Abstract P3-03-02: Redo sentinel lymph node biopsy for ipsilateral breast tumor recurrence after breast conserving surgery with negative sentinel nodes: A pooled analysis from a systematic review and two institutes

Background:Cases of redo sentinel lymph node biopsy (SLNB) are growing according to the increase of ipsilateral breast tumor recurrence (IBTR) after breast-conservative surgery (BCS). To evaluate a feasibility of redo SLNB in patients with IBTR after negative SLN, we conveyed a pooled analysis using data from a systematic review and two institutes.

Materials and methods:A systematic search of PubMed was conducted to identify data of patient level from publications evaluating redo SLNB for cases with IBTR. Eligible patients who underwent BCS and were confirmed as negative axilla after SLNB were identified. An identification rate (IR) and a false-negative rate (FNR) were calculated. To identify FNR, we only included cases with back-up axillary node dissection (ALND) from retrieved data.

Results:In a systematic review, a total of 197 peer-reviewed publications were retrieved, of which 19 papers included patients who met eligibility criteria. Data from 464 patients were collected. In two-institutes, 38 cases with same criteria were identified. A total of 502 patient's data were pooled. The IR of redo-SLNB was 71.7% (360/502) in pooled data. For the FNR, data from 147 patients with back-up ALND after SLNB was analyzed. The FNR and accuracy of redo-SLNB were 9.8% (5/51) and 97% (142/147).

Conclusions:We found that the IR and the FNR of redo SLNB were 71.7% and 9.8%, respectively. Redo SLNB is reliable procedure for axillary staging in patients with IBTR after negative SLNB.

Citation Format: Yoon C-i, Bae SJ, Choi JE, Cha CH, Park SE, Ahn SG, Jeong J. Redo sentinel lymph node biopsy for ipsilateral breast tumor recurrence after breast conserving surgery with negative sentinel nodes: A pooled analysis from a systematic review and two institutes [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P3-03-02.

Paradigm for diagnosing mycobacterial disease: direct detection and differentiation of Mycobacterium tuberculosis complex and non-tuberculous mycobacteria in clinical specimens using multiplex real-time PCR

AIMS

Mycobacterium tuberculosis and non-tuberculous mycobacteria (NTM) are clinically different, and the rapid detection and differentiation of M. tuberculosis complex (MTBC) and NTM is crucial for patient management and infection control. Given the slow growth of most pathogenic mycobacteria, nucleic acid amplification assays are excellent tools for direct identification of mycobacteria in clinical specimens. Recently, a multiplex real-time PCR assay was developed that can directly detect 20 mycobacterial species in clinical specimens. Here, we evaluated the diagnostic performance of the assay for diagnosing mycobacterial disease under routine laboratory conditions.

METHODS

A total of 3334 specimens collected from 1437 patients suspected of tuberculosis infection were subjected to acid-fast bacilli staining, conventional culture and the multiplex real-time PCR assay. To evaluate the sensitivity and specificity of the assay, the overall diagnosis of tuberculosis was defined by positive culture plus medical history, and the 2007 American Thoracic Society and Infectious Disease Society of America diagnostic criteria for NTM disease were applied.

RESULTS

The sensitivity, specificity, positive predictive value and negative predictive value were 87.5%, 99.6%, 96.1% and 98.5%, respectively, for the detection of MTBC isolates and 53.3%, 99.9%, 95.2%, and 98.9%, respectively, for detecting NTM isolates.

CONCLUSIONS

Thus, the assay can correctly differentiate between MTBC and NTM isolates in clinical specimens and would be a useful tool for the rapid differentiation of tuberculosis and NTM disease, despite its limited sensitivity for the diagnosis of NTM disease.

Direct identification of mycobacteria from liquid media using a triplex real-time PCR coupled with pyrosequencing method

Culture in enriched broth, as well as on a solid medium, is recommended for primary isolation of mycobacteria. With the introduction of liquid mycobacterial culture methods, a substantial workload regarding the identification of culture-recovered mycobacterial species, particularly Mycobacterium tuberculosis complex (MTC), has been imposed on our laboratory. We thus developed a triplex, real-time PCR coupled with pyrosequencing assay that can directly identify mycobacterial species from liquid media, which can reduce the workload. In this assay, real-time PCR simultaneously detects MTC and Mycobacterium xenopi, and amplifies the region of 16S rRNA gene containing hypervariable region A for pyrosequencing analysis; subsequent, pyrosequencing identifies many other nontuberculous mycobacteria. The assay was evaluated using 333 DNA samples directly prepared from liquid media, including 24 reference strains and 309 clinical isolates. Three hundred and twenty-eight (98.5%) of the 333 samples were correctly identified. The remaining five were determined as indeterminate. In conclusion, this coupled assay would be an alternative method for rapid identification of mycobacteria directly from liquid media in a clinical laboratory with a high workload in regions where tuberculosis is endemic.

Direct identification of mycobacteria from clinical specimens by multiplex real-time PCR

AIMS

To directly identify clinically relevant mycobacteria from clinical specimens, we have developed a multiplex real-time PCR assay with hydrolysis probes that can identify 20 mycobacterial species.

METHODS AND RESULTS

The assay was initially evaluated using 248 strains, including both reference strains and clinical isolates. Then, the assay was implemented according to a scheme in our laboratory. The scheme based on the clinical differences between the Mycobacterium tuberculosis complex (MTC) and nontuberculous mycobacteria (NTM) consisted of three stepwise PCRs. MTC and NTM were differentially detected in the step 1 PCR, and the NTM species were identified in the step 2 and step 3 PCRs. During a 2·5-year period, 1877 isolates of MTC (1142 directly recovered from clinical specimens) and 596 isolates of NTM (143 directly recovered from clinical specimens) were detected, and the species of 590 (99·0%) of the 596 NTM isolates were identified.

CONCLUSIONS

Our experience shows that this is a new paradigm for rapidly and accurately identifying clinically relevant mycobacteria, in which a multiplex real-time PCR assay is directly applied to clinical specimens in a stepwise fashion.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is the first report of a multiplex real-time PCR assay for identifying clinically important mycobacterial species directly from clinical specimens and its application in a clinical microbiology laboratory.

A challenging diagnosis: crystal-storing histiocytosis in plasma cell myeloma

OBJECTIVES

Crystal-storing histiocytosis (CSH) is an uncommon finding in plasma cell neoplasms. CSH is thought to be an intralysosomal deposition of secreted paraproteins or immunoglobulins, which usually express κ immunoglobulin light chains that finally aggregate in crystals. Because of its rarity, CSH in bone marrow often makes diagnosis difficult.

METHODS

A 57-year-old woman had IgA κ monoclonal proteinemia and monoclonal proteinuria. In the bone marrow aspirate, plasma cells were initially counted less than what would be expected, whereas histiocytes with intracellular crystals were increased. Then, we used α-naphthyl acetate esterase (ANAE) staining to distinguish between true histiocytes and plasma cells. Immunostaining for κ, CD138, CD56, and CD68 was performed on a bone marrow biopsy specimen.

RESULTS

True histiocytes containing crystalline inclusions were stained strongly for ANAE, while unstained cells with intracytoplasmic crystals represented plasma cells. The biopsy specimen revealed diffuse infiltration of CD138-positive plasma cells. We also confirmed the presence of plasma cells, histiocytes, and their crystallized inclusions with the immunostaining. The patient was finally diagnosed with plasma cell myeloma.

CONCLUSIONS

The diagnosis was challenging; the bone marrow findings resembled features of other histiocytic disorders. The use of immunohistochemistry enabled the diagnosis of CSH in the presence of plasma cell myeloma.

CD34 and p53 immunohistochemical stains differentiate hypocellular myelodysplastic syndrome (hMDS) from aplastic anemia and a CD34 immunohistochemical stain provides useful survival information for hMDS

Background

The presence of significant dysplasia in bone marrow (BM) aspirates helps to distinguish between hypocellular myelodysplastic syndrome (hMDS) and aplastic anemia (AA). Occasionally, diluted BM aspirates make it difficult to recognize dysplastic changes and can also negatively affect the detection of cytogenetic abnormalities in hMDS. We evaluated the usefulness of CD34 and p53 immunoreactivity for discriminating between hMDS and AA and for estimating survival outcomes in hMDS patients.

Methods

BM clot section (BMC) or BM biopsy (BMB) specimens were obtained from 64 hMDS/AA patients (33 with hMDS and 31 with AA) and seven controls. Immunohistochemical (IHC) staining for CD34 and p53 was performed by using the EnVision detection system (Dako, Denmark). We compared the results of IHC staining, BM findings, and chromosomal analyses, and determined overall survival outcomes.

Results

The number of CD34- and p53-positive BM cells was higher among the patients with hMDS than among the patients with AA (P<0.001 and P=0.001, respectively). hMDS patients with increased CD34-positive cells had significantly poorer survival outcomes compared with those with normal number of CD34-positive cells (P=0.013).

Conclusions

CD34 and p53 IHC stains of BMC or BMB provide useful information for differentiating between hMDS and AA. CD34 IHC staining of BMC or BMB also provides useful information for estimating survival outcomes in hMDS patients.

A combination of CD15/CD10, CD64/CD33, CD16/CD13 or CD11b flow cytometric granulocyte panels is sensitive and specific for diagnosis of myelodysplastic syndrome

Flow cytometry (FCM) is a reproducible and objective technique that may be useful in the diagnosis of myelodysplastic syndrome (MDS) by detecting abnormal immunophenotypes specific to MDS. We investigated 5 granulocyte/monocyte panels by FCM to find a sensitive and specific combination of panels in order to discriminate MDS from non-clonal hematologic disorders. Bone marrow aspirates from 35 patients with MDS and 25 patients with non-clonal hematologic disorders were studied. We performed FCM using 5 granulocyte/monocyte panels (CD15/CD10/CD45, CD64/CD33/CD45, CD16/CD13/CD45, CD16/CD11b/CD45, and CD56/CD19/CD7/CD45) to examine the positive rate in MDS and controls, and to find an optimal combination that maximizes the detection rate of MDS. In MDS, the number of abnormal immunophenotypes per 5 granulocytic and 5 monocytic panels were 2.1 ± 1.2 and 2.2 ± 1.4. The rates were higher than the controls (P< 0.001, respectively). As the number of employed panels increased, the percent values of abnormal immunophenotypes increased (P=0.002). The maximum rate of abnormal immunophenotype was 89.7% in MDS patients, especially 100.0% in normal karyotype, when a combination of three panels, CD15/CD10/CD45, CD64/CD33/CD45, and either CD16/CD13/CD45 or CD16/CD11b/CD45 was used. This study demonstrates that a combination of CD15/CD10, CD64/CD33, CD16/CD13 or CD11b granulocyte panels in FCM is sensitive and specific for diagnosis of MDS.

[Minimal residual disease detection in acute leukemia patients by flow cytometric assay of cross-lineage antigen expression]

BACKGROUND

It has been demonstrated that flow cytometric detection of minimal residual disease (MRD) has a prognostic significance in the treatment of patients with acute leukemia. We investigated the significance of flow cytometric MRD detection for the first time in Korea.

METHODS

We analyzed the results of MRD detection in morphologically complete remission bone marrow aspirates from 89 patients with newly-diagnosed or relapsed acute leukemia, in which leukemic cells had cross-lineage antigen expression. Patients were grouped based on MRD frequencies: ≥ 1.0%, high MRD; <1.0%, low MRD.

RESULTS

Forty-seven ALL patients consisted of 10 with high and 37 with low MRD levels. Patients with high MRD levels showed a tendency of more frequent relapse than those with low MRD levels (40.0% and 13.5%, respectively) (P=0.08). High MRD group showed a tendency of short relapse-free survival (RFS) and overall survival (OS), although the differences were not statistically significant. Forty-two AML patients consisted of 16 with high and 26 with low MRD levels. There were no correlations between the MRD levels and relapse rate, RFS or OS. AML patients with high MRD levels showed significantly higher rate of unfavorable cytogenetic risk categories and lower rate of favorable risk categories (P=0.03).

CONCLUSIONS

MRD detection by flow cytometric assay of cross-lineage antigen expression would be useful in predicting treatment outcome in patients with ALL rather than AML. We expect that the establishment of the standardization of methods, time to test or antibody combination would be achieved through further trials in this country.

Evaluation of the TEST 1 erythrocyte sedimentation rate system and intra- and inter-laboratory quality control using new latex control materials

BACKGROUND

The erythrocyte sedimentation rate (ESR) test has been considered to be a simple procedure, not requiring quality control (QC). However, QC is essential for accuracy and precision. We evaluated the TEST 1 ESR system and performed QC procedures using newly developed latex control materials in three hospitals.

METHODS

Using tripotassium ethylenediaminetetraacetic acid blood samples (n=184), we compared TEST 1 ESR values with Westergren ESR data and evaluated intra-assay precision. Three levels of latex control materials were used to assess inter-assay precision. Reference range assessment was done using samples from 220 healthy individuals. Inter-laboratory QC with latex control materials in three hospitals was performed.

RESULTS

Correlation between TEST 1 ESR and Westergren ESR results was good (p<0.001). Intra-assay precision [coefficients of variation (CV) 6.6%-21.7%] with patient samples and inter-assay precision (CV 0.0%-6.8%) with latex control materials were satisfactory. The reference ranges of 2-10 mm/h for males and 2-19 mm/h for females were established. Inter-laboratory QC data with latex control materials in three hospitals demonstrated good accuracy and satisfactory precision (CV 0.0%-14.4%).

CONCLUSIONS

Our results demonstrate that the TEST 1 QC is reliable and the latex control materials are valuable for inter-laboratory proficiency testing.

Erythrocyte sedimentation rate measurements by TEST 1 better reflect inflammation than do those by the Westergren method in patients with malignancy, autoimmune disease, or infection

We compared the TEST 1 (Alifax, Padova, Italy) and Westergren methods of measuring the erythrocyte sedimentation rate (ESR) to assess inflammation. The ESR was measured by both methods in 154 blood samples from patients with malignancy (n = 69), autoimmune disease (n = 44), or infection (n = 41). Total protein, albumin, and C-reactive protein (CRP) levels were measured in each plasma sample, and albumin and alpha(1)-, alpha(2)-, beta(1)-, beta(2)-, and gamma-globulin fractions were measured by capillary electrophoresis. TEST 1 ESR values were significantly lower than the Westergren values, by 10.9 mm/h. We found that the correlations of TEST 1 ESR values with inflammatory protein levels (total protein, globulin, CRP, and alpha(1)-, alpha(2)-, beta(2)-, and gamma-globulin) were better than those obtained using the Westergren method. These findings indicate that ESR measurements by TEST 1 reflect inflammation better than do those by the Westergren method in patients with malignancy, autoimmune disease, or infection.

[Experience on identification of cross-reactive group specificity performed by anti-human globulin panel reactive antibody tests]

BACKGROUND

Panel reactive antibody (PRA) is to screen and identify HLA antibody. Majority of antibody specificities in high-PRA are directed against cross reactive group (CREG). Thus, this study was to know the advantage of identifying CREG specificity and whether antibody specificities are changed according to CREG classification.

METHODS

HLA class I antibodies were identified from 159 sera from 108 patients in Asan Medical Center, who had shown more than 5% PRA by anti-human globulin (AHG)-complement-dependent cytotoxicity (CDC). Tail analysis-based computer program was developed to identify specificities, applying both Rodey (R-ABC) and Takemoto (T-ABC) classification. The results were also compared with those obtained when without CREG application (ABC).

RESULTS

Among 151 cases in which HLA specificities was identified, the frequency of CREG specificity was 22.5% in R-ABC and 27.2% in T-ABC. Eleven cases showed CREG specificities only in one classification. However, the individual antigen specificities in one hand were all included in the CREG identified in the other hand. CREG specificities in samples with PRA >50% (60%) were more frequently identified than those in samples with PRA < or =50% (9%) (in R-ABC, P<0.0001). Without applying CREG to interpretation, specificity was not identified in 9 cases.

CONCLUSIONS

Application of CREG enhanced the rate of antibody identification. Antibody specificities of those cases where CREG specificities were different between Rodey and Takemoto classifications were almost the same when compared at the individual antigen level. Therefore, it was thought that it makes no difference to use any one of these two classifications in interpreting PRA.

Identification of hepatitis C virus genotype 6 in Korean patients by analysis of 5' untranslated region using a matrix assisted laser desorption/ionization time of flight-based assay, restriction fragment mass polymorphism

Previous surveys of the prevalence of hepatitis C virus (HCV) in Korea have identified types 1 and 2, but little has been said of other genotypes and viral subtypes. In this study, HCV genotypes in Korea were investigated using Restriction Fragment Mass Polymorphism (RFMP) assay, a sensitive and specific method for genotyping based on MALDI-TOF mass spectrometry. A total of 1,043 independent serum samples from HCV-infected patients were analyzed. Of interest, 15 subjects (1.4%) were determined to contain HCV genotype 6 and 46 subjects (4.4%) contained mixed genotypes with the most prevalent genotypes being HCV 1b and 2a/c (45.0% and 35.4%, respectively). The 15 subjects with HCV genotype 6 comprised eight cases of subtype 6c, including one case of mixed infection with 1b, three cases of HCV 6a, and six cases of unassigned subtypes. Sequencing corroborated the identity of genotype 6 from 13 subjects, while the line probe assay (LiPA) mis-identified them as genotype 1b. The majority (7/9) of the genotype 6 patients enrolled for interferon/ribavirin therapy, achieved a sustained virologic response. The ability of the RFMP assay to differentiate various HCV genotypes should enable better analysis of the relationship between HCV genotype and disease prognosis.

Meta-analysis of the association between HLA-DRB1 allele and rheumatoid arthritis susceptibility in Asian populations

The aims of this study were to summarize results on the association of HLA-DRB1 with rheumatoid arthritis (RA) in Asians and to determine if the shared epitope (SE) hypothesis could explain the meta-analysis results. Among the papers published between January 1987 and July 2006 on RA susceptibility in Asian-Mongoloid populations (Korean, Japanese, Chinese, and Thai), 12 were selected for the metaanalysis. Mongoloid-Asian patients with RA had significantly higher frequencies of HLA-DRB1*0101, *0401, *0410, and *1001 than controls (OR 1.5-2.1, p<0.05 for association). When analyses were restricted to more ethnically homogeneous populations, HLA-DRB1*0405 showed a significant susceptibility to RA in Koreans (OR 5.65, 95% CI 4.32-7.39), whereas the HLA-DRB1*0301, *0403, *0406, *0701, *1301, and *1405 alleles showed protective association with RA (OR 0.32-0.70, p<0.05 for association). In conclusion, it was found that HLA-DRB1 *0101, *0401, *0405, *0410, and *1001 are susceptible, while HLA-DRB1* 0301, *0403, *0406, *0701, *1301, and *1405 are protective in Asian-Mongoloids. All the RA-associated alleles except DRB1*0301 could be explained by the structural model supporting the SE hypothesis that RA susceptibility is determined by the combination of amino acid residues at HLA-DR beta71 and beta74, not by beta71 alone.

CT versus sonography for monitoring radiofrequency ablation in a porcine liver

OBJECTIVE

The objective of this study was to compare CT and sonography for monitoring radiofrequency (RF) lesions in porcine livers.

SUBJECTS AND METHODS

RF lesions (n = 12) were created in three pig livers by applying 13 min of current to a multielectrode RF probe with a target temperature of 95 degrees C. Helical unenhanced CT and corresponding axial sonography were performed before ablation, at 2 min, 8 min, and immediately after ablation. Contrast-enhanced CT was performed after ablation. CT scans and sonograms were evaluated by blinded observers for conspicuity of the RF lesion, edge detection (visibility of liver-lesion interface), and artifacts. Hounsfield units were recorded at every time interval, and radiologic-pathologic correlation for lesion size and configuration was performed.

RESULTS

Mean size of RF lesions was 3. 03 +/- 0.9 cm. On CT, RF lesions had consistently lower attenuation values than surrounding liver (22.2 H lower than liver at 8 min, p < 0.0001). Echogenicity was variable with sonography (hypoechoic = 59%, hyperechoic = 25%, isoechoic = 16%). Unenhanced CT significantly improved conspicuity, edge detection of RF lesions, and decreased artifacts compared with sonography (p < 0.05). Contrast-enhanced CT improved RF lesion detection, border discrimination, and artifacts compared with sonography (p < 0.05). Unenhanced CT had the best correlation to pathologic size (r = 0.74), followed by contrast-enhanced CT (r = 0.72) and sonography (r = 0.56). Contrast-enhanced CT best correlated with lesion shape, but slightly overestimated size because of areas of ischemia peripheral to the RF lesion.

CONCLUSION

In this animal model, unenhanced CT was an effective way to monitor RF ablation compared with sonography because of increased lesion discrimination, reproducible decreased attenuation during ablation, and improved correlation to pathologic size.

Metastatic breast cancer

The unfortunate reality of metastatic breast cancer is that all treatment is palliative in nature. This is a disease that currently has no cure and for which therapy is directed towards accentuating survival and relieving symptoms. Current technology allows the prediction and detection of metastases earlier and with greater accuracy. These achievements need to be consolidated by the discovery of innovative therapies that can alter the inevitable outcome of this disease.

Chronic cryptogenic sensory polyneuropathy: clinical and laboratory characteristics

BACKGROUND

Chronic sensory-predominant polyneuropathy (PN) is a common clinical problem confronting neurologists. Even with modern diagnostic approaches, many of these PNs remain unclassified.

OBJECTIVE

To better define the clinical and laboratory characteristics of a large group of patients with cryptogenic sensory polyneuropathy (CSPN) evaluated in 2 university-based neuromuscular clinics.

DESIGN

Medical record review of patients evaluated for PN during a 2-year period. We defined CSPN on the basis of pain, numbness, and tingling in the distal extremities without symptoms of weakness. Sensory symptoms and signs had to evolve for at least 3 months in a roughly symmetrical pattern. Identifiable causes of PN were excluded by history, physical examination findings, and results of laboratory studies. We analyzed clinical and laboratory data from patients with CSPN and compared findings in patients with and without pain.

RESULTS

Of 402 patients with PN, 93 (23.1%) had CSPN and stable to slowly progressive PN syndrome. These patients presented with a mean age of 63.2 years and a mean duration of symptoms of 62.9 months. Symptoms almost always started in the feet and included distal numbness or tingling in 86% of patients and pain in 72% of patients. Despite the absence of motor symptoms at presentation, results of motor nerve conduction studies were abnormal in 60% of patients, and electromyographic evidence of denervation was observed in 70% of patients. Results of laboratory studies were consistent with axonal degeneration. Patients with and without pain were similar regarding physical findings and laboratory test abnormalities. Only a few patients (<5%) had no evidence of large-fiber dysfunction on physical examination or electrophysiologic studies. All 66 patients who had follow-up examinations (mean, 12.5 months) remained ambulatory.

CONCLUSIONS

Cryptogenic sensory polyneuropathy is a common, slowly progressive neuropathy that begins in late adulthood and causes limited motor impairment. Isolated small-fiber involvement is uncommon in this group of patients. Management should focus on rational pharmacotherapy of neuropathic pain combined with reassurance of CSPN's benign clinical course.

Amyotrophic lateral sclerosis: abnormalities of the tongue on magnetic resonance imaging

We compared the magnetic resonance images of the tongues of 16 amyotrophic lateral sclerosis (ALS) patients with those of 20 control patients and found the tongue in ALS patients is more frequently and more severely involved than suspected clinically, with major abnormalities of size, shape, position, and internal structure. The tongue size in ALS, as measured in the sagittal plane, can be reduced by as much as two-thirds of normal. The shape of the tongue in ALS tends to be rectangular or square rather than curved as is normal. As severity of the disease increases, the position of the tongue changes so that the bulk of the muscle falls away from the incisors and no longer is in contact with the hard or soft palate. The normal radial bands from the anterior floor of the mouth to the mucosal surface are often missing in ALS as are the two curvilinear bands that run parallel to the mucosal surface and intersect the radial bands. Also, there is a mottled disorganization of the internal structure of the tongue with areas of increased and decreased signal intensity.

Charge asymmetry of the purple membrane measured by uranyl quenching of dansyl fluorescence

Purple membrane was covalently labeled with 5-(dimethylamino) naphthalene-1-sulfonyl hydrazine (dansyl hydrazine) by carbodiimide coupling to the cytoplasmic surface (carboxyl-terminal tail: 0.7 mol/mol bacteriorhodopsin) or by periodate oxidation and dimethylaminoborane reduction at the extracellular surface (glycolipids: 1 mol/mol). In 2 mM acetate buffer, pH 5.6, micromolar concentrations of UO2 +(2) were found to quench the dansyl groups on the cytoplasmic surface (maximum = 26%), while little quenching was observed at the extracellular surface (maximum = 4%). Uranyl ion quenched dansyl hydrazine in free solution at much higher concentrations. Uranyl also bound tightly to unmodified purple membrane, (apparent dissociation constant = 0.8 microM) as measured by a centrifugation assay. The maximum stoichiometry was 10 mol/mol of bacteriorhodopsin, which is close to the amount of phospholipid phosphorus in purple membrane. The results were analyzed on the assumptions that UO2 +(2) binds in a 1:1 complex with phospholipid phosphate and that the dansyl distribution and quenching mechanisms are the same at both surfaces. This indicates a 9:1 ratio of phosphate between the cytoplasmic and extracellular surfaces. Thus, the surface change density of the cytoplasmic side of the membrane is more negative than -0.010 charges/A2.