Biochemical investigation of the tryptophan biosynthetic enzyme anthranilate phosphoribosyltransferase in plants

While mammals require the essential amino acid tryptophan (Trp) in their diet, plants and microorganisms synthesize Trp de novo. The five-step Trp pathway starts with the shikimate pathway product, chorismate. Chorismate is converted to the aromatic compound anthranilate, which is then conjugated to a phosphoribosyl sugar in the second step by anthranilate phosphoribosyltransferase (PAT1). As a single-copy gene in plants, all fixed carbon flux to indole and Trp for protein synthesis, specialized metabolism, and auxin hormone biosynthesis proceeds through PAT1. While bacterial PAT1s have been studied extensively, plant PAT1s have escaped biochemical characterization. Using a structure model, we identified putative active site residues that were variable across plants and kinetically characterized six PAT1s (Arabidopsis thaliana (thale cress), Citrus sinensis (sweet orange), Pistacia vera (pistachio), Juglans regia (English walnut), Selaginella moellendorffii (spike moss), and Physcomitrium patens (spreading earth-moss)). We probed the catalytic efficiency, substrate promiscuity, and regulation of these six enzymes and found that the C. sinensis PAT1 is highly specific for its cognate substrate, anthranilate. Investigations of site-directed mutants of the A. thaliana PAT1 uncovered an active site residue that contributes to promiscuity. While Trp inhibits bacterial PAT1 enzymes, the six plant PAT1s that we tested were not modulated by Trp. Instead, the P. patens PAT1 was inhibited by tyrosine, and the S. moellendorffii PAT1 was inhibited by phenylalanine. This structure-informed biochemical examination identified variations in activity, efficiency, specificity, and enzyme-level regulation across PAT1s from evolutionarily diverse plants.

Two independently evolved natural mutations additively deregulate TyrA enzymes and boost tyrosine production in planta

l-Tyrosine is an essential amino acid for protein synthesis and is also used in plants to synthesize diverse natural products. Plants primarily synthesize tyrosine via TyrA arogenate dehydrogenase (TyrA_a or ADH), which are typically strongly feedback inhibited by tyrosine. However, two plant lineages, Fabaceae (legumes) and Caryophyllales, have TyrA enzymes that exhibit relaxed sensitivity to tyrosine inhibition and are associated with elevated production of tyrosine-derived compounds, such as betalain pigments uniquely produced in core Caryophyllales. Although we previously showed that a single D222N substitution is primarily responsible for the deregulation of legume TyrAs, it is unknown when and how the deregulated Caryophyllales TyrA emerged. Here, through phylogeny-guided TyrA structure-function analysis, we found that functionally deregulated TyrAs evolved early in the core Caryophyllales before the origin of betalains, where the E208D amino acid substitution in the active site, which is at a different and opposite location from D222N found in legume TyrAs, played a key role in the TyrA functionalization. Unlike legumes, however, additional substitutions on non-active site residues further contributed to the deregulation of TyrAs in Caryophyllales. The introduction of a mutation analogous to E208D partially deregulated tyrosine-sensitive TyrAs, such as Arabidopsis TyrA2 (AtTyrA2). Moreover, the combined introduction of D222N and E208D additively deregulated AtTyrA2, for which the expression in Nicotiana benthamiana led to highly elevated accumulation of tyrosine in planta. The present study demonstrates that phylogeny-guided characterization of key residues underlying primary metabolic innovations can provide powerful tools to boost the production of essential plant natural products.

The plant pathogen enzyme AldC is a long-chain aliphatic aldehyde dehydrogenase

Aldehyde dehydrogenases are versatile enzymes that serve a range of biochemical functions. Although traditionally considered metabolic housekeeping enzymes because of their ability to detoxify reactive aldehydes, like those generated from lipid peroxidation damage, the contributions of these enzymes to other biological processes are widespread. For example, the plant pathogen Pseudomonas syringae strain PtoDC3000 uses an indole-3-acetaldehyde dehydrogenase to synthesize the phytohormone indole-3-acetic acid to elude host responses. Here we investigate the biochemical function of AldC from PtoDC3000. Analysis of the substrate profile of AldC suggests that this enzyme functions as a long-chain aliphatic aldehyde dehydrogenase. The 2.5 Å resolution X-ray crystal of the AldC C291A mutant in a dead-end complex with octanal and NAD⁺ reveals an apolar binding site primed for aliphatic aldehyde substrate recognition. Functional characterization of site-directed mutants targeting the substrate- and NAD(H)-binding sites identifies key residues in the active site for ligand interactions, including those in the "aromatic box" that define the aldehyde-binding site. Overall, this study provides molecular insight for understanding the evolution of the prokaryotic aldehyde dehydrogenase superfamily and their diversity of function.

Structural and biochemical analysis of phosphoethanolamine methyltransferase from the pine wilt nematode Bursaphelenchus xylophilus

In free-living and parasitic nematodes, the methylation of phosphoethanolamine to phosphocholine provides a key metabolite to sustain phospholipid biosynthesis for growth and development. Because the phosphoethanolamine methyltransferases (PMT) of nematodes are essential for normal growth and development, these enzymes are potential targets of inhibitor design. The pine wilt nematode (Bursaphelenchus xylophilus) causes extensive damage to trees used for lumber and paper in Asia. As a first step toward testing BxPMT1 as a potential nematicide target, we determined the 2.05 Å resolution x-ray crystal structure of the enzyme as a dead-end complex with phosphoethanolamine and S-adenosylhomocysteine. The three-dimensional structure of BxPMT1 served as a template for site-directed mutagenesis to probe the contribution of active site residues to catalysis and phosphoethanolamine binding using steady-state kinetic analysis. Biochemical analysis of the mutants identifies key residues on the β1d-α6 loop (W123F, M126I, and Y127F) and β1e-α7 loop (S155A, S160A, H170A, T178V, and Y180F) that form the phosphobase binding site and suggest that Tyr127 facilitates the methylation reaction in BxPMT1.

Independent evolution of ancestral and novel defenses in a genus of toxic plants (Erysimum, Brassicaceae)

Phytochemical diversity is thought to result from coevolutionary cycles as specialization in herbivores imposes diversifying selection on plant chemical defenses. Plants in the speciose genus Erysimum (Brassicaceae) produce both ancestral glucosinolates and evolutionarily novel cardenolides as defenses. Here we test macroevolutionary hypotheses on co-expression, co-regulation, and diversification of these potentially redundant defenses across this genus. We sequenced and assembled the genome of E. cheiranthoides and foliar transcriptomes of 47 additional Erysimum species to construct a phylogeny from 9868 orthologous genes, revealing several geographic clades but also high levels of gene discordance. Concentrations, inducibility, and diversity of the two defenses varied independently among species, with no evidence for trade-offs. Closely related, geographically co-occurring species shared similar cardenolide traits, but not glucosinolate traits, likely as a result of specific selective pressures acting on each defense. Ancestral and novel chemical defenses in Erysimum thus appear to provide complementary rather than redundant functions.

Plants are often attacked by insects and other herbivores. As a result, they have evolved to defend themselves by producing many different chemicals that are toxic to these pests. As producing each chemical costs energy, individual plants often only produce one type of chemical that is targeted towards their main herbivore. Related species of plants often use the same type of chemical defense so, if a particular herbivore gains the ability to cope with this chemical, it may rapidly become an important pest for the whole plant family.

To escape this threat, some plants have gained the ability to produce more than one type of chemical defense. Wallflowers, for example, are a group of plants in the mustard family that produce two types of toxic chemicals: mustard oils, which are common in most plants in this family; and cardenolides, which are an innovation of the wallflowers, and which are otherwise found only in distantly related plants such as foxglove and milkweed. The combination of these two chemical defenses within the same plant may have allowed the wallflowers to escape attacks from their main herbivores and may explain why the number of wallflower species rapidly increased within the last two million years.

Züst et al. have now studied the diversity of mustard oils and cardenolides present in many different species of wallflower. This analysis revealed that almost all of the tested wallflower species produced high amounts of both chemical defenses, while only one species lacked the ability to produce cardenolides. The levels of mustard oils had no relation to the levels of cardenolides in the tested species, which suggests that the regulation of these two defenses is not linked. Furthermore, Züst et al. found that closely related wallflower species produced more similar cardenolides, but less similar mustard oils, to each other. This suggests that mustard oils and cardenolides have evolved independently in wallflowers and have distinct roles in the defense against different herbivores.

The evolution of insect resistance to pesticides and other toxins is an important concern for agriculture. Applying multiple toxins to crops at the same time is an important strategy to slow the evolution of resistance in the pests. The findings of Züst et al. describe a system in which plants have naturally evolved an equivalent strategy to escape their main herbivores. Understanding how plants produce multiple chemical defenses, and the costs involved, may help efforts to breed crop species that are more resistant to herbivores and require fewer applications of pesticides.

Natural variation in the expression and catalytic activity of a naringenin 7-O-methyltransferase influences antifungal defenses in diverse rice cultivars

Phytoalexins play a pivotal role in plant-pathogen interactions. Whereas leaves of rice (Oryza sativa) cultivar Nipponbare predominantly accumulated the phytoalexin sakuranetin after jasmonic acid induction, only very low amounts accumulated in the Kasalath cultivar. Sakuranetin is synthesized from naringenin by naringenin 7-O-methyltransferase (NOMT). Analysis of chromosome segment substitution lines and backcrossed inbred lines suggested that NOMT is the underlying cause of differential phytoalexin accumulation between Nipponbare and Kasalath. Indeed, both NOMT expression and NOMT enzymatic activity are lower in Kasalath than in Nipponbare. We identified a proline to threonine substitution in Kasalath relative to Nipponbare NOMT as the main cause of the lower enzymatic activity. Expanding this analysis to rice cultivars with varying amounts of sakuranetin collected from around the world showed that NOMT induction is correlated with sakuranetin accumulation. In bioassays with Pyricularia oryzae, Gibberella fujikuroi, Bipolaris oryzae, Burkholderia glumae, Xanthomonas oryzae, Erwinia chrysanthemi, Pseudomonas syringae, and Acidovorax avenae, naringenin was more effective against bacterial pathogens and sakuranetin was more effective against fungal pathogens. Therefore, the relative amounts of naringenin and sakuranetin may provide protection against specific pathogen profiles in different rice-growing environments. In a dendrogram of NOMT genes, those from low-sakuranetin-accumulating cultivars formed at least two clusters, only one of which involves the proline to threonine mutation, suggesting that the low sakuranetin chemotype was acquired more than once in cultivated rice. Strains of the wild rice species Oryza rufipogon also exhibited differential sakuranetin accumulation, indicating that this metabolic diversity predates rice domestication.

Brassicaceae-specific Gretchen Hagen 3 acyl acid amido synthetases conjugate amino acids to chorismate, a precursor of aromatic amino acids and salicylic acid

To modulate responses to developmental or environmental cues, plants use Gretchen Hagen 3 (GH3) acyl acid amido synthetases to conjugate an amino acid to a plant hormone, a reaction that regulates free hormone concentration and downstream responses. The model plant Arabidopsis thaliana has 19 GH3 proteins, of which 8 have confirmed biochemical functions. One Brassicaceae-specific clade of GH3 proteins was predicted to use benzoate as a substrate and includes AtGH3.7 and AtGH3.12/PBS3. Previously identified as a 4-hydroxybenzoic acid-glutamate synthetase, AtGH3.12/PBS3 influences pathogen defense responses through salicylic acid. Recent work has shown that AtGH3.12/PBS3 uses isochorismate as a substrate, forming an isochorismate-glutamate conjugate that converts into salicylic acid. Here, we show that AtGH3.7 and AtGH3.12/PBS3 can also conjugate chorismate to cysteine and glutamate, which act as precursors to aromatic amino acids and salicylic acid, respectively. The X-ray crystal structure of AtGH3.12/PBS3 in complex with AMP and chorismate at 1.94 Å resolution, along with site-directed mutagenesis, revealed how the active site potentially accommodates this substrate. Examination of Arabidopsis knockout lines indicated that the gh3.7 mutants do not alter growth and showed no increased susceptibility to the pathogen Pseudomonas syringae, unlike gh3.12 mutants, which were more susceptible than WT plants, as was the gh3.7/gh3.12 double mutant. The findings of our study suggest that GH3 proteins can use metabolic precursors of aromatic amino acids as substrates.

Arabidopsis: the original plant chassis organism

Arabidopsis thaliana (thale cress) has a past, current, and future role in the era of synthetic biology. Arabidopsis is one of the most well-studied plants with a wealth of genomics, genetics, and biochemical resources available for the metabolic engineer and synthetic biologist. Here we discuss the tools and resources that enable the identification of target genes and pathways in Arabidopsis and heterologous expression in this model plant. While there are numerous examples of engineering Arabidopsis for decreased lignin, increased seed oil, increased vitamins, and environmental remediation, this plant has provided biochemical tools for introducing Arabidopsis genes, pathways, and/or regulatory elements into other plants and microorganisms. Arabidopsis is not a vegetative or oilseed crop, but it is as an excellent model chassis for proof-of-concept metabolic engineering and synthetic biology experiments in plants.

Structural basis for substrate recognition and inhibition of prephenate aminotransferase from Arabidopsis

Aromatic amino acids are protein building blocks and precursors to a number of plant natural products, such as the structural polymer lignin and a variety of medicinally relevant compounds. Plants make tyrosine and phenylalanine by a different pathway from many microbes; this pathway requires prephenate aminotransferase (PAT) as the key enzyme. Prephenate aminotransferase produces arogenate, the unique and immediate precursor for both tyrosine and phenylalanine in plants, and also has aspartate aminotransferase (AAT) activity. The molecular mechanisms governing the substrate specificity and activation or inhibition of PAT are currently unknown. Here we present the X-ray crystal structures of the wild-type and various mutants of PAT from Arabidopsis thaliana (AtPAT). Steady-state kinetic and ligand-binding analyses identified key residues, such as Glu108, that are involved in both keto acid and amino acid substrate specificities and probably contributed to the evolution of PAT activity among class Ib AAT enzymes. Structures of AtPAT mutants co-crystallized with either α-ketoglutarate or pyridoxamine 5'-phosphate and glutamate further define the molecular mechanisms underlying recognition of keto acid and amino acid substrates. Furthermore, cysteine was identified as an inhibitor of PAT from A. thaliana and Antirrhinum majus plants as well as the bacterium Chlorobium tepidum, uncovering a potential new effector of PAT.

Reaction Mechanism of Prephenate Dehydrogenase from the Alternative Tyrosine Biosynthesis Pathway in Plants

Unlike metazoans, plants, bacteria, and fungi retain the enzymatic machinery necessary to synthesize the three aromatic amino acids l-phenylalanine, l-tyrosine, and l-tryptophan de novo. In legumes, such as soybean, alfalfa, and common bean, prephenate dehydrogenase (PDH) catalyzes the tyrosine-insensitive biosynthesis of 4-hydroxyphenylpyruvate, a precursor to tyrosine. The three-dimensional structure of soybean PDH1 was recently solved in complex with the NADP+ cofactor. This structure allowed for the identification of both the cofactor- and ligand-binding sites. Here, we present steady-state kinetic analysis of twenty site-directed active-site mutants of soybean (Glycine max) PDH compared to wild-type. Molecular docking of the substrate, prephenate, into the active site of the enzyme revealed its potential interactions with the active site residues and made a case for the importance of each residue in substrate recognition and/or catalysis, most likely through transition state stabilization. Overall, these results suggested that the active site of the enzyme is highly sensitive to any changes, as even subtle alterations substantially reduced the catalytic efficiency of the enzyme.

Molecular basis of the evolution of alternative tyrosine biosynthetic routes in plants

L-Tyrosine (Tyr) is essential for protein synthesis and is a precursor of numerous specialized metabolites crucial for plant and human health. Tyr can be synthesized via two alternative routes by different key regulatory TyrA family enzymes, prephenate dehydrogenase (PDH, also known as TyrA_p) or arogenate dehydrogenase (ADH, also known as TyrA_a), representing a unique divergence of primary metabolic pathways. The molecular foundation underlying the evolution of these alternative Tyr pathways is currently unknown. Here we characterized recently diverged plant PDH and ADH enzymes, obtained the X-ray crystal structure of soybean PDH, and identified a single amino acid residue that defines TyrA substrate specificity and regulation. Structures of mutated PDHs co-crystallized with Tyr indicate that substitutions of Asn222 confer ADH activity and Tyr sensitivity. Reciprocal mutagenesis of the corresponding residue in divergent plant ADHs further introduced PDH activity and relaxed Tyr sensitivity, highlighting the critical role of this residue in TyrA substrate specificity that underlies the evolution of alternative Tyr biosynthetic pathways in plants.

Dissonance Strikes a Chord in Stilbene Synthesizers

In this issue of Cell Chemical Biology, Mori et al. (2016) combine X-ray crystallography and biochemistry to discover a new mechanism for stilbene synthesis in bacteria. The dialkyl-condensing enzyme StlD catalyzes formation of cyclohexanediones using a non-canonical β-ketosynthase active site. Aromatization by StlC completes production of the stilbene product.

Delivery of bevacizumab to atheromatous porcine carotid tissue using echogenic liposomes

Ultrasound is both a valuable diagnostic tool and a promoter of beneficial tissue bioeffects for the treatment of cardiovascular disease. Vascular effects can be mediated by mechanical oscillations of circulating microbubbles that may also encapsulate and shield therapeutic agents in the bloodstream. Here, the effect of color-Doppler ultrasound exposure on bevacizumab-loaded liposome delivery into the vascular bed was assessed in atheromatous porcine carotids. Bevacizumab, an anti-angiogenic antibody to vascular endothelial growth factor (VEGF-A), was loaded into echogenic liposomes (BEV-ELIP) and confirmed to be immunoreactive. BEV-ELIP flowing within the lumen were exposed to color-Doppler ultrasound at three acoustic pressures for 3.5 min during treatment at physiologic temperature and fluid pressure. To confirm the presence of bubble activity, cavitation was detected within the lumen by a single-element passive cavitation detector. After treatment, the artery was fixed at physiologic pressure and subjected to immunohistochemical analysis to assess the penetration of bevacizumab within the carotid wall. The results suggest that other factors may more strongly influence the deposition of bevacizumab into carotid tissue than color-Doppler ultrasound and cavitation. In both sets of arteries, preferential accumulation of bevacizumab occurred in locations associated with atheroma progression and neointimal thickening: fibrous tissue, necrotic plaque and areas near macrophage infiltration. The delivery of bevacizumab to carotid vascular tissue correlated with the properties of the tissue bed, such as permeability, or affinity for growth-factor binding. Future investigations using this novel therapeutic strategy may focus on characterizing the spatial extent of delivery and bevacizumab colocalization with biochemical markers of atheroma.

Identification of a Noroxomaritidine Reductase with Amaryllidaceae Alkaloid Biosynthesis Related Activities

Amaryllidaceae alkaloids are a large group of plant natural products with over 300 documented structures and diverse biological activities. Several groups of Amaryllidaceae alkaloids including the hemanthamine- and crinine-type alkaloids show promise as anticancer agents. Two reduction reactions are required for the production of these compounds: the reduction of norcraugsodine to norbelladine and the reduction of noroxomaritidine to normaritidine, with the enantiomer of noroxomaritidine dictating whether the derivatives will be the crinine-type or hemanthamine-type. It is also possible for the carbon-carbon double bond of noroxomaritidine to be reduced, forming the precursor for maritinamine or elwesine depending on the enantiomer reduced to an oxomaritinamine product. In this study, a short chain alcohol dehydrogenase/reductase that co-expresses with the previously discovered norbelladine 4'-O-methyltransferase from Narcissus sp. and Galanthus spp. was cloned and expressed in Escherichia coli Biochemical analyses and x-ray crystallography indicates that this protein functions as a noroxomaritidine reductase that forms oxomaritinamine from noroxomaritidine through a carbon-carbon double bond reduction. The enzyme also reduces norcraugsodine to norbelladine with a 400-fold lower specific activity. These studies identify a missing step in the biosynthesis of this pharmacologically important class of plant natural products.

Pulsed ultrasound enhances the delivery of nitric oxide from bubble liposomes to ex vivo porcine carotid tissue

Ultrasound-mediated drug delivery is a novel technique for enhancing the penetration of drugs into diseased tissue beds noninvasively. By encapsulating drugs into microsized and nanosized liposomes, the therapeutic can be shielded from degradation within the vasculature until delivery to a target site by ultrasound exposure. Traditional in vitro or ex vivo techniques to quantify this delivery profile include optical approaches, cell culture, and electrophysiology. Here, we demonstrate an approach to characterize the degree of nitric oxide (NO) delivery to porcine carotid tissue by direct measurement of ex vivo vascular tone. An ex vivo perfusion model was adapted to assess ultrasound-mediated delivery of NO. This potent vasodilator was coencapsulated with inert octafluoropropane gas to produce acoustically active bubble liposomes. Porcine carotid arteries were excised post mortem and mounted in a physiologic buffer solution. Vascular tone was assessed in real time by coupling the artery to an isometric force transducer. NO-loaded bubble liposomes were infused into the lumen of the artery, which was exposed to 1 MHz pulsed ultrasound at a peak-to-peak acoustic pressure amplitude of 0.34 MPa. Acoustic cavitation emissions were monitored passively. Changes in vascular tone were measured and compared with control and sham NO bubble liposome exposures. Our results demonstrate that ultrasound-triggered NO release from bubble liposomes induces potent vasorelaxation within porcine carotid arteries (maximal relaxation 31%±8%), which was significantly stronger than vasorelaxation due to NO release from bubble liposomes in the absence of ultrasound (maximal relaxation 7%±3%), and comparable with relaxation due to 12 μM sodium nitroprusside infusions (maximal relaxation 32%±3%). This approach is a valuable mechanistic tool for assessing the extent of drug release and delivery to the vasculature caused by ultrasound.

Cavernous sinus and inferior petrosal sinus flow signal on three-dimensional time-of-flight MR angiography

BACKGROUND AND PURPOSE

Venous flow signal in the cavernous sinus and inferior petrosal sinus has been shown on MR angiograms in patients with carotid cavernous fistula (CCF). We, however, identified flow signal in some patients without symptoms and signs of CCF. This review was performed to determine the frequency of such normal venous flow depiction at MR angiography.

METHODS

Twenty-five 3D time-of-flight (TOF) MR angiograms obtained on two different imaging units (scanners A and B) were reviewed with attention to presence of venous flow signal in the cavernous sinus or inferior petrosal sinus or both. Twenty-five additional MR angiograms were reviewed in patients who had also had cerebral arteriography to document absence of CCF where venous MR angiographic signal was detected, as well as to gain insight into venous flow patterns that might contribute to MR angiographic venous flow signal. Differences in scanning technique parameters were reviewed.

RESULTS

Nine (36%) of the 25 MR angiograms obtained on scanner A but only one (4%) of the 25 obtained on scanner B showed flow signal in the cavernous or inferior petrosal sinus or both in the absence of signs of CCF. On review of 25 patients who had both MR angiography and arteriography, three patients with venous signal at MR angiography failed to exhibit CCF at arteriography.

CONCLUSION

Identification of normal cavernous sinus or inferior petrosal sinus venous signal on 3D TOF MR angiograms may occur frequently, and is probably dependent on technical factors that vary among scanners. The exact factors most responsible, however, were not elucidated by this preliminary review.

Lower extremity volumetric arterial blood flow in normal subjects

The objective of this clinical study was to establish normal values for volumetric blood flow in the leg at rest using Doppler ultrasound, and to determine what biophysical factors influence resting volumetric flow. Arterial blood flow was measured at four sites in the legs of 40 healthy subjects using an ATL Ultramark 9 HDI system. All subjects were nonhypertensive nonsmokers with ankle brachial index values greater than 1 and no history of vascular disease. The subjects, 20 of each gender, in age ranging from 20 to 64 y were examined. Blood flow was calculated from the time-averaged, intensity-weighted mean velocity Doppler waveforms and vessel cross-sectional area at the same site. Thigh and calf circumference measurements were used to estimate muscle masses. The mean flow and standard error measured in four arteries in the leg were: 284+/-21 mL/min in the common femoral (CFA); 152+/-10 mL/min in the superficial femoral (SFA); 72+/-5 mL/min in the popliteal; and 3+/-1 mL/min in the dorsalis pedis. Although women tended to have higher time-averaged mean velocities in the CFA and SFA than men (t-test, p < 0.008), their arterial cross-sectional areas tended to be smaller (t-test, p < 0.004) and no statistically significant difference was found between men and women in volumetric flow at any site. No correlation was found between age, weight, height, muscle mass and volumetric flow at all four sites. These estimates of lower extremity volumetric flow in healthy subjects provide a baseline for future studies of flow rates in patients with vascular disease.

Inertial cavitation produced by pulsed ultrasound in controlled host media

The purpose of this work is to provide measurements in well-characterized media in order to better understand inertial (transient) cavitation phenomena. Focused transducers with megahertz center frequencies (2.5 MHz, 4.3 MHz) and a clinical diagnostic ultrasound system (4.0 MHz) was used to generate pulsed ultrasound to induce cavitation. An improved active cavitation detector which utilizes a highly focused transducer with much higher center frequency (30 MHz) was used to measure the threshold of inertial cavitation. In order to study the effect of the concentration of nucleation agents on cavitation thresholds, experiments were conducted in distilled water seeded with polystyrene particles. Inertial cavitation thresholds were measured for various concentrations of polystyrene particles. It was observed that the threshold decreased from 2.5 MPa at concentration of about 10(6) particles/ml to 1.6 MPa at a concentration of about 10(9) particles/ml. The effect of the concentration is not significant for smaller changes of concentration. Measurements of the cavitation thresholds were then made in specially developed phantom materials to study the effect of viscosity on the cavitation threshold when surface tension and other mechanical properties of the materials are kept relatively constant. Experimental results show that the threshold increases with increasing viscosity, consistent with theoretical predictions. Cavitation was also detected in water seeded with polystyrene particles using a clinical ultrasound system at an acoustic pressure of 3.84 MPa. Results are comparable with those obtained in the laboratory using a 4.3-MHz focused transducer.

In vitro measurements of inertial cavitation thresholds in human blood

Inertial cavitation thresholds were measured in human blood exposed to pulsed ultrasound. Freshly drawn blood, bank blood and aqueous dilutions of both were used in this experimental study. Micrometer-sized polystyrene particles were used as extra potential nuclei in some samples. Focused transducers with megahertz center frequencies (2.5 MHz, 4.3 MHz) were employed to generate pulsed ultrasound to induce cavitation. Specially designed cells for hosting the blood samples were made to adapt to the experimental environment. Cavitation threshold measurements were achieved by using an active cavitation detection scheme which utilizes a highly focused transducer with a much higher center frequency (30 MHz). In 50% diluted blood samples, when no polystyrene particles were added to the samples, the threshold for cavitation was about 4.1 MPa at 2.5 MHz, while no cavitation was detected at 4.3 MHz. Generally, the measured thresholds decrease in samples with lower volume concentration of red blood cells or when polystyrene particles were added to the samples. Results show that the measured thresholds in some circumstances are in the range of output pressure of diagnostic ultrasound instrumentation; but for whole, freshly drawn blood, our apparatus was unable to detect cavitation, even at 6.3 MPa.

Volumetric flow estimation in vivo and in vitro using pulsed-Doppler ultrasound

The measurement of volumetric blood flow in small vessels in vitro and in vivo poses a significant technological challenge. In this study, two pulsatile flow models were developed, one with a 3.2-mm lumen diameter and one with a 12.7-mm lumen diameter, to assess the accuracy of volumetric flow estimation of two pulsed-Doppler devices, a Crystal Biotech VF1 20-MHz system with either a cuff-mounted or a needle-mounted probe and an Advanced Technology Laboratories Ultramark 9 High Definition Imaging system with a 5-MHz linear array transducer. The VF1 volumetric flow error was measured in the 3.2-mm phantom over a variety of pulsatile and continuous waveforms. The accuracy of the VF1 was also tested in porcine femoral and renal arteries. VF1 volumetric flow error ranged from 4.8% to 54.3% in the in vivo studies. The ATL demonstrated similar volumetric flow errors in the porcine femoral artery (approximately 3.2 mm diameter), but these errors were reduced to < or = 17.4% in the 12.7-mm-diameter in vitro flow model.

Direct evidence of cavitation in vivo from diagnostic ultrasound

Recent increases in the pressure output of diagnostic ultrasound scanners have led to an interest in establishing thresholds for bioeffects in many organs including the lungs of mammals. Damage may be mediated by inertial cavitation, yet there have been no such direct observations in vivo. To explore the hypothesis of cavitation-based bioeffects from diagnostic ultrasound, research has been performed on the thresholds of damage in rat lungs exposed to 4.0-MHz pulsed Doppler and color Doppler ultrasound. A 30-MHz active cavitation detection scheme complementing these studies provides the first direct evidence of cavitation in vivo from diagnostic ultrasound pulses.

Quantitative Doppler assessment of acute scrotal inflammation

PURPOSE

To establish quantitative Doppler criteria for acute unilateral epididymitis and/or orchitis.

MATERIALS AND METHODS

Triplex ultrasonography (US) was used to prospectively evaluate 31 patients with acute hemiscrotal pain and 15 asymptomatic control subjects. Morphology and perfusion were assessed, and peak systolic velocities (PSVs) were recorded. Ratios of PSV were calculated by using values from the right and left sides.

RESULTS

PSVs in control subjects differed significantly from those in patients (P < .0001). A PSV > or = 15 cm/sec produced diagnostic accuracy of 90% for orchitis and 93% for epididymitis: Five of six false-negative findings were in patients younger than age 15 years. Epididymal PSV ratios > or = 1.7 or testicular ratios > or = 1.9 were diagnostic of acute inflammation (P < .0001). In three cases, morphologic and color Doppler findings were normal and only the PSV ratios were diagnostic.

CONCLUSION

PSV and PSV ratios provide diagnostic criteria for acute unilateral epididymitis and/or orchitis that are more accurate than morphologic evaluation and color flow imaging either alone or in combination.

Validation of volume flow measurements with cine phase-contrast MR imaging for peripheral arterial waveforms

A flow phantom was used to study MR volume flow measurements for monophasic and triphasic waveforms over the flow range expected in peripheral arteries at rest and with exercise (2-24 mL/sec, n = 50). The improvement in accuracy with phase-correction image processing to eliminate errors caused by eddy currents was measured. Volume flow estimates with Doppler sonography were also measured. MR volume flow measurements correlated with volume collection with r = .996 and mean error = 4.6%. Phase-correction processing decreased mean error from 12.6% to 4.6% (P < .001, paired t-test). Doppler sonography had a higher mean error of 10.3% (P < .001, unpaired t-test). Cine phase-contrast MR imaging provides accurate estimates of volume blood flow for waveforms and flow ranges expected in peripheral arteries.

Characterization of focal hepatic lesions with duplex sonography: findings in 198 patients

OBJECTIVE

The purpose of this study was to determine the value of duplex Doppler sonography in the characterization of focal hepatic lesions.

SUBJECTS AND METHODS

Duplex Doppler sonography with a 3-MHz transducer was performed prospectively on 198 patients with focal hepatic lesions. One hundred thirty-two patients had pathologically proved diagnoses, confirming 46 hepatomas and 86 metastatic lesions. Twenty-three hemangiomas were confirmed either with tissue, tagged RBC study, MR imaging, or hemangioma CT protocol. Forty-three patients with no history of cancer had benign lesions that were established on the basis of follow-up imaging and at least a 2-year clinical history with no evidence of cancer.

RESULTS

Thirty-two hepatomas and four metastatic lesions had Doppler shifts of 4.5 kHz or more, giving a 70% sensitivity, 95% specificity, and 89% positive predictive value in distinguishing hepatomas from metastatic lesions. Forty-three hepatomas, 47 metastatic lesions, one hemangioma, and three benign lesions had Doppler shifts of 1.75 kHz or more, giving a 68% sensitivity, 94% specificity, and 96% positive predictive value in distinguishing malignant from benign lesions. Doppler shifts (mean +/- SD) were as follows: hepatomas, 4.72 +/- 1.72 kHz; metastases, 1.99 +/- 1.63 kHz; and hemangiomas, 0.53 +/- 0.75 kHz. These differences in mean Doppler shifts were statistically significant (p < .0001).

CONCLUSION

Duplex Doppler sonography aids in distinguishing hepatomas from metastases and malignant from benign hepatic lesions with a high degree of specificity but low sensitivity.

An ethnographic study of nursing culture as an exploration for determining the existence of a system of ritual

The idea that much of nursing is 'ritualized' activity which is harmful to patient care assumes that 'ritual' itself is unacceptable behaviour or practice. At a time when market forces are clearly influencing the delivery of care and, in turn, changes in nursing practice, it has become important both to clarify what 'ritual' is and to determine its existence and 'form' within nursing. This study explored nursing culture for 'ritual' in a ward setting and used ethnography as both method and description. Rituals were found to exist in the working day of the nurses studied, but was not an indication that 'ritualized behaviour' is harmful to individualized patient care. There is a clear need, however, to determine specifically the difference between 'unsafe outdated practices' and ritual in a cultural 'sense'. This would ensure that what had to be relinquished would in no way jeopardize the future existence of nursing and nurses as socially cohesive groups with their own culture.

Endovaginal US and Doppler findings after first-trimester abortion

Endovaginal ultrasound (US) imaging and color Doppler flow imaging techniques were used to evaluate the uterus and its contents and to establish characteristics of a normal post-abortion appearance in 19 women who underwent elective first-trimester abortions. Twenty-two examinations were performed between 2 and 17 days after the procedure. Thirteen of the 22 examinations (59%) showed different amounts of intrauterine material of varying echogenicity. Seven of the 22 examinations (32%) showed a thick endometrial stripe, and only two showed a normal stripe. Color Doppler flow imaging demonstrated typical peritrophoblastic flow in four of eight patients on the second and third days after the abortions were performed. After the third day, flow was observed in only two of 11 patients, and intrauterine material was also seen. These results indicate that intrauterine material and low-impedance flow are frequently observed after an abortion and do not necessarily indicate clinically important retained products of conception.

Endovaginal pulsed and color Doppler in first-trimester pregnancy

Endovaginal pulsed and color Doppler techniques were used to evaluate 23 unselected first-trimester pregnancies prior to elective abortion. Mean estimated gestational age was 8.9 +/- 1.6 weeks (range = 6.6-13.0). Low impedance, high velocity peritrophoblastic flow was seen in all cases. Mean peak systolic and end-diastolic velocities were 41.0 +/- 19.7 and 22.9 +/- 6.5 cm/s; mean resistance index (RI) was 0.41 +/- 0.10. Forty-five uterine arteries were evaluated with mean peak systolic and end-diastolic velocities of 44.1 +/- 14.7 and 7.6 +/- 4.3 cm/s; mean RI was 0.81 +/- 0.10. The peritrophoblastic and uterine artery velocities did not correlate with gestational age. Both ovaries were evaluated in 14 patients. One had no detectable flow in either ovary. For the active ovary in the other 13 patients, mean peak systolic and end-diastolic velocities were 27.4 +/- 9.9 and 15.2 +/- 5.9 cm/s; mean RI was 0.44 +/- 0.09. The corresponding values for the inactive ovary were 8.9 +/- 3.8 cm/s, 2.0 +/- 2.0 cm/s, and 0.76 +/- 0.22. These results can be used as a baseline for future studies of abnormal pregnancies.

In vitro detection of cavitation induced by a diagnostic ultrasound system

Experiments were performed to determine whether a clinical diagnostic scanner, a Hewlett-Packard (HP) 77020A, could produce cavitation in water containing suspensions of either 0.245-mum polystyrene spheres or Albunex, 1-10 mum albumin-coated microbubbles. Two calibrated, phased-array HP imaging transducers with 2.5- and 5.0-MHz operating frequencies were driven in M-mode (single cycle) and Doppler mode 4 cycles by the HP imaging system. Cavitation was detected in the water with polystyrene spheres at 2.5 MHz in both M-mode and Doppler mode at a peak negative acoustic pressure of 1.1 MPa or greater. Insonification at 5.0 MHz in either mode did not produce a detectable amount of cavitation, even with peak negative pressures as high as 1.2 MPa. Cavitation was not detected in water with the Albunex spheres at either frequency.

Gauging the likelihood of cavitation from short-pulse, low-duty cycle diagnostic ultrasound

Although no deleterious effects form diagnostic ultrasound have been reported in epidemiologic studies and surveys of widespread clinical usage (Ziskin and Petitti 1988), the conditions for the onset of transient cavitation must be investigated in the total evaluation of potential risks associated with diagnostic ultrasound applications. An extension of the results from the approximate theory developed by Holland and Apfel (1989) is applied in this paper to a population of nuclei to predict the onset of cavitation in host fluids with physical properties similar to those of biological fluids. From this analysis and from results of recent in vitro cavitation experiments, an index is developed which can gauge the likelihood of substantial microbubble growth in the presence of short-pulse, low-duty cycle diagnostic ultrasound.

Thresholds for transient cavitation produced by pulsed ultrasound in a controlled nuclei environment

Transient cavitation is a discrete phenomenon that relies on the existence of stabilized nuclei, or pockets of gas within a host fluid, for its genesis. A convenient descriptor for assessing the likelihood of transient cavitation is the threshold pressure, or the minimum acoustic pressure necessary to initiate bubble growth and subsequent collapse. An automated experimental apparatus has been developed to determine thresholds for cavitation produced in a fluid by short tone bursts of ultrasound at 0.76, 0.99, and 2.30 MHz. A fluid jet was used to convect potential cavitation nuclei through the focal region of the insonifying transducer. Potential nuclei tested include 1-microns polystyrene spheres, microbubbles in the 1- to 10-microns range that are stabilized with human serum albumin, and whole blood constituents. Cavitation was detected by a passive acoustical technique that is sensitive to sound scattered from cavitation bubbles. Measurements of the transient cavitation threshold in water, in a fluid of higher viscosity, and in diluted whole blood are presented. These experimental measurements of cavitation thresholds elucidate the importance of ultrasound, host fluid, and nuclei parameters in determining these thresholds. These results are interpreted in the context of an approximate analytical theory for the prediction of the onset of cavitation.

An improved theory for the prediction of microcavitation thresholds

An approximate analytical formulation is presented that allows for the calculation of acoustic pressure thresholds for transient cavitation over a variety of frequencies and host fluid parameters. Specifically, R.E. Apfel's (1986) theory is extended to include an estimate of the time delay associated with the Laplace pressure, 2sigma/R(0), where sigma is the surface tension and R(0) is the initial radius. Also presented is a correction factor for the time-averaged pressure difference, across the bubble wall during growth. An optimum size distribution of nuclei for the predisposition of a sample to microcavitation is exhibited. The role of transient cavitation in medical ultrasound is discussed.

Thresholds for cavitation produced in water by pulsed ultrasound

The threshold for transient cavitation produced in water by pulsed ultrasound was measured as a function of pulse duration and pulse repetition frequency at both 0.98 and 2.30 MHz. The cavitation events were detected with a passive acoustic technique which relies upon the scattering of the irradiation field by the bubble clouds associated with the events. The results indicate that the threshold is independent of pulse duration and acoustic frequency for pulses longer than approximately 10 acoustic cycles. The threshold increases for shorter pulses. The cavitation events are likely to be associated with bubble clouds rather than single bubbles.