Exploring the Chain Release Mechanism from an Atypical Apicomplexan Polyketide Synthase.

Polyketide synthases (PKSs) are megaenzymes that form chemically diverse polyketides and are found within the genomes of nearly all classes of life. We recently discovered the type I PKS from the apicomplexan parasite Toxoplasma gondii, TgPKS2, which contains a unique putative chain release mechanism that includes ketosynthase (KS) and thioester reductase (TR) domains. Our bioinformatic analysis of the thioester reductase of TgPKS2, TgTR, suggests differences compared to other systems and hints at a possibly conserved release mechanism within the apicomplexan subclass Coccidia. To evaluate this release module, we first isolated TgTR and observed that it is capable of 4 electron (4e-) reduction of octanoyl-CoA to the primary alcohol, octanol, utilizing NADH. TgTR was also capable of generating octanol in the presence of octanal and NADH, but no reactions were observed when NADPH was supplied as a cofactor. To biochemically characterize the protein, we measured the catalytic efficiency of TgTR using a fluorescence assay and determined the TgTR binding affinity for cofactor and substrates using isothermal titration calorimetry (ITC). We additionally show that TgTR is capable of reducing an acyl carrier protein (ACP)-tethered substrate by liquid chromatography mass spectrometry and determine that TgTR binds to holo-TgACP4, its predicted cognate ACP, with a KD of 5.75 ± 0.77 µM. Finally, our transcriptional analysis shows that TgPKS2 is upregulated ∼4-fold in the parasite's cyst-forming bradyzoite stage compared to tachyzoites. Our study identifies features that distinguish TgPKS2 from well-characterized systems in bacteria and fungi and suggests it aids the T. gondii cyst stage.

Seasonal temperatures in South Eleuthera, The Bahamas, have considerable impacts on the cardiorespiratory function and swimming performance of Nassau grouper (Epinephelus striatus).

Surprisingly, the impacts of environmental changes on the physiology of tropical/subtropical marine fishes have received limited attention. Given that (i) temperature is considered to be a key factor controlling the biology of fishes; (ii) no published data are available on the swimming performance, metabolic capacity or cardiac function of any of the ~165 grouper species worldwide; and (iii) the Nassau grouper is an endangered species of great ecological and socioeconomic significance in The Bahamas, we investigated how current summer/early fall (30°C) and winter (22°C) temperatures in South Eleuthera affected the aerobic metabolism and heart function of wild Nassau grouper when swum to exhaustion (i.e. to their critical swimming speed, Ucrit). The Nassau grouper had a very low Ucrit at 30°C (i.e. <1 body lengths s-1), and a 30% lower swimming performance during the winter (at 22°C), and this was that was indicative of a reduced absolute aerobic scope (~185 vs. 290 mg O2 kg-1 h-1) and values of maximum heart rate ([Formula: see text]HMax) and scope for [Formula: see text]H that were only one-half of that achieved at 30°C (~60 vs. 120 and 29 vs. 61 beats min-1, respectively). Overall, these data reveal that the Nassau grouper's aerobic and swimming capacity are well below values reported for other tropical/subtropical fishes and suggest that, despite a compensatory (~30-40%) increase in stroke volume, constraints on [Formula: see text]H near this species' lower thermal limit negatively affect its cardiac output and swimming performance. These findings have considerable ecological implications as Bahamian grouper populations migrate over long distances to spawn during the winter months, and given the predicted increase in temperature variability with climate change.

Acute and chronic cold exposure differentially affect cardiac control, but not cardiorespiratory function, in resting Atlantic salmon (Salmo salar).

No studies have examined the effects of cold temperatures (∼0-1 °C) on in vivo cardiac function and control, and metabolism, in salmonids. Thus, we examined: 1) how acclimation to 8 °C vs. acclimation (>3 weeks) or acute exposure (8-1 °C at 1 °C h-1) to 1 °C influenced cardiorespiratory parameters in resting Atlantic salmon; and 2) if/how the control of cardiac function was affected. Oxygen consumption ( M ˙ O 2 ) and cardiac function [i.e., heart rate (f H) and cardiac output ( Q ˙ ) ] were 50% lower in the acutely cooled and 1oC-acclimated salmon as compared to 8 °C fish, whereas stroke volume (VS) was unchanged. Intrinsic f H was not affected by whether the fish were acutely exposed or acclimated to 1 °C (values ∼51, 24 and 21 beats min-1 in 8 and 1 °C-acclimated fish, and 8-1 °C fish, respectively), and in all groups f H was primarily under adrenergic control/tone (cholinergic tone 13-18%; adrenergic tone 37-70%). However, ß-adrenergic blockade resulted in a 50% increase in VS in the 1oC-acclimated group, and this was surprising as circulating catecholamine levels were ∼1-3 nM in all groups. Overall, the data suggest that this species has a limited capacity to acclimate to temperatures approaching 0 °C. However, we cannot exclude the possibility that cardiac and metabolic responses are evoked when salmon are cooled to ∼ 0-1 °C, and that this prevented further declines in these parameters (i.e., they 'reset' quickly). Our data also provide further evidence that VS is temperature insensitive, and strongly suggest that changes in adrenoreceptor mediated control of venous pressure/capacitance occur when salmon are acclimated to 1 °C.

Hydrogel particles improve detection of SARS-CoV-2 RNA from multiple sample types.

Here we present a rapid and versatile method for capturing and concentrating SARS-CoV-2 from contrived transport medium and saliva samples using affinity-capture magnetic hydrogel particles. We demonstrate that the method concentrates virus from 1 mL samples prior to RNA extraction, substantially improving detection of virus using real-time RT-PCR across a range of viral titers (100-1,000,000 viral copies/mL) and enabling detection of virus using the 2019 nCoV CDC EUA Kit down to 100 viral copies/mL. This method is compatible with commercially available nucleic acid extraction kits (i.e., from Qiagen) and a simple heat and detergent method that extracts viral RNA directly off the particle, allowing a sample processing time of 10 min. We furthermore tested our method in transport medium diagnostic remnant samples that previously had been tested for SARS-CoV-2, showing that our method not only correctly identified all positive samples but also substantially improved detection of the virus in low viral load samples. The average improvement in cycle threshold value across all viral titers tested was 3.1. Finally, we illustrate that our method could potentially be used to enable pooled testing, as we observed considerable improvement in the detection of SARS-CoV-2 RNA from sample volumes of up to 10 mL.

Interactions of native and non-native lady beetle species (Coleoptera: Coccinellidae) with aphid-tending ants in laboratory arenas.

Interactions between lady beetles and the European fire ant (Myrmica rubra L.) tending potato aphids [Macrosiphum euphorbiae (Thomas)] were compared in the laboratory. Lady beetle species native to North America (Coccinella trifasciata perplexa Mulsant, Coleomegilla maculata lengi Timberlake, Hippodamia convergens Guérin-Méneville) and non-native species of Palearctic origin [Coccinella septempunctata L., Harmonia axyridis (Pallas), Hippodamia variegata (Goeze), Propylea quatuordecimpunctata L.)] were evaluated. Harmonia axyridis consumed a significantly greater number of aphids compared with all other species but C. septempunctata. Ant stings affected H. variegata and C. septempunctata to a greater extent than other species. Ants showed a significantly greater amount of aggression toward H. convergens and H. variegata compared with P. quatuordecimpunctata. P. quatuordecimpunctata, C. trifasciata, and H. axyridis reacted significantly less to ants compared with H. variegata, H. convergens, C. maculata, and C. septempunctata. Differences in interactions with natural enemies may explain, in part, the successful establishment of some non-native coccinellids in new habitats.

Diagnostic accuracy of contrast-enhanced computed tomography for assessment of mandibular and medial retropharyngeal lymph node metastasis in dogs with oral and nasal cancer.

The diagnostic accuracy of contrast-enhanced CT for detection of cervical lymph node metastasis in dogs is unknown. The purpose of this retrospective, observational, diagnostic accuracy study was to assess the efficacy of CT for detection of mandibular and medial retropharyngeal lymph node metastasis in dogs. Histopathology of dogs with cancer of the head, CT and bilateral mandibular and medial retropharyngeal lymphadenectomy was reviewed. A single radiologist measured lymph nodes to derive short axis width and long-short axis ratios. Two blinded radiologists separately assessed lymph node margins, attenuation and contrast enhancement and each provided a final subjective interpretation of each node site as benign or neoplastic. Where radiologists' opinions differed, a consensus was reached. Sensitivity, specificity and accuracy were calculated for mandibular and medial retropharyngeal sites. Agreement between radiologists was assessed. Fisher's exact test and the Kruskal-Wallis H-test were used to assess associations between variables. Forty-one primary tumours were recorded in 40 dogs. Metastasis to mandibular or retropharyngeal lymph nodes occurred in 16 out of 40 dogs (43/160 nodes). Agreement between radiologists was almost perfect for margination, attenuation and enhancement, strong for interpretation of mandibular lymph node metastasis, and weak for interpretation of medial retropharyngeal lymph node metastasis. Sensitivity of CT was 12.5% and 10.5%, specificity was 91.1% and 96.7%, and accuracy was 67.5% and 76.3% for mandibular and medial retropharyngeal lymph nodes respectively. No individual CT findings were predictive of nodal metastasis. Given the low sensitivity of CT, this modality cannot be relied upon alone for assessment of cervical lymph node metastasis in dogs.

Neonatal nephrocalcinosis: long term follow up.

AIMS: To assess the spontaneous resolution of neonatal nephrocalcinosis and its long term effects on renal function. METHODS: Fourteen very low birthweight preterm babies with nephrocalcinosis were followed up at 5-7 years of age; 14 controls were matched for sex, gestation, and birth weight. Height, weight, blood pressure, and renal symptomatology were recorded, and a renal ultrasound scan was performed. Early morning urine osmolality and creatinine ratios of albumin, phosphate, calcium, oxalate and beta microglobulin were determined. Urea and electrolytes in the study group were determined, and glomerular filtration rate (GFR) and TmP/GFR (tubular reabsorption of phosphate per GFR) were calculated. Statistical analysis was performed on a group basis using the Mann-Whitney confidence interval. RESULTS: Mean age was 6.9 years (range 5.81-7.68). An early morning urine osmolality >700 mOsm/kg was achieved in all cases. In two cases and four controls, the calcium/creatinine ratio was >0.7 mmol/mmol. In all cases, the GFR was normal (median 132.6 ml/min/1.73 m(2) (range 104.1-173.1)). Median TmP/GFR was 1.22 mmol/l (0.73-1.61), with two having levels below the normal range. These did not have persisting nephrocalcinosis. Nephrocalcinosis was found in three of the 12 cases scanned and one control. There were no significant differences in urine biochemistry. CONCLUSIONS: Resolution of nephrocalcinosis occurred in 75% of cases. No evidence was found to suggest that nephrocalcinosis is associated with renal dysfunction in the long term. There was evidence of hypercalciuria in the cases and controls, suggesting that prematurity may be a risk factor.

Purification and kinetic characterization of a specific glucokinase from Streptococcus mutans OMZ70 cells.

Glucokinase (ATP-D-glucose 6-phosphotransferase, EC 2.7.1.2) was purified 144-fold from extracts of sucrose-grown Streptococcus mutans OMZ70 (ATCC 33535) cells. Twenty compounds were tested as potential substrates; only glucose (Km = 0.61 mM) was phosphorylated. The reaction catalyzed by the purified enzyme was dependent on the presence of glucose, nucleoside triphosphate and metal ion; glucose 6-phosphate and ADP were the products. Of the seven nucleoside triphosphates tested, ATP (Km = 0.21 mM) was the most efficient phosphate donor in the enzyme-catalyzed formation of glucose 6-phosphate. Both Mn2+ (relative activity, 173%) and Co2+ (264%) were more efficient than Mg2+ (100%) in supporting the enzyme reaction. The enzyme exhibited a broad maximal activity in the pH range from 7.5 to 9.5. The apparent molecular weight of glucokinase, as determined by gel filtration, was 41 000. With glucose held constant at either saturating or subsaturating levels, ADP was a noncompetitive inhibitor of ATP (Ki = 0.67 mM). ADP was an uncompetitive inhibitor of glucose (Ki = 0.71 mM) when ATP was held constant at either a saturating or subsaturating concentration. Glucose 6-phosphate was a competitive inhibitor of glucose (Ki = 0.31 mM) at saturating ATP and exhibited noncompetitive or mixed inhibition at a subsaturating ATP concentration. Glucose 6-phosphate was not an inhibitor toward ATP at saturating glucose concentrations, but exhibited noncompetitive inhibition at subsaturating glucose concentrations. The kinetic data support the postulation of a sequential mechanism for the glucokinase reaction; they are consistent with an ordered mechanism in which glucose binds first and glucose 6-phosphate dissociates last. Furthermore, the data suggest the existence of more than one enzyme binding site for the substrates of the glucokinase reaction.

Partial purification and properties of a specific glucokinase from Streptococcus mutans SL-1.

The presence of glucokinase (ATP:D-glucose 6-phosphotransferase, EC 2.7.1.2) activity in seven strains of oral streptococci is demonstrated. The glucokinase purified from Streptococcus mutans SL-1 cells is shown to be a highly specific enzyme, phosphorylating only glucose (eight sugars tested). The enzyme is a true glucokinase: formation of the product, shown here to be glucose 6-phosphate, is dependent on the presence of glucose, ATP, divalent metal ion and enzyme. The Km for glucose is 1.40 mM, the pH optimum for the enzyme is a broad plateu from pH 7.1 to 9.5 and the molecular weight is estimated to be 40 000. The finding of a glucokinase in oral streptococci indicates the existence of an intracellular mechanism of glucose phosphorylation. The implications of this observation are discussed.

The solubility of anesthetic gases in lipid bilayers.

We have measured the lipid/gas partition coefficients at various temperatures of eight anesthetic agents in two sonicated lipid bilayers containing either 96% egg phosphatidylcholine/4% phosphatidic acid or 64% egg phosphatidylcholine/3% phosphatidic acid/33% cholesterol. The Bunsen lipid/gas partition coefficients in the pure phospholipid bilayer at 25 degrees C were: methoxyflurane 820 (interpolated), halothane 150, isoflurane 140, fluroxene 52, xenon 1.4, sulfur hexafluoride 0.24, carbon tetrafluoride 0.056 and hexafluoroethane 0.34. These partition coefficients were close to those in a bulk hydrophobic solvent (olive oil) but were reduced by about 20% in the cholesterol-containing bilayer preparation. In biomembranes the partition coefficient for halothane was lower than in lipid bilayers by about half an order of magnitude. As in olive oil, the partition coefficients mostly increased with decreasing temperature. The enthalpy, entropy and free energy associated with transfer of 1 mol of these agents from the gas phase at 1 atmosphere partial pressure and 25 degrees C into the lipid bilayers under the same conditions were calculated from the temperature variation of the partition coefficients. All of these compounds, with the exception of methoxyflurane, fit the Barclay Butler relationship between entropy and enthalpy of partitioning. The Bunsen partition coefficients were correlated with the anesthetic potencies of seven of these agents in mice and in dogs. Comparisons were made between the different bilayers and olive oil and between hypotheses of anesthesia based on concentration of anesthetic at the active site (Meyer-Overton) and based on the product of concentration and molar volume of anesthetic at the active site (Mullins). Excellent correlations between anesthetic potency and lipid bilayer partition were obtained in all cases. The most consistent fits to the predicted slopes were achieved when both molar volume and partitioning of the anesthetic into the cholesterol-containing bilayer were taken into account, but the differences between the models were small.

Human defensin 5 expression in intestinal metaplasia of the upper gastrointestinal tract.

BACKGROUND: Upper gastrointestinal tract intestinal metaplasia (IM) is termed Barrett's oesophagus (BO) or gastric intestinal metaplasia (GIM), depending on its location. BO and GIM are associated with chemical exposure resulting from gastro-oesophageal reflux and chronic Helicobacter pylori infection, respectively. Paneth cells (PCs), characterised by cytoplasmic eosinophilic granules, are found in a subset of IM at these sites, but histology may not accurately detect them. AIM: To determine human defensin 5 (HD5; an antimicrobial peptide produced by PCs) expression in BO and GIM, and to investigate its association with H pylori infection. METHODS: Endoscopic biopsies from 33 patients with BO and 51 with GIM, and control tissues, were examined by routine histology and for H pylori infection and HD5 mRNA and protein expression. RESULTS: In normal tissues, HD5 expression was specific for PCs in the small intestine. Five patients with BE and 42 with GIM expressed HD5, but few HD5 expressing cells in IM had the characteristic histological features of PCs. Most HD5 positive specimens were H pylori infected and most HD5 negative specimens were not infected. CONCLUSIONS: HD5 immunohistochemistry was often positive in IM when PCs were absent by conventional histology. Thus, HD5 immunohistochemistry may be superior to histology for identifying metaplastic PCs and distinguishing GIM from BO. The higher frequency of HD5 expression in GIM than in BO is associated with a higher frequency of H pylori infection, suggesting that in IM PCs may form part of the mucosal antibacterial response.

Calcium supplementation prevents seasonal bone loss and changes in biochemical markers of bone turnover in elderly New England women: a randomized placebo-controlled trial.

Elderly women are at increased risk for bone loss and fractures. In previous cross-sectional and longitudinal studies of women residing in northern latitudes, bone loss was most pronounced during winter months and in those consuming less than 1 g calcium per day. In this study we sought to test the hypothesis that calcium supplementation by either calcium carbonate or dietary means would prevent seasonal bone loss and preserve bone mass. Sixty older postmenopausal women without osteoporosis were randomized to one of three treatment arms: Dietary milk supplementation (D-4 glasses of milk/day), Calcium carbonate (CaCO3-1000 mg/day in two divided doses), or placebo (P). After 2 yr, placebo-treated women consumed a mean of 683 mg/day of calcium and lost 3.0% of their greater trochanteric (GT) bone mineral density (BMD) (P < 0.03 vs. baseline); Dietary supplemented women averaged a calcium intake of 1028 mg/day and sustained minimal loss from the GT (-1.5%; P = 0.30), whereas CaCO3-treated women (total Ca intake, 1633 mg/day) suffered no bone loss from the GT and showed a significant increase in spinal and femoral neck BMD (P < 0.05). Femoral bone loss occurred exclusively during the two winters of the study (i.e. total loss, -3.2%; P < 0.02 in placebo-treated women) with virtually no change in GT BMD during summer. Serum 25-OH vitamin D declined by more than 20% (P < 0.001) in all groups during the winter months but returned to baseline in summer; PTH levels rose approximately 20% (P < 0.001) during winter but did not return to baseline during the summers. Urine N-telopeptide and osteocalcin levels increased significantly but only in the P-treated women and only during winter. Serum insulin growth factor binding protein 4, an inhibitory insulin growth factor binding protein, rose 15% (P < 0.03) from summer to winter, but this increase was significant only in those women consuming <1000 mg/day of calcium. By multivariate analysis, total calcium intake was the strongest predictor of bone loss from the hip. Urinary N-telopeptide also closely correlated with GT BMD but only during winter (P = 0.003). We conclude that calcium supplementation prevents bone loss in elderly women by suppressing bone turnover during the winter when serum 25-OH vitamin D declines and serum PTH increases. The precise amount of calcium necessary to preserve BMD in elderly women requires further studies, although in this study, at least 1000 mg/day of supplemental calcium was adequate prophylaxis against femoral bone loss.

Nucleotide sequence of the beta-D-phosphogalactoside galactohydrolase gene of Lactobacillus casei: comparison to analogous pbg genes of other gram-positive organisms.

Lactose metabolism in Lactobacillus casei occurs via phosphoenolpyruvate-dependent phosphotransferase uptake of lactose and subsequent cleavage of lactose-6-phosphate by beta-D-phosphogalactoside galactohydrolase (P-beta Gal). The genes for lactose uptake and P-beta Gal have been shown to be plasmid-associated in L. casei 64H [Chassy et al., Curr. Microbiol. 1 (1978) 141-144]. The cloned P-beta Gal-coding gene (pbg) previously described [Lee et al., J. Bacteriol. 152 (1982) 1138-1146] was subcloned on a 2.9-kb KpnI-Bg/II fragment isolated from pLZ605. Sequence analysis of this fragment revealed an open reading frame of 1422 bp capable of coding for a protein product containing 474 amino acids and having an Mr of 53,989. The L. casei protein showed a high degree of homology to the proteins whose sequence was deduced from the nucleotide sequence of the pbg genes of Staphylococcus aureus and Streptococcus lactis. Because of the significant homologies observed, as reflected in amino acid content as well as predicted structural characteristics of the three proteins, we suggest a common origin for the P-beta Gals of these three organisms.

Isolation of human intestinal defensins from ileal neobladder urine.

We describe the isolation of naturally occurring human intestinal defensins HD-5 and HD-6 from ileal neobladder urine and ileal mucosa. Using an antibody-based detection assay, we found multiple N-terminally processed forms of HD-5. The predominant HD-5 forms in tissue were longer than those in neobladder urine (amino acid (aa) 23-94 and 29-94 versus aa 36-94, 56-94 and 63-94) suggesting that Paneth cells store prodefensin that is processed to mature defensin during or after degranulation. Search for mature HD-6 yielded aa 69-100 as the predominant form in both sources. The ileal neobladder is a promising model to study human Paneth cell secretion.

Latitudinal gradients in colony size for social insects: termites and ants show different patterns.

On the basis of a comparison of Nearctic and Neotropical ants, social insects have been proposed to show a latitudinal gradient in colony size. Further, the "fasting endurance hypothesis," which predicts larger colonies in areas with extended periods of low food availability, was proposed as the mechanism driving the gradient. To test the generality of the pattern and its mechanism, we examined the relationships between termite colony size and both latitude and annual evapotranspiration, a measure of plant productivity. We found no evidence that colony size increases with increasing latitude or decreasing plant productivity. We conclude that the pattern identified for ants cannot be generalized to include social insects as a whole. As is the case for ecogeographic gradients in insect body sizes, a pattern that is reported for one taxon may not be consistent for other taxa at the global level.

Enhancement of tumor radiation response by the combination of a perfluorochemical emulsion and hyperbaric oxygen.

The presence of radioresistant hypoxic cells in tumors is believed to be one of the limiting factors in achieving local tumor control by radiotherapy. Treatment with hyperbaric oxygen during irradiation has been shown to improve the radiation response of many solid tumors in rodents and of some tumors in patients. Intravenous administration of perfluorochemical emulsions combined with oxygen breathing at atmospheric pressure has also been shown to improve the radiation response of several rodent tumors. Theoretical considerations suggest that the combination of a perfluorochemical emulsion and hyperbaric oxygen should be significantly more effective than either agent alone. This hypothesis was tested by examining the radiation response of BA1112 rhabdomyosarcomas growing in WAG/rij-Y rats. Treatment with a perfluorochemical emulsion, Fluosol-DA, plus hyperbaric oxygen (3 Atmospheres O2) significantly increased the radiation response of the malignant cells in these solid tumors. The observed changes in the tumor cell survival curve suggest that the combination of Fluosol-DA and HBO decreases the proportion of severely hypoxic cells in the tumor to less than 1.5% of the original value. The effect of the Fluosol-DA dose and the duration of pretreatment with HBO are described.

Effects of hyperbaric oxygen and a perfluorooctylbromide emulsion on the radiation responses of tumors and normal tissues in rodents.

Perfluorochemical emulsions are being examined in many laboratory and clinical studies as possible adjuncts to radiotherapy and chemotherapy. The studies reported here examine the clinical potential of hyperbaric oxygen (HBO) in combination with a highly concentrated perfluorochemical emulsion (Oxygent) containing 100% w/v perfluorooctylbromide (PFOB). HBO alone produced only a small improvement in the radiation response of BA1112 tumors in WAG/rij rats, while regimens combining HBO with Oxygent produced much greater radiation sensitization. A sham emulsion, formulated without the O2-carrying PFOB, did not alter the radiation response of the tumors in comparison with that seen with HBO alone. Neither HBO nor Oxygent plus HBO altered the radiosensitivity of bone marrow progenitor cells in BALB/c mice. HBO alone augmented skin reactions in BALB/c mice, but addition of Oxygent did not alter the skin reactions in comparison to those seen with HBO alone. Regimens combining Oxygent with HBO selectively increased the radiation sensitivity of tumors relative to normal tissues, thereby enhancing the therapeutic ratio. These results support the potential usefulness of perfluorochemical emulsions and HBO in clinical radiation therapy.

Modulation of tumor oxygenation and radiosensitivity by a perfluorooctylbromide emulsion.

The effect of a concentrated perfluorooctylbromide emulsion (Oxygent) on the radiosensitivity and oxygenation of solid tumors was examined using EMT6 mammary tumors in BALB/c mice and BA1112 rhabdomyosarcomas in WAG/rij rats. Treatment with Oxygent plus carbogen or oxygen breathing increased the radiosensitivity of both tumors. Analysis of tumor cell survival data and polarographic measurements of intratumoral pO2 indicated that this potentiation reflected an increase in the proportion of well-oxygenated tumor cells. Treatments with carbogen breathing alone, with Oxygent plus air-breathing, or with a vehicle emulsion containing all the components except the perfluorocarbon did not produce comparable improvements in tumor radiosensitivity. Concentrated perfluorooctylbromide emulsions appear to warrant further development and preclinical testing as adjuncts to cancer therapy.