forms channels indistinguishable in ion-conducting properties from those formed by the entire T-domain. In the crystal structure of the toxin's water-soluble form, the bulk of this stretch is an alpha-helical hairpin, designated TH8-9. The present study was directed at determining which residues in TH8-9 line the ion-conducting pathway of the channel; i.e., its lumen or entrances. To this end, we singly mutated 49 of TH8-9's 51 residues (328-376) to cysteines, formed channels with the mutant T-domain proteins in planar lipid bilayers, and then determined whether they reacted with small, charged, lipid-insoluble, sulfhydryl-specific methanethiosulfonate (MTS) derivatives added to the bathing solutions. The indication of a reaction, and that the residue lined the ion-conducting pathway, was a sudden change in single-channel conductance and/or flickering behavior. The results of this study were surprising in two respects. First, of the 49 cysteine-substituted residues in TH8-9 tested, 23 reacted with MTS derivatives in a most unusual pattern consisting of two segments: one extending from 329 to 341 (11 of 13 reacted), and the other from 347 to 359 (12 of 13 reacted); none of the residues outside of these two segments appeared to react. Second, in every cysteine mutant channel manifesting an MTS effect, only one transition in single-channel conductance (or flickering behavior) occurred, not the several expected for a multimeric channel. Our results are not consistent with an alpha-helical or beta-strand model for the channel, but instead suggest an open, flexible structure. Moreover, contrary to common sense, they indicate that the channel is not multimeric but is formed from only one TH8-9 unit of the T-domain.

Conformation of T4 lysozyme in solution. Hinge-bending motion and the substrate-induced conformational transition studied by site-directed spin labeling

T4 lysozyme and mutants thereof crystallize in different conformations that are related to each other by a bend about a hinge in the molecule. This observation suggests that the wild type protein may undergo a hinge-bending motion in solution to allow substrate access to an otherwise closed active site cleft [Faber, H.R., & Matthews, B.W. (1990) Nature 348, 263-266]. To test this hypothesis, either single or pairs of nitroxide side chains were introduced into the protein to monitor tertiary contact interactions and inter-residue distances, respectively, in solution. A set of constraints for these structural parameters was derived from a reference state, a covalent enzyme-substrate adduct where the enzyme is locked in the closed state. In the absence of substrate, differences in both inter-residue distances and tertiary contact interactions relative to this reference state are consistent with a hinge-bending motion that opens the active site cleft. Quantitative analysis of spin-spin interactions between nitroxide pairs reveals an 8 A relative domain movement upon substrate binding. In addition, it is demonstrated that the I3P mutation, which produces a large hinge-bending angle in the crystal, has no effect on the solution conformation. Thus, the hinge motion is not the result of the mutation but is an integral part of T4 lysozyme catalysis in solution, as suggested recently [Zhang, X.J., Wozniak, J.A., & Matthews, B.W. (1995) J. Mol. Biol. 250, 527-552]. The strategy employed here, based on site-directed spin labeling, should be generally applicable to the study of protein conformation and conformational changes in solution.

Organization of diphtheria toxin T domain in bilayers: a site-directed spin labeling study

The diphtheria toxin transmembrane (T) domain was spin-labeled at consecutive residues in a helical segment, TH9. After binding of the T domain to membranes at low pH, the nitroxide side chains generated by spin labeling were measured with respect to their frequency of collision with polar and nonpolar reagents. The data showed that the helical structure of TH9 in solution is conserved, with one face exposed to water and the other to the hydrophobic interior of the bilayer. Measurement of the depth of the nitroxide side chains from the membrane surfaces revealed an incremental change of about 5 angstroms per turn, which is consistent with a transmembrane orientation of an alpha helix. These results indicate that the helix forms the lining of a transmembrane water-filled channel.

Interaction of the isolated transmembrane domain of diphtheria toxin with membranes

Insertion of diphtheria toxin's T (transmembrane) domain into the endosomal membrane under acidic conditions is known to promote translocation of its catalytic domain across the membrane and into the cytosol. The T domain, a cysteine-free bundle of alpha-helices, was expressed as a discrete protein in Escherichia coli and purified. The isolated domain was stable and largely monomeric at pH 8.0. Like the holotoxin it bound the hydrophobic fluorophore, 2-p-toluidinylnaphthalene 6-sulfonate, upon acidification, but the transition pH was higher than with the holotoxin (pH 5.6 vs 5.1) and broader, reflecting the absence of interdomain interactions. The domain also permeabilized large unilamellar vesicles under acidic conditions, as demonstrated by release of entrapped solutes. Mutant forms of T domain, each with a single residue replaced by cysteine, were derivatized with a thiol-reactive nitroxide-containing spin label and analyzed by electron paramagnetic resonance (EPR). EPR spectra and solvent accessibilities of the labels at pH 8.0 were consistent with the environments predicted from the toxin's crystallographic structure. Acidification in the presence of large unilamellar vesicles caused a nitroxide label at position 332 on helix TH8 to move from a buried site in the water soluble state to a lipid-exposed surface site at a depth of approximately 15 A within the bilayer. This is consistent with the concept that the TH8-TH9 helix pair inserts into the bilayer.(ABSTRACT TRUNCATED AT 250 WORDS)

Protein thiol-disulfide interchange and interfacing with biological systems

Disulfide-containing proteins offer unique advantages for mechanistic studies of the formation of native three-dimensional structure from unordered, reduced precursors. The main advantage is that covalent intermediates are formed; by characterizing these intermediates, one obtains substantial information about the reaction pathway. Thiol-disulfide interchange is a major component of most oxidative mechanisms carrying thiol to disulfide; thus, it required some attention in its own right. Afinsen's descriptions of a "shuffle-ase" enzyme led us to examine the rates of the uncatalyzed exchange under physiologically plausible conditions. Somewhat surprisingly, we found that the rates for formation of several native proteins in uncatalyzed systems containing GSSG and GSH are as great as with the "shuffle-ase" enzyme, suggesting that a substantial portion of biological thiol oxidations proceed by uncatalyzed exchange. While thiol-disulfide exchange of course results in no net change in the oxidation level of a system, catalytic linkage of thiol or disulfide to other redox systems provides a mechanism for achieving net changes.

Formation of three-dimensional structure in protein fragments. Reactivation of reduced hen egg lysozyme fragment 1-127

Regeneration of enzymic activity from reduced hen egg lysozyme peptide 1-127 was effected with a glutathione oxidation-reduction buffer. The rate of regeneration was nearly as great for peptide 1-127 as for reduced lysozyme itself, and the yields were the same (greater than 80%). The regenerated fragment 1-127 was shown to be indistinguishable from fragment 1-127 before reduction by ion exchange chromatography, amino acid analysis, polyacrylamide gel electrophoresis, and disulfide analysis. These results show that the COOH-terminal dipeptide Arg-Leu is not essential for the acquisition of the native three-dimensional structure of lysozyme.

Nonequivalent binding sites in cystathionase. Nanosecond and steady fluorescence studies

The analogs P-pyridoxyl-L-alanine and P-pyridoxyl-L-homoserine bind to the apoprotein of the enzyme cystathionase and inhibit the reactivation of enzymatic activity after addition of pyridoxyl-5-P. The binding of the inhibitors was monitored by measuring the fluorescence emitted by the P-pyridoxyl moiety at 395 nm (excitation 325 nm). The fluorometric titration results indicate the presence of nonequivalent binding sites in the apoprotein. A model based on two classes of independent binding sites fits the fluorometric data reasonably well. The presence of nonequivalent fluorescent sites in reduced cystathionase was also detected by nanosecond spectroscopy. In contrast to the model compound P-pyridoxyl-epsilon-lysine (tau equals 2.6 ns), the P-pyridoxyl residues of cystathionase display multiexponential fluorescence decay. Two fluorescence lifetimes (tau2 equals 4.1 ns and tau2 equals 15 ns) fit the deconvoluted decay results obtained by pulse fluorimetry. It is proposed that the P-pyridoxyl chromophores of reduced cystathionase have different environments.