Functional domains of a zinc metalloprotease from Vibrio vulnificus

Vibrio vulnificus, an opportunistic human pathogen causing wound infection and septicemia, secretes a 45-kDa metalloprotease (V. vulnificus protease; VVP). A plasmid which carries the entire vvp gene subcloned into pBluescriptIIKS+ was transformed into Escherichia coli DH5alpha for overproduction of the protease. The 45-kDa recombinant protease (rVVP) was isolated from the periplasmic fraction of the transformant by ammonium sulfate precipitation followed by column chromatography on phenyl Sepharose. Biochemical characterization of the isolated rVVP showed that the recombinant protease was identical to that produced by V. vulnificus. When rVVP was incubated at 37 degrees C, a 35-kDa fragment was generated through autoproteolytic removal of the C-terminal peptide. This 35-kDa fragment (rVVP-N) was found to have sufficient proteolytic activity toward oligopeptides and soluble proteins but had markedly reduced activity toward insoluble proteins. Lineweaver-Burk plot analysis indicated increased Km values of rVVP-N for all of the protein substrates. rVVP, but not rVVP-N, was shown to agglutinate rabbit erythrocytes, bind to the erythrocyte ghosts, and digest the ghost membrane proteins. These results strongly suggest that rVVP (and VVP) consists of at least two functional domains: an N-terminal 35-kDa polypeptide mediating proteolysis and a C-terminal 10-kDa polypeptide which may be essential for efficient attachment to protein substrates and erythrocyte membranes.

Lithium deficiency reduces thymus weight and alters differential blood count in rats

Effects of lithium (Li) deficiency and/or immobilization stress on the thymus weight and differential blood count of rats were studied. The thymus weight of the rats fed a low Li diet were lighter than those of rats fed a Li supplemented diet, whether the rats were exposed to stress or not. Of the rats not exposed to stress, those in the low Li diet group showed a significant decrease in the ratios of neutrophils and T helper lymphocytes. However, there was an increase in the total number of lymphocytes in the low Li dietary group. It was shown that Li deficiency altered the responses to stress in rats.

Effect of different concentrations of amino acids in human serum and follicular fluid on the development of one-cell mouse embryos in vitro

As a model for establishing an optimized medium for human in vitro fertilization (IVF), modified human tubal fluid (HTF) media containing amino acids at concentrations found in human serum and follicular fluid were prepared, and the effect of the media on development of random-bred (ICR) and F1 hybrid (CBF1) mice embryos was studied. The total concentrations of amino acids found in serum and follicular fluid were about one-third to one-half the concentrations present in two conventional media used in human IVF: Ham's F-10 and Eagle's minimal essential medium (MEM). When ICR mouse embryos were cultured in the HTF medium containing 21 amino acids at concentrations found in follicular fluid, the number of embryos developing to morulae at 72 h and to blastocysts at 96 h increased in comparison with those cultured in HTF medium. When HTF containing amino acids at concentrations found in serum was used, only induced morula formation at 72 h was enhanced. The number of hatching blastocysts at 96 h also increased when CBF1 mouse embryos were cultured with HTF supplemented with amino acids at concentrations found in follicular fluid. When ICR mouse embryos were cultured in modified HTF media containing concentrations of amino acids found in Ham's F-10 and MEM that contained higher concentrations of glutamine, embryo development was inhibited. The amount of ammonium produced during incubation for 3 days was significantly less when embryos were cultured in media containing concentrations of amino acids found in follicular fluid compared with when Ham's F-10 or MEM was the culture medium. Ammonium is produced by the breakdown of glutamine in the culture medium during incubation with or without embryos. These results suggest that the concentrations of amino acids found in follicular fluid are more effective and safer for embryo culture than those in other media currently in use.

Cloning and sequencing of the Vibrio parahaemolyticus fur gene

A ferric uptake regulatory gene (fur) was cloned from Vibrio parahaemolyticus WP1 by a polymerase chain reaction-based technique followed by functional complementation of a fur mutation in Escherichia coli. A sequence analysis showed that, at the amino acid level, the V. parahaemolyticus Fur protein is 81% identical with the Fur protein from E. coli and over 90% identical with those of the Vibrio species.

Some properties of nicked Vibrio vulnificus hemolysin

Vibrio vulnificus, an opportunistic human pathogen, secretes the 50 kDa single-chain hemolysin. When incubated with an exocellular protease from this vibrio, the 50 kDa hemolysin was cleaved in some peptides joined with the disulfide bond(s); the 40 kDa fragment and the small fragment(s) undetectable in SDS-PAGE. The nicked hemolysin induced comparable hemolysis through the same process as that of the intact toxin. However, the nicked hemolysin was found to be more stable against inactivation due to autoaggregation, so that it formed a larger precipitation zone in the single radial immunodiffusion test using the antiserum against the intact hemolysin. These results suggest that V. vulnificus hemolysin is modified to be a more hydrophilic protein by nicking, while it is not accompanied by loss of the hemolytic activity.

Vibrio mimicus attaches to the intestinal mucosa by outer membrane hemagglutinins specific to polypeptide moieties of glycoproteins

Vibrio mimicus is the closest organism to Vibrio cholerae. V. mimicus E-33, which is a highly adhesive and enteropathogenic strain, is known to produce three types of hemagglutinins (HAs), i.e., a 31-kDa exocellular metalloprotease (Vm-HA/protease), lipopolysaccharide (Vm-LPSHA), and a 39-kDa major outer membrane protein (Vm-OMPHA). Hemagglutination induced by Vm-LPSHA and Vm-OMPHA was inhibited by glycoproteins, including mucin, fetuin, and asialofetuin, but not by monosaccharides, disaccharides, or N-acetylated saccharides. The inhibitory potential of each glycoprotein for Vm-OMPHA was greatly augmented by treatment with a glycolytic enzyme such as beta-D-galactosidase or beta-D-glucosidase, while pronase treatment achieved complete abolition of the inhibitory potential. The inhibitory ability of the glycoproteins for Vm-LPSHA was also abolished by pronase treatment; however, glycolytic enzyme treatment showed no effect. Hence, the polypeptide portion of glycoproteins may directly associate with Vm-OMPHA and Vm-LPSHA, but the sugar moiety may act as a barrier to interaction with Vm-OMPHA. The glycoproteins as well as Fab antibodies against Vm-OMPHA and Vm-LPSHA eliminated the ability of E-33 cells to agglutinate rabbit erythrocytes and to attach to rabbit intestinal mucosa. Additionally, expression of the hemagglutinating ability by the bacterial cells was accompanied by efficient bacterial adherence to the intestinal mucosa. Finally, the hemagglutinating activity of Vm-OMPHA was markedly increased by incubation with Vm-HA/protease. These results indicate that all three HAs may have significant roles in the glycoprotein-mediated intestinal adherence of V. mimicus E-33.

Hemagglutination is a novel biological function of lipopolysaccharide (LPS), as seen with the Vibrio cholerae O139 LPS

It has been generally thought that the polysaccharide moiety of lipopolysaccharide (LPS) maintains only serological specificity, while the lipid A portion determines various biological functions. However, we found that hemagglutination was a common function of the polysaccharide moiety of LPSs from important human enteropathogenic bacteria. Of the LPSs examined, Vibrio cholerae O139 LPS showed the highest hemagglutinating activity. Glycoproteins, such as mucin and fetuin, showed efficient inhibition of the hemagglutinating ability. Since cell-mediated hemagglutination is known to be correlated with bacterial adherence, hemagglutination induced by the polysaccharide moiety is interpreted to indicate that cell-surface LPS is a potential adhesin.

A novel point mutation in the steroid sulfatase gene in X-linked ichthyosis

We analyzed the steroid sulfatase (STS) gene in nine Japanese patients with X-linked ichthyosis (XLI) by a polymerase chain reaction technique and subsequent DNA sequencing. Eight of nine patients showed complete deletion of the STS gene. In a patient of XLI exhibiting a normal amplifying pattern with predicted sizes of the STS gene, a novel mutation was found resulting in the appearance of a stop codon in exon 7 of the STS gene. This suggests that exon 7 or an area in its downstream region is important for STS activity.

Intracerebral Schwannoma--case report

A 15-year-old girl presented with a rare intracerebral schwannoma manifesting as epileptic seizure. Computed tomography, magnetic resonance imaging, and cerebral angiography showed a right parietooccipital lobe tumor. The preoperative diagnosis was malignant glioma, but immunohistochemical and ultrastructural examinations showed the tumor was indistinguishable from peripheral schwannoma. Only 29 cases of schwannomas not related to the cranial nerve have been reported, mostly in children and young adults. The origin appears to be ectopic or perivascular elements in the brain. Such intracerebral schwannomas can be classified according to origins into intra-axial, periventricular, dural attachment, and other types.

Magnetic resonance imaging of osteonecrosis in divers: comparison with plain radiographs

OBJECTIVE

To assess the diagnostic value of magnetic resonance imaging (MRI) as compared with radiographic findings in osteonecrosis in divers.

DESIGN AND PATIENTS

MRI scans and conventional radiographs of the shoulder, hip and knee joints of 23 professional male scuba divers were reviewed together with their clinical findings and personal histories. Correlations between the MRI findings and the radiographic evaluation, clinical symptoms, and personal history were then investigated.

RESULTS AND CONCLUSIONS

Lesions found on MRI in 23 divers included 27 in 39 proximal humeri, 17 in 36 proximal femora, 13 in 32 distal femora, and 12 in 32 proximal tibiae. Diffuse, marginated, or irregular patterns were observed. No lesions were seen in epiphyses of the distal femur or proximal tibia. We tried to classify these MRI findings by location and appearance. MRI showed no patients with only one affected bone. A close correlation between the MRI findings and maximum diving depth was observed in the proximal humerus. MRI depicted bone lesions that could not be detected on the radiographs. A routine MRI investigation of the hip joints should be performed in every diver in whom osteonecrosis is diagnosed at another site, for early detection of femoral head osteonecrosis. MRI of the shoulder joint is also the best surveillance in divers who dive deeper than 15 m.

Purification and characterization of a hemolysin produced by Vibrio mimicus

Vibrio mimicus is a causative agent of human gastroenteritis. This pathogen secretes a pore-forming toxin, V. mimicus hemolysin (VMH), which causes hemolysis by three sequential steps: binding to an erythrocyte membrane, formation of a transmembrane pore, and disruption of the cell membrane. VMH with a molecular mass of 63 kDa was purified by ammonium sulfate precipitation and column chromatography with phenyl Sepharose HP and Superose 6 HR. The hemolytic reaction induced by VMH continued up to disruption of all erythrocytes in the assay system. Moreover, VMH that bound preliminarily to erythrocyte ghosts showed a sufficient ability to attack intact erythrocytes. These results suggest reversible binding of the toxin molecule to the membrane. The final cell-disrupting stage was effectively inhibited by various divalent cations. Additionally, some cations, such as Zn2+ and Cu2+, blocked the pore-forming stage at high concentrations. Although VMH could disrupt all kinds of mammalian erythrocytes tested, those from horses were most sensitive to the hemolysin. Horse erythrocytes were found to have the most toxin-binding sites and to be hemolyzed by the least amount of membrane-bound toxin molecules, suggesting that toxin binding to and pore formation on erythrocytes are more effective in horses than in other mammals. Purified VMH induced fluid accumulation in a ligated rabbit ileal loop in a dose-dependent manner. In addition, the antibody against the hemolysin obviously reduced enteropathogenicity of living V. mimicus cells. These findings clearly demonstrate that VMH is probably involved in the virulence of this human pathogen.

Pathogenic vibrios in the natural aquatic environment

In recent years, members belonging to the genus Vibrio of the family Vibrionaceae have acquired increasing importance because of the association of several of its members with human disease. The most feared of the Vibrio species is Vibrio cholerae, the causative agent of cholera, a devastating disease of global significance. Other important vibrios of medical importance are V. parahemolyticus, V. vulnificus, V. mimicus, and to a lesser extent V. fluvialis, V. furnissii, V. hollisae, and V. damsela. Recent studies have also implicated V. alginolyticus and V. metschnikovii in human disease, although their complete significance has not yet been established. The virulence of all medically important vibrios is aided by a variety of traits that help breach human defenses. In this review, we provide an overview of the environmental distribution of the pathogenic vibrios and the important virulence traits that enable them to cause disease.

Vacuolar-type H(+)-ATPase in mouse bladder epithelium is responsible for urinary acidification

The urine in the mouse bladder was found to be acidic, ranging from pH 5.3 to 5.5 in the daytime and pH 6.0 to 6.3 at night. Administration of bafilomycin A1 or concanamycin A, specific inhibitors of vacuolar-type H(+)-ATPase, into bladder lumen caused neutralization of urinary pH at least for 36 h, whereas inhibitors of mitochondrial ATP synthase (F-type H(+)-ATPase) or P-type H(+)-ATPases did not. Bafilomycin A1-sensitive proton secretion from isolated inside-out bladder was also observed. Immuno-electron microscopy with antibodies against vacuolar H(+)-ATPase revealed that vacuolar-type H(+)-ATPase is rich in luminal plasma membrane and endosomes of superficial cells of the bladder epithelium. These results indicate that vacuolar-type H(+)-ATPases present in luminal plasma membrane of the superficial epithelial cells secrete protons so as to acidify the urine in mouse bladder.

Characterization of a mutant of Vibrio vulnificus for heme utilization

Vibrio vulnificus, an opportunistic human pathogen, can obtain iron from a variety of heme proteins. This process involves the digestion of heme proteins by an exoprotease to liberate protoheme (iron-protoporphyrin IX). In the present study, we isolated and characterized a mutant for protoheme utilization. One mutant isolated by treatment with a chemical mutagen was shown to be unable to use either protoheme or heme proteins, but multiplied in a medium supplemented with an iron siderophore, such as iron-vulnibactin. Like a wild-type strain, the mutant sensed iron depletion, so that the 74- and 79-kDa outer membrane proteins were expressed under iron-regulated conditions. Both the parent and mutant strains secreted hemolysin independent of the iron concentration of the medium. Whole cells of either of the strains were equally capable of binding of hematin. Taken together, the data suggest that the mutant may have a mutation in a gene encoding an inner membrane or a periplasmic protein which transports protoheme or iron dissociated from protoporphyrin IX into the cell.

Combinations of IgE values and lymphocyte proliferative responses for consideration of the clinical course of infantile hen's-egg-sensitive atopic dermatitis

We have attempted a new approach, using peripheral blood mononuclear cells (PBMCs), to predict the clinical course of infantile food-sensitive atopic dermatitis (AD). In this study, we investigated the relationships between the clinical course of infantile hen's-egg-sensitive AD and laboratory data obtained at the early stage of AD, particularly on combinations of specific IgE antibodies to hen's egg and proliferative responses of PBMCs to ovalbumin (OA). Total IgE concentrations, specific IgE antibodies to hen's egg and proliferative responses of PBMCs to OA were measured in 31 hen's-egg-sensitive AD patients within 6 months after development of AD symptoms. After the acquisition of laboratory data, the clinical courses of all patients were followed up for 1 year. The stimulation index of proliferative responses of PBMCs to OA in hen's-egg-sensitive AD patients whose symptoms did not improve was significantly (p < 0.05) higher than that in hen's-egg-sensitive AD patients whose symptoms improved. On the other hand, the radioallergosorbent test (RAST) scores for hen's egg were not statistically different between the two groups. The degree of refractoriness of AD symptoms tended to be higher in the patients who showed a higher total IgE concentration at the beginning of the clinical course than in those who showed a lower one. These results might be attributed to the underlying cause of AD, i.e., a cell-mediated and an IgE-mediated allergy. We concluded that, in addition to total IgE concentrations, combinations of RAST values and lymphocyte proliferative responses to OA are important to predict the clinical course of infantile hen's-egg-sensitive AD.

Pulmonary hemosiderosis with hypersensitivity to buckwheat

BACKGROUND

Adverse reactions after ingesting buckwheat are known to be IgE-mediated. Further, hypersensitivity reactions may be involved in some patients with pulmonary hemosiderosis related to cow milk sensitivity. We, however, encountered a patient with pulmonary hemosiderosis related to buckwheat protein without high levels of buckwheat-specific IgE antibodies.

OBJECTIVE

The aim of this study was to investigate the mechanisms.

METHODS

RAST for anti-buckwheat IgE, skin prick test, skin patch test, and proliferative responses of peripheral blood mononuclear cells to buckwheat were investigated in this patient.

RESULTS

RAST values for buckwheat protein were negative, and skin prick test for buckwheat protein also gave negative results. On the other hand, skin patch testing for buckwheat protein elicited positive responses. Further, peripheral blood mononuclear cells of our patient responded to buckwheat protein.

CONCLUSIONS

Our patient had pulmonary hemosiderosis related to non-immediate buckwheat protein hypersensitivity.

Analysis of the structural gene encoding a hemolysin in Vibrio mimicus

An environmental isolate of V. mimicus, strain E-33, has been reported to produce and secrete a hemolysin of 63 kDa. The hemolysin is enterotoxic in test animals. The nucleotide sequence of the structural gene of the hemolysin was determined. We found a 2,232 bp open reading frame, which codes a peptide of 744 amino acids, with a calculated molecular weight of 83,903 Da. The sequence for the structural gene was closely related to the V. cholerae el tor hlyA gene, coding an exocellular hemolysin. The amino terminal amino-acid sequence of the 63 kDa hemolysin, purified from V. mimicus, was determined by the Edman degradation method and found to be NH2-S-V-S-A-N-N-V-T-N-N-N-E-T. This sequence is identified from S-152 to T-164 predicted from the nucleotide sequence. So, it seems that the mature hemolysin in V. mimicus is processed upon deleting the first 151 amino acids, and the molecular mass is 65,972 Da. Analyzing the deduced amino-acid sequence, we also found a potential signal sequence of 24 amino acids at the amino terminal. Our results suggest that, like V. cholerae hemolysin, two-step processing also exists in V. Mimicus hemolysin.

[Report of a case of intraosseous meningioma: a review of the literature and diagnostic value using MRI study]

A 40-year-old woman with intraosseous meningioma is reported. She was admitted because of a slowly growing, bony hard, fixed mass in the temporal area that was covered by the normal scalp. The neurological examination on admission was negative. Plain skull radiogram demonstrated an area of hyperostotic change in the temporal region. Bone-targeted CT demonstrated irregularity in the structure of the skull bone. Selective external carotid angiogram showed a homogeneous tumor stain in the region of the skull thickening. Macroscopically complete tumor resection (Simpson's grade 1) was carried out. Her postoperative course was uneventful. The skull tumor had the typical feature of meningotheliomatous meningioma. MRI showed a mass lesion of the temporal bone on T1- and T2-weighted images, and a well-enhanced mass lesion of the outer dura and outer table of the skull bone on Gd-DTPA enhanced T1 images. The findings of the preoperative MRI study using Gd-DTPA were identical with the tumor area on histological study. Therefore, MRI with Gd-DTPA enhancement was considered a useful diagnostic modality.

Elevated expression of DNA polymerase beta gene in glioma cell lines with acquired resistance to 1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3- nitrosourea

We investigated the expression of DNA polymerase beta (beta-pol) and O6-methylguanine-DNA methyltransferase (MGMT) in human glioma cells with acquired resistance to 1-(4-amino-2-methyl-5-pyrimidinyl)-methyl-3-(2-chloroethyl)-3-nitrosoure a (ACNU) and in the parent cells. ACNU-resistant T430 (T430R) and A172 (A172R) glioma cell lines were established following repeated exposure to ACNU. The level of MGMT mRNA expression was elevated in T430R, but not in A172R. In contrast, the level of beta-pol mRNA expression and the level of beta-pol protein were elevated in A172R, compared with the parent cells. While the mechanism of MGMT repair has been considered to be important in the drug resistance of human brain tumors to ACNU, our present results demonstrate that beta-pol may also play an important role in the acquisition of tumor cell resistance to ACNU in human gliomas.

Purification and characterization of novel hemagglutinins from Vibrio mimicus: a 39-kilodalton major outer membrane protein and lipopolysaccharide

Two hemagglutinins (HAs) mediating the agglutinability to rabbit erythrocytes were isolated from 32-h culture supernatant of enterotoxigenic strain E-33 of Vibrio mimicus by ultrafiltration followed by gel filtration and anion-exchange column chromatography. The HAs were designated R-HA and C-HA on the basis of specific hemagglutinating activity towards rabbit erythrocytes only (R-HA) and towards chicken and rabbit erythrocytes (C-HA). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and subsequent staining with Coomassie brilliant blue revealed no detectable protein band and a single band of Mr 39,000 in the case of R-HA and C-HA, respectively. However, silver staining of the gel containing R-HA revealed the appearance of low-molecular-weight material. These two HAs differed from each other and from previously reported HA/protease in receptor specificity, molecular composition, and biochemical and immunochemical properties. No simple sugar other than glycoproteins, including mucin, inhibited hemagglutinating activities of both C-HA and R-HA. Rabbit antibody against R-HA or C-HA could agglutinate E-33 whole cells, implying a possible cell surface origin of the two HAs. The isolated E-33 lipopolysaccharide (LPS) or its polysaccharide moiety conferred biochemical and immunochemical properties identical to those of R-HA, confirming that the R-HA represents polysaccharide of LPS. The LPS preparations from heterologous strains of Vibrio mimicus and Vibrio cholerae non-O1 confirmed that the hemagglutinating ability is a common function of LPS. On the other hand, the antibody against C-HA specifically recognized a major outer membrane protein (OMP) with an Mr of around 39,000 in both homologous and heterologous strains of V. mimicus, suggesting an OMP origin of C-HA. Furthermore, the antibody recognized a major OMP with an Mr of around 37,000 in V. cholerae. Although the immunogenicity of LPS and OMP is well documented for important intestinal pathogens, the hemagglutinating properties of such attractive cell surface components are hitherto unrecognized and will definitely contribute towards understanding their role in bacterial adherence.

Expression of virulence-related properties by, and intestinal adhesiveness of, Vibrio mimicus strains isolated from aquatic environments

A study of the major pathogenic characteristics of Vibrio mimicus was carried out with 77 strains isolated from aquatic environments in Okayama, Japan. Of the strains tested, 96% demonstrated in vitro adherence to the rabbit intestinal mucosa, of which 36, 20, and 43% belonged to the strongly, moderately, and weakly adhesive groups, respectively. Of the 27 strains which appeared to be enterotoxigenic in the experiments using rabbit ileal loops, 74% belonged to the strongly adhesive group. All strains of V. mimicus at early log phase showed cell-mediated hemagglutination, and 70% of strongly hemagglutinative strains belonged to the strongly adhesive group, implying a possible correlation between cell-mediated hemagglutination and bacterial adherence. However, no significant correlation could be detected in the production of putative exocellular pathogenic factors and bacterial adherence or enterotoxigenicity.

Regulation of sulphate assimilation in Saccharomyces cerevisiae

We examined how the activity of O-acetylserine and O-acetylhomoserine sulphydrylase (OAS/OAH) SHLase of Saccharomyces cerevisiae is affected by sulphur source added to the growth medium and genetic background of the strain. In a wild-type strain, the activity was repressed if methionine, cysteine or glutathione was added to the growth medium. However, in a strain deficient of cystathionine gamma-lyase, cysteine and glutathione were repressive, but methionine was not. In strains deficient of serine O-acetyltransferase (SATase), OAS/OAH SHLase activity was low regardless of sulphur source and was further lowered by cysteine and glutathione, but not by methionine. From these observations, we concluded that S-adenosylmethionine should be excluded from being the effector for regulation of OAS/OAH SHLase. Instead, we suspected that S. cerevisiae would have the same regulatory system as Escherichia coli for sulphate assimilation; i.e. cysteine inhibits SATase to lower the cellular concentration of OAS which is required for induction of the sulphate assimilation enzymes including OAS/OAH SHLase. Subsequently, we obtained data supporting this speculation.

Comparison of a fluorogenic assay with a conventional method for rapid detection of Vibrio parahaemolyticus in seafoods

A conventional method and a fluorogenic assay for the detection of Vibrio parahaemolyticus were compared. Among 29 seafood samples examined for the presence of V. parahaemolyticus, 17 samples harbored V. parahaemolyticus, and trypsinlike activity was noticed in 19 seafoods. The added fluorogenic substrate was cleaved in single samples of shrimp, turbo, and cuttlefish from which V. parahaemolyticus could not be isolated by the conventional method. Vibrio alginolyticus, in addition to V. parahaemolyticus, was found to exhibit intracellular trypsinlike activity. Trypsinlike activity in seafoods was observed after the most probable number for the initial density of V. parahaemolyticus-like organisms was found to have reached > 10(2) per g. A V. parahaemolyticus inoculum at 10(4) CFU/ml in arabinose-glucuronate medium was required to attain growth to 10(6) CFU/ml, which is the level necessary for the release of detectable amounts of fluorescent compound from the added substrate.