Importance of non-medical reasons for dropout in patients on peritoneal dialysis

BACKGROUND

To describe the associated factors for non-medical reasons for dropout in peritoneal dialysis (PD) patients.

METHODS

A retrospective cohort study was performed using registry data of adult patients commencing PD as their initial renal replacement therapy in one hospital-facilitated PD center in Taiwan between 2014 and 2018. The collected data included socio-demographics and relevant medical and PD-related parameters. Kaplan-Meier analysis was used to determine the impact of non-medical reasons and medical reasons on PD dropout.

RESULTS

The analysis included 224 PD patients, of whom 37 dropped out for non-medical reasons and 187 for medical reasons during the study period. There was significant difference between the two cohorts in age (62.3 years vs. 56.1 years, P = 0.010) and PD vintage (median 3.4 years vs. 4.8 years, P = 0.001). Diabetes was more predominant in the cohort for non-medical reasons than in the one for medical reasons (54.1% vs. 27.3% respectively, P = 0.001). In non-medical reason cohort, two leading reasons given for dropping out were lacking of caregivers (n = 12) and losing confidence (n = 10), whereas PD-related peritonitis (n = 101) was the main medical reason for PD dropout. Using Kaplan-Meier curve analysis, patients in the non-medical reason cohort demonstrated higher cumulative dropout rate compared to patients in the medical reason cohort during a 10-year period (P < 0.001).

CONCLUSIONS

The main characteristics of PD dropout patients for non-medical reasons are age, diabetes, patients' perception and caregiver support.

Time-varying serum albumin levels and all-cause mortality in prevalent peritoneal dialysis patients: a 5-year observational study

BACKGROUND

In this study, we investigated the association of time-varying serum albumin levels with mortality over a 5-year period in one cohort of patients undergoing long-term peritoneal dialysis (PD) therapy.

METHODS

The participants in this study enrolled 302 patients who underwent long-term PD at a single PD center in Taiwan. We reviewed medical records from 2011 to 2015 retrospectively. Time-averaged albumin level and serum albumin reach rate (defined as the percentage of serum albumin measurements that reached ≥3.5 g/dL) were applied as the predictor variables in the first 2 years (2011-2012). All-cause mortality was used as the outcome variable in the subsequent 3 years (2013-2015). Hazard function of all-cause mortality in the study participants was examined by using Cox proportional hazard regression models .

RESULTS

Patients with different albumin reach rates (75-< 100%, 50-< 75%, 1-< 50%) did not exhibit a significantly increased risk for all-cause mortality. Patients with a 0% albumin reach rate exhibited a significantly increased risk for all-cause mortality (hazard ratio [HR] 7.59, 95% confidence interval [CI], 2.38-24.21) by fully adjusted analysis. Patients with time-averaged albumin levels of < 3.5 g/dL (HR 15.49, 95% CI 1.74-137.72) exhibited a higher risk for all-cause mortality than those with serum albumin levels ≥4.0 g/dL.

CONCLUSIONS

This study demonstrated that higher serum albumin reach rates and higher time-averaged serum albumin levels are associated with a lower mortality rate over a 5-year period among patients undergoing long-term PD.

Viridans streptococcus peritonitis in peritoneal dialysis: clinical characteristics and comparison with concurrent polymicrobial infection

Background

The clinical course of Viridans streptococci (VS) peritonitis in patients undergoing peritoneal dialysis (PD) is rarely reported. This study examined the association of clinical factors with VS peritonitis.

Methods

We retrospectively reviewed clinical data from patients with VS peritonitis from March 1990 to February 2016 in a PD center in Taiwan and evaluated clinical profiles and treatment outcomes.

Results

A total of 109 episodes of VS peritonitis in 71 patients identified. Among these patients, 57 had mono-VS peritonitis and 14 had concurrent polymicrobial infections. The median time interval from PD initiation to the first VS peritonitis episode was 18 months (range, 0.6–144 months). Among clinical outcomes, most VS peritonitis episodes were completely cured regardless of a history of peritonitis. All episodes with catheter removal occurred in those without a history of recent antibiotic use.

Conclusion

VS peritonitis in patients undergoing PD typically has favorable treatment outcomes. Antibiotic therapy should be started promptly.

Association of fatty liver disease with nonfatal cardiovascular events in patients undergoing maintenance hemodialysis

BACKGROUND/AIMS

The prevalence of cardiovascular (CV) disease in patients undergoing maintenance hemodialysis (HD) is reportedly higher than that in healthy individuals. In the present study, we aimed to investigate whether ultrasonographically documented fatty liver disease (FLD) is an independent risk factor for nonfatal CV events in patients undergoing HD.

METHODS

A retrospective cohort study was conducted in a medical center in southern Taiwan. The medical records of 490 patients undergoing HD who were enrolled between July 1998 and October 2012 were screened. Finally, 278 patients who had undergone hepatic ultrasonography and had available data were recruited in the present study. The patients included 130 men and 148 women; their mean age was 59.9 years. The primary endpoint was nonfatal CV events in the observation period. The comparable data included epidemiological, hematological, and biochemical profiles. A time-dependent statistical method was used to analyze the associated factors.

RESULTS

The prevalence of nonfatal CV events was significantly increased in the patients with FLD compared with those without FLD (CV events: 32 vs. 18%, respectively; p = 0.008). After adjusting for associated risk factors (sex, age, body mass index, smoking, diabetes, hypertension, dyslipidemia, and Kt/V), multivariate analyses identified FLD (CV events: hazard ratio 2.84, 95% confidence interval 1.52-5.28, p = 0.001), advanced age, and diabetes to be independently associated with nonfatal CV events.

CONCLUSION

The study suggests that FLD was an independent risk factor for nonfatal CV events in patients undergoing maintenance HD.

Lower education level is a major risk factor for peritonitis incidence in chronic peritoneal dialysis patients: a retrospective cohort study with 12-year follow-up

BACKGROUND

Peritoneal dialysis (PD)-related peritonitis remains an important complication in PD patients, potentially causing technique failure and influencing patient outcome. To date, no comprehensive study in the Taiwanese PD population has used a time-dependent statistical method to analyze the factors associated with PD-related peritonitis.

METHODS

Our single-center retrospective cohort study, conducted in southern Taiwan between February 1999 and July 2010, used time-dependent statistical methods to analyze the factors associated with PD-related peritonitis.

RESULTS

The study recruited 404 PD patients for analysis, 150 of whom experienced at least 1 episode of peritonitis during the follow-up period. The incidence rate of peritonitis was highest during the first 6 months after PD start. A comparison of patients in the two groups (peritonitis vs null-peritonitis) by univariate analysis showed that the peritonitis group included fewer men (p = 0.048) and more patients of older age (≥65 years, p = 0.049). In addition, patients who had never received compulsory education showed a statistically higher incidence of PD-related peritonitis in the univariate analysis (p = 0.04). A proportional hazards model identified education level (less than elementary school vs any higher education level) as having an independent association with PD-related peritonitis [hazard ratio (HR): 1.45; 95% confidence interval (CI): 1.01 to 2.06; p = 0.045). Comorbidities measured using the Charlson comorbidity index (score >2 vs ≤2) showed borderline statistical significance (HR: 1.44; 95% CI: 1.00 to 2.13; p = 0.053).

CONCLUSIONS

A lower education level is a major risk factor for PD-related peritonitis independent of age, sex, hypoalbuminemia, and comorbidities. Our study emphasizes that a comprehensive PD education program is crucial for PD patients with a lower education level.

Model-based segmentation of flexor tendons from magnetic resonance images of finger joints

Trigger finger is a common hand disease, causing swelling, painful popping and clicking in moving the affected finger joint. To better evaluate patients with trigger finger, segmentation of flexor tendons from magnetic resonance (MR) images of finger joints, which can offer detailed structural information of tendons to clinicians, is essential. This paper presents a novel model-based method with three stages for automatically segmenting the flexor tendons. In the first stage, a set of tendon contour models (TCMs) is initialized from the most proximal cross-sectional image via two-step ellipse estimation. Each of the TCMs is then propagated to its distally adjacent image by affine registration. The propagation is sequentially performed along the proximal-distal direction until the most distal image is reached, as the second stage of segmentation. The TCMs on each cross-sectional image are refined in the last stage with the snake deformation. MR volumes of three subjects were used to validate the segmentation accuracy. Compared with the manual results, our method showed good accuracy with small average margins of errors (within 0.5 mm) and large overlapping ratio (dice similarity coefficient above 0.8). Overall, the proposed method has great potential for morphological change assessment of flexor tendons and pulley-tendon system modeling for image guided surgery.

Factors associated with blood concentrations of indoxyl sulfate and p-cresol in patients undergoing peritoneal dialysis

BACKGROUND

Accumulating evidence supports the important role of protein-bound uremic toxins such as indoxyl sulfate and p-cresol in uremic syndrome. They exert direct deleterious effects on a variety of cells and could link to clinical outcome. Factors relevant to indoxyl sulfate and p-cresol levels in peritoneal dialysis (PD) patients have rarely been investigated. We conducted a cross-sectional study to analyze the factors that correlate with both total and free indoxyl sulfate and p-cresol.

METHODS

182 stable PD patients with mean PD therapy duration 38.5 +/- 33.3 months were enrolled. Their mean age was 48.9 +/- 13.5 years; 62.6% (114/182) were female patients. Demographic data, including age, gender, and PD therapy duration, were reviewed and recorded. PD-associated features such as residual kidney function (RKF), peritoneal transport property, and dialysis modality were also recorded. Hemoglobin, blood urea nitrogen (BUN), serum creatinine, C-reactive protein, interleukin (IL)-6, and IL-10 were measured. Levels of total and free indoxyl sulfate and p-cresol were determined.

RESULTS

Patients without RKF had lower Kt/V and weekly creatinine clearance and higher serum creatinine and IL-6 levels. These patients also had higher total and free indoxyl sulfate levels. There was no difference in indoxyl sulfate or p-cresol levels compared to patients with different peritoneal transport properties or with different treatment modalities. Multivariate regression analysis revealed that weekly creatinine clearance and serum creatinine were independent associates of total indoxyl sulfate level; IL-6, total indoxyl sulfate, and free p-cresol were associated with free indoxyl sulfate level. Weekly creatinine clearance and free p-cresol level independently correlated with total p-cresol; while gender, total p-cresol, and free indoxyl sulfate were associated with free p-cresol level.

CONCLUSION

The free forms of indoxyl sulfate and p-cresol constituted a small portion of their total forms. The presence of RKF affected levels of free and total indoxyl sulfate. IL-6 level was significantly associated with free indoxyl sulfate level. There was a close relationship between indoxyl sulfate and p-cresol levels in their free forms in PD patients.

Photonic logic NOR gate based on two symmetric microring resonators

We demonstrate an all-optical NOR logic gate based on symmetric GaAs-AlGaAs microring resonators whose resonances are closely matched. Two input pump data streams are tuned close to one resonance of the symmetric microrings to switch a probe beam tuned to another resonance by two-photon absorption. The switching energy of the gate is 20 pJ/pulse, and the switching window is 40 ps, limited by the carrier lifetime. The use of two rings provides for better cascading in photonic logic circuits because of the higher number of available ports.

Host cell factor requirement for hepatitis C virus enzyme maturation

The cellular chaperone, HSP90, is identified here as an essential factor for the activity of NS2/3 protease of hepatitis C virus. The cleavage activity of NS2/3 protease synthesized in reticulocyte lysate is ATP-dependent, as evidenced by ATP depletion experiments and inhibition with nonhydrolyzable ATP analogs. Geldanamycin and radicicol, ATP-competitive inhibitors of the chaperone HSP90, also inhibit the cleavage of in vitro-synthesized NS2/3. Furthermore, these HSP90 inhibitors prevent NS2/3 cleavage when the protease is expressed in mammalian cells. The physical association of NS2/3 with HSP90 is demonstrated by immunoprecipitation. Thus, by way of a chaperone/folding activity, an HSP90-containing complex is required for maturation of the polyprotein that encodes the enzymes essential for hepatitis C virus replication.

Identification of MK-944a: a second clinical candidate from the hydroxylaminepentanamide isostere series of HIV protease inhibitors

Recent results from human clinical trials have established the critical role of HIV protease inhibitors in the treatment of acquired immune-deficiency syndrome (AIDS). However, the emergence of viral resistance, demanding treatment protocols, and adverse side effects have exposed the urgent need for a second generation of HIV protease inhibitors. The continued exploration of our hydroxylaminepentanamide (HAPA) transition-state isostere series of HIV protease inhibitors, which initially resulted in the identification of Crixivan (indinavir sulfate, MK-639, L-735,524), has now yielded MK-944a (L-756,423). This compound is potent, is selective, and competitively inhibits HIV-1 PR with a K(i) value of 0.049 nM. It stops the spread of the HIV(IIIb)-infected MT4 lymphoid cells at 25.0-50.0 nM, even in the presence of alpha(1) acid glycoprotein, human serum albumin, normal human serum, or fetal bovine serum. MK-944a has a longer half-life in several animal models (rats, dogs, and monkeys) than indinavir sulfate and is currently in advanced human clinical trials.

Only a small fraction of purified hepatitis C RNA-dependent RNA polymerase is catalytically competent: implications for viral replication and in vitro assays

The enzymatic activity of a C-terminally truncated form of the RNA-dependent RNA polymerase, termed NS5B(Delta21), of the hepatitis C virus (strain BK) has been investigated using both homopolymeric and heteropolymeric RNA templates. Incorporation of nucleotides into a heteropolymeric RNA template as catalyzed by NS5B(Delta21) is characterized by biphasic reaction time courses. At high concentrations of nucleoside triphosphate in reactions allowing a preincubation of NS5B(Delta21) and RNA template, an initial rapid phase of the reaction is followed by a slower linear phase. The amplitude of the first phase of the reaction varies directly with the concentration of the enzyme in the reaction. It is shown here that full-length copies of the template are produced during the first phase of the reaction. Our results reveal that NS5B(Delta21) is processive but only a small fraction, less than 1%, of the purified enzyme present participates productively in the reaction. Most importantly, the turnover number for the hepatitis C NS5B(Delta21) is comparable to those observed for other polymerases such as the HIV-1 reverse transcriptase. The combined results reconcile in part the apparent discrepancy of the low, observed specific activity of the purified enzyme and the rapid generation of HCV in vivo.

Combinatorial diversification of indinavir: in vivo mixture dosing of an HIV protease inhibitor library

An efficient combination solution-phase/solid-phase route enabling the diversification of the P1', P2', and P3 subsites of indinavir has been established. The synthetic sequence can facilitate the rapid generation of HIV protease inhibitors possessing more favorable pharmacokinetic properties as well as enhanced potencies. Multiple compound dosing in vivo may also accelerate the identification of potential drug candidates.

An alternate binding site for the P1-P3 group of a class of potent HIV-1 protease inhibitors as a result of concerted structural change in the 80s loop of the protease

Structures of the complexes of HIV protease inhibitor L--756,423 with the HIV-1 wild-type protease and of the inhibitors Indinavir, L-739,622 and Saquinavir with the mutant protease (9X) containing nine point mutations (Leu10Val, Lys20Met, Leu24Ile, Ser37Asp, Met46Ile, Ile54Val, Leu63Pro, Ala71Val, Val82Thr) have been determined. Comparative analysis of these structures reveals an alternate binding pocket for the P1-P3 group of Indinavir and L--756, 423. The alternate binding pocket is a result of concerted structural change in the 80s loop (residues 79-82) of the protease. The 80s loop is pulled away from the active site in order to accommodate the P1-P3 group, which is sandwiched between the flap and the 80s loop. This structural change is observed for the complexes of the wild type as well as the 9X mutant protease. The study reveals that the 80s loop is an intrinsically flexible loop in the wild-type HIV-1 protease and that mutations in this loop are not necessary to result in conformational changes. Conformation of this loop in the complex depends primarily upon the nature of the bound inhibitor and may be influenced by mutations in the protease. The results underscore the need to understand the intrinsic structural plasticity of the protease for the design of effective inhibitors against the wild-type and drug-resistant enzyme forms. In addition, the alternate binding pocket for the P1-P3 group of Indinavir and L--756,423 may be exploited for the design of potent inhibitors.

X-ray structure of simian immunodeficiency virus integrase containing the core and C-terminal domain (residues 50-293)--an initial glance of the viral DNA binding platform

The crystal structure of simian immunodeficiency virus (SIV) integrase that contains in a single polypeptide the core and the C-terminal deoxyoligonucleotide binding domain has been determined at 3 A resolution with an R-value of 0.203 in the space group P2(1)2(1)2(1). Four integrase core domains and one C-terminal domain are found to be well defined in the asymmetric unit. The segment extending from residues 114 to 121 assumes the same position as seen in the integrase core domain of avian sarcoma virus as well as human immunodeficiency virus type-1 (HIV-1) crystallized in the absence of sodium cacodylate. The flexible loop in the active site, composed of residues 141-151, remains incompletely defined, but the location of the essential Glu152 residue is unambiguous. The residues from 210-218 that link the core and C-terminal domains can be traced as an extension from the core with a short gap at residues 214-215. The C(alpha) folding of the C-terminal domain is similar to the solution structure of this domain from HIV-1 integrase. However, the dimeric form seen in the NMR structure cannot exist as related by the non-crystallographic symmetry in the SIV integrase crystal. The two flexible loops of the C-terminal domain, residues 228-236 and residues 244-249, are much better fixed in the crystal structure than in the NMR structure with the former in the immediate vicinity of the flexible loop of the core domain. The interface between the two domains encompasses a solvent-exclusion area of 1500 A(2). Residues from both domains purportedly involved in DNA binding are narrowly distributed on the same face of the molecule. They include Asp64, Asp116, Glu152 and Lys159 from the core and Arg231, Leu234, Arg262, Arg263 and Lys264 from the C-terminal domain. A model for DNA binding is proposed to bridge the two domains by tethering the 228-236 loop of the C-terminal domain and the flexible loop of the core.

Inhibition of hepatitis C virus NS2/3 processing by NS4A peptides. Implications for control of viral processing

The NS2/3 protease of hepatitis C virus is responsible for a single cleavage in the viral polyprotein between the nonstructural proteins NS2 and NS3. The minimal protein region necessary to catalyze this cleavage includes most of NS2 and the N-terminal one-third of NS3. Autocleavage reactions using NS2/3 protein translated in vitro are used here to investigate the inhibitory potential of peptides likely to affect the reaction. Peptides representing the cleaved sequence have no effect upon reaction rates, and the reaction rate is insensitive to dilution. Both results are consistent with prior suggestions that the NS2/3 cleavage is an intramolecular reaction. Surprisingly, peptides containing the 12-amino acid region of NS4A responsible for binding to NS3 inhibit the NS2/3 reaction with K(i) values as low as 3 microM. Unrelated peptide sequences of similar composition are not inhibitory, and neither are peptides containing incomplete segments of the NS4A region that binds to NS3. Inhibition of NS2/3 by NS4A peptides can be rationalized from the organizing effect of NS4A on the N terminus of NS3 (the NS2/3 cleavage point) as suggested by the known three-dimensional structure of the NS3 protease domain (Yan, Y., Li, Y., Munshi, S., Sardana, V., Cole, J. L., Sardana, M., Steinkuhler, C., Tomei, L., De Francesco, R., Kuo, L. C., and Chen, Z. (1998) Protein Sci. 7, 837-847). These findings may imply a sequential order to proteolytic maturation events in hepatitis C virus.

Purification, solution properties and crystallization of SIV integrase containing a continuous core and C-terminal domain

The C-terminal two-thirds segment of integrase derived from the simian immunodeficiency virus has been cloned, expressed in Escherichia coli, and purified to greater than 95% homogeneity. The protein encompasses amino-acid residues 50-293 and contains a F185H substitution to enhance solubility. In dilute solutions at concentrations below 1 mg ml(-1), the enzyme is predominantly dimeric. At the higher concentrations (>10 mg ml(-1)) required to enable crystallization, the enzyme self-associates to form species with molecular weights greater than 200 kDa. Despite the apparent high aggregation in solution, the enzyme crystallizes from a 8%(v/v) polyethylene glycol (molecular weight 6000) solution in a form suitable for X-ray diffraction studies. The resulting single crystals belong to the space group P2(1)2(1)2(1), with unit-cell parameters a = 79.76, b = 99.98, c = 150.2 A, alpha = beta = gamma = 90 degrees and Z = 4. Under X-ray irradiation generated with a rotating-anode generator, the crystals diffract to 2.8 A resolution and allow collection of a native 3 A resolution diffraction data set.

Non-active site changes elicit broad-based cross-resistance of the HIV-1 protease to inhibitors

Three high level, cross-resistant variants of the HIV-1 protease have been analyzed for their ability to bind four protease inhibitors approved by the Food and Drug Administration (saquinavir, ritonavir, indinavir, and nelfinavir) as AIDS therapeutics. The loss in binding energy (DeltaDeltaG(b)) going from the wild-type enzyme to mutant enzymes ranges from 2.5 to 4.4 kcal/mol, 40-65% of which is attributed to amino acid substitutions away from the active site of the protease and not in direct contact with the inhibitor. The data suggest that non-active site changes are collectively a major contributor toward engendering resistance against the protease inhibitor and cannot be ignored when considering cross-resistance issues of drugs against the HIV-1 protease.

An uniquely purified HCV NS3 protease and NS4A(21-34) peptide form a highly active serine protease complex in peptide hydrolysis

The N-terminal domain of the hepatitis C virus (HCV) polyprotein containing the NS3 protease (residues 1027 to 1206) was expressed in Escherichia coli as a soluble protein under the control of the T7 promoter. The enzyme has been purified to homogeneity with cation exchange (SP-Sepharose HR) and heparin affinity chromatography in the absence of any detergent. The purified enzyme preparation was soluble and remained stable in solution for several weeks at 4 degrees C. The proteolytic activity of the purified enzyme was examined, also in the absence of detergents, using a peptide mimicking the NS4A/4B cleavage site of the HCV polyprotein. Hydrolysis of this substrate at the expected Cys-Ala scissile bond was catalyzed by the recombinant protease with a pseudo second-order rate constant (k(cat)/K(M)) of 205 and 196,000 M(-1) s(-1), respectively, in the absence and presence of a central hydrophobic region (sequence represented by residues 21 to 34) of the NS4A protein. The rate constant in the presence of NS4A peptide cofactor was two orders of magnitude greater than reported previously for the NS3 protease domain. A significantly higher activity of the NS3 protease-NS4A cofactor complex was also observed with a substrate mimicking the NS4B/5A site (k(cat)/K(M) of 5180 +/- 670 M(-1) s(-1)). Finally, the optimal formation of a complex between the NS3 protease domain and the cofactor NS4A was critical for the high proteolytic activity observed.

Rapid X-ray diffraction analysis of HIV-1 protease-inhibitor complexes: inhibitor exchange in single crystals of the bound enzyme

The ability to replace an inhibitor bound to the HIV-1 protease in single crystals with other potent inhibitors offers the possibility of investigating a series of protease inhibitors rapidly and conveniently with the use of X-ray crystallography. This approach affords a fast turnaround of structural information for iterative rational drug designs and obviates the need for studying the complex structures by co-crystallization. The replacement approach has been successfully used with single crystals of the HIV-1 protease complexed with a weak inhibitor. The structures of the complexes obtained by the replacement method are similar to those determined by co-crystallization.

Complex of NS3 protease and NS4A peptide of BK strain hepatitis C virus: a 2.2 A resolution structure in a hexagonal crystal form

The crystal structure of the NS3 protease of the hepatitis C virus (BK strain) has been determined in the space group P6(3)22 to a resolution of 2.2 A. This protease is bound with a 14-mer peptide representing the central region of the NS4A protein. There are two molecules of the NS3(1-180)-NS4A(21'-34') complex per asymmetric unit. Each displays a familiar chymotrypsin-like fold that includes two beta-barrel domains and four short alpha-helices. The catalytic triad (Ser-139, His-57, and Asp-81) is located in the crevice between the beta-barrel domains. The NS4A peptide forms an almost completely enclosed peptide surface association with the protease. In contrast to the reported H strain complex of NS3 protease-NS4A peptide in a trigonal crystal form (Kim JL et al., 1996, Cell 87:343-355), the N-terminus of the NS3 protease is well-ordered in both molecules in the asymmetric unit of our hexagonal crystal form. The folding of the N-terminal region of the NS3 protease is due to the formation of a three-helix bundle as a result of crystal packing. When compared with the unbound structure (Love RA et al., 1996, Cell 87:331-342), the binding of the NS4A peptide leads to the ordering of the N-terminal 28 residues of the NS3 protease into a beta-strand and an alpha-helix and also causes local rearrangements important for a catalytically favorable conformation at the active site. Our analysis provides experimental support for the proposal that binding of an NS4A-mimicking peptide, which increases catalytic rates, is necessary but not sufficient for formation of a well-ordered, compact and, hence, highly active protease molecule.

Activation of RNase L by 2',5'-oligoadenylates. Biophysical characterization

Ribonuclease L (RNase L) is an endoribonuclease that is activated upon binding of adenosine oligomers linked 2' to 5' to cleave viral and cellular RNAs. We recently proposed a model for activation in which activator A binds to monomer, E, to form EA, which subsequently dimerizes to the active form, E2A2 (Cole, J. L., Carroll, S. S., and Kuo, L. C. (1996) J. Biol. Chem. 271, 3978-3981). Here, we have employed this model to define the equilibrium constants for activator binding (Ka) and dimerization of EA to E2A2 (Kd) by equilibrium analytical ultracentrifugation and fluorescence measurements. Multi-wavelength sedimentation data were globally fit to the model above, yielding values of Ka = 1.69 microM and Kd = 17. 8 nM for 2',5'-linked adenosine trimer. Fluorescent conjugates of 2',5'-linked adenosine trimer with 7-hydroxycoumarin have been prepared. The coumarin emission anisotropy shows a large increases upon binding to RNase L. Analysis of anisotropy titrations yields values of Ka and Kd close to those obtained by sedimentation. The sedimentation parameters for unmodified 2',5'-linked adenosine trimer also agree with those obtained by enzyme kinetic methods (Carroll, S. S., Cole, J. L., Viscount, T., Geib, J., Gehman, J., and Kuo, L. C. (1997) J. Biol. Chem. 272, 19193-19198). Thus, the data presented here clearly define the energetics of RNase L activation and support the minimal activation model.

Activation of RNase L by 2',5'-oligoadenylates. Kinetic characterization

Ribonuclease L (RNase L), the 2',5'-oligoadenylate-dependent ribonuclease, is one of the cellular antiviral systems with enhanced activity in the presence of interferon. A reaction scheme has been developed to model the sequence of steps necessary for the activation of RNase L (Cole, J. L., Carroll, S. S., Blue, E. S., Viscount, T., and Kuo, L. C. (1997) J. Biol. Chem. 272, 19187-19192). The model comprises three sequential binding steps: the binding of activator to enzyme monomer, the subsequent dimerization of the activated monomer to form the active enzyme dimer, followed by the binding of substrate prior to catalysis. The model is used to evaluate the activation of RNase L by several synthetic analogs of the native activator. The 5'-phosphate of the activator has been determined to be an important structural determinant for the efficient activation of RNase L, and its loss caused a loss of activator affinity of 2-3 orders of magnitude. The length of activator is not an important determinant of activator potency for the activator analogs examined. The specific activity of the enzyme under conditions of saturation of activator binding and complete dimerization of the activated monomers varies only by about a factor of 3 for the activators examined, indicating that once dimerized in the presence of any of these activators, the enzyme exhibits a similar catalytic activity.

Mutational anatomy of an HIV-1 protease variant conferring cross-resistance to protease inhibitors in clinical trials. Compensatory modulations of binding and activity

Site-specific substitutions of as few as four amino acids (M46I/L63P/V82T/I84V) of the human immunodeficiency virus type 1 (HIV-1) protease engenders cross-resistance to a panel of protease inhibitors that are either in clinical trials or have recently been approved for HIV therapy (Condra, J. H., Schleif, W. A., Blahy, O. M. , Gadryelski, L. J., Graham, D. J., Quintero, J. C., Rhodes, A., Robbins, H. L., Roth, E., Shivaprakash, M., Titus, D., Yang, T., Teppler, H., Squires, K. E., Deutsch, P. J., and Emini, E. A. (1995) Nature 374, 569-571). These four substitutions are among the prominent mutations found in primary HIV isolates obtained from patients undergoing therapy with several protease inhibitors. Two of these mutations (V82T/I84V) are located in, while the other two (M46I/L63P) are away from, the binding cleft of the enzyme. The functional role of these mutations has now been delineated in terms of their influence on the binding affinity and catalytic efficiency of the protease. We have found that the double substitutions of M46I and L63P do not affect binding but instead endow the enzyme with a catalytic efficiency significantly exceeding (110-360%) that of the wild-type enzyme. In contrast, the double substitutions of V82T and I84V are detrimental to the ability of the protease to bind and, thereby, to catalyze. When combined, the four amino acid replacements institute in the protease resistance against inhibitors and a significantly higher catalytic activity than one containing only mutations in its active site. The results suggest that in raising drug resistance, these four site-specific mutations of the protease are compensatory in function; those in the active site diminish equilibrium binding (by increasing Ki), and those away from the active site enhance catalysis (by increasing kcat/KM). This conclusion is further supported by energy estimates in that the Gibbs free energies of binding and catalysis for the quadruple mutant are quantitatively dictated by those of the double mutants.