[Contribution of the University Clinical Research Center's laboratoryin the diagnosis of SARS-CoV-2 in Mali]. / Contribution du laboratoire du Centre Universitaire de Recherche Clinique (UCRC) au diagnostic de SRAS-CoV-2 au Mali.

INTRODUCTION: The rapid diagnostic capacities of laboratories in Mali have been an essential element in the response to COVID-19. The University Clinical Research center (UCRC) diagnosed the first cases of Mali COVID-19. OBJECTIVE: The objective was to describe the contribution of the UCRC in the diagnosis of Covid-19 and to clinically and epidemiologically characterize the patients tested in the UCRC laboratory. MATERIALS AND METHODS: A cross-sectional study was conducted during eight months of intense activity. The samples were sent from the National Institute of Public Health (INSP) to the UCRC. RESULTS: The UCRC tested 12,406 contacts and suspected samples and confirmed the diagnosis in 1091 patients, or 9%. The most common symptoms were cough (48.78%), headache (34.14%), fatigue / weakness (34.14%), while (33.33%) of the patients were asymptomatic. The sample positivity rate among new cases decreased from May to September 2020, despite almost 230% of the number of samples tested. CONCLUSION: The laboratory played a major role in the response and there may be a low transmission of the virus in the Malian community.

INTRODUCTION: Les capacités de diagnostic rapide des laboratoires au Mali ont été un élément essentiel dans la riposte contre la COVID-19. Le Centre Universitaire de Recherche Clinique (UCRC)a diagnostiqué les premiers cas du Mali. OBJECTIF: Etait de décrire l'apport de l'UCRC dans le diagnostic de la Covid-19 et de caractériser cliniquement et épidémiologiquement les patients testés au laboratoire de l'UCRC. MATÉRIELS ET MÉTHODES: Une étude transversale a été conduite pendant huit mois d'activité intense. Les échantillons ont été envoyés de l'Institut National de Santé Publique (INSP) à l'UCRC. RÉSULTATS: L'UCRC a testé 12 406 échantillons contacts et suspects et a confirmé le diagnostic chez 1091 patients soit 9%. Les symptômes les plus rencontrés ont été la toux (48,78%), les maux de tête (34,14%), la fatigue/faiblesse (34,14%), tandis que (33,33%) des patients étaient asymptomatiques. Le taux de positivité des échantillons a diminué entre mai et août et avec une légère diminution en septembre 2020,avec près de 230% du nombre d'échantillons testés. CONCLUSION: Le laboratoire a joué un grand rôle dans la riposte et il y'aurait une faible transmission du virus dans la communauté Malienne.

Diabetes Mellitus among new tuberculosis patients in Bamako, Mali.

INTRODUCTION: Diabetes Mellitus (DM) increases worldwide, mostly in low- and middle-income countries. In Mali, the prevalence in the adult population is estimated at 1.8%, but tuberculosis (TB) patients are not systematically screened. The goal of our study was to determine the prevalence of DM among newly diagnosed TB patients. METHODS: We conducted a cross sectional study and a pilot prospective cohort study in four health centers in Bamako. All patients underwent fasting capillary-blood glucose (FCBG) test at Day 0, and repeated after one-week of TB treatment. Venous FBG test was performed for discrepancies between the two FCBG results. Thereafter, FCBG was performed for pilot study at month-2 (M2) and M5 of TB treatment. RESULTS: Two hundred and one patients were enrolled in this study. Impaired fasting blood glucose was identified in 17 (8.5%), of whom 11 (5.5%) had DM (VFBG >7 mmol/L). Among patients with DM, seven (63.6%) had successful TB treatment outcome, versus 142 (74.7%) of those without DM (p = 0.64), and (OR: 1.69, 95%CI 0.47-6.02). CONCLUSION: The prevalence of DM among TB patients in Bamako exceeds that of the general population and screening at TB diagnosis suffices to identify those with DM. Systematic screening of both diseases will allow better treatment.

Intestinal transport of weak electrolytes. Evidence in favor of a three-compartment system.

A study has been made of the transmural fluxes of benzoic, phenylacetic, and pentanoic acids, benzylamine, hexylamine, and D-amphetamine across rat jejunum incubated in vitro. The M to S fluxes of the weak acids were greater than their corresponding S to M fluxes, and the S to M fluxes of the weak bases were larger than their M to S fluxes. These patterns of asymmetric movements were observed when the transmural electrical potential difference was clamped at 0 mV, and when the pH values of the mucosal and serosal fluids were identical. The effects of a weak acid on the fluxes of other weak electrolytes were qualitatively similar when the effector weak acid was added to the mucosal fluid, and when it was added to the serosal fluid. But the effects of a weak base on the fluxes of other weak electrolytes were dependent upon its location, and the interactions observed when the effector weak base was added to the mucosal fluid were qualitatively different than those seen when it was added to the serosal fluid. The interactions between weak electrolytes could readily be explained in terms of the function of a system of three compartments in series, in which the pH of the intermediate compartment is greater than that of the bulk phases. But these observations could not be explained in terms of an analogous system involving an intermediate compartment of low pH, or in terms of a carrier mediated system. The transport function of the three-compartment system can be described in the form of an equation, and it is found that a pH difference of less than 0.5 unit may explain our observations on weak electrolyte transport.

Treatment with field bean protease inhibitor can effectively repress ethylnitrosourea (ENU)-induced neoplasms of the nervous system in Sprague-Dawley rats.

The ability of field bean protease inhibitor (FBPI) to inhibit ethylnitrosourea (ENU)-induced tumours of the nervous system of Sprague-Dawley rats was investigated. Groups of 1-day-old rats were injected intraperitoneally (i.p.) with neurocarcinogenic amounts of ENU and a few hours later, one group was treated i.p. with 80 mg of FBPI per kg body weight. This treatment was carried out three times a week for the first month and five times a week for the next month. Animals were killed when they were neurologically ill and their neural tissues were assessed for lesions. Those FBPI-treated rats which showed no illness were also killed to terminate the experiment about 8 weeks after the last rat of the control group was affected with paralysis. The neural tumours induced in all groups were predominantly large tumours found in the cerebrum of the rats. ENU-treated rats showed a 100% incidence of nervous system tumours with a mean time of manifestation of neurological symptoms of 282 days, which was significantly shorter in comparison to that noted in the FBPI-treated group. The latter group showed an incidence of 58.3%, i.e. a significant reduction of 41% in the incidence of neural tumours, as well as a lower mean value for the number of tumours per rat. All these aspects indicated that FBPI is a potential neurooncopreventive agent. A neural tumour incidence of 100% in the rats treated with heat-inactivated FBPI confirmed that the tumour suppressive activity of FBPI is related to its protease inhibitory activity.

The field bean protease inhibitor has the potential to suppress B16F10 melanoma cell lung metastasis in mice.

Metastasis is a characteristic and fatal feature of human malignancies. Its regulation is therefore of the utmost significance to clinicians. The present study was undertaken to determine whether a legume-derived protease inhibitor (PI) of trypsin/chymotrypsin, the field bean PI (FBPI), also has plasmin inhibitory activity and can inhibit pulmonary metastasis of B16F10 melanoma cells systemically injected into BDF1 mice. Two approaches to the problem were made. In the first, the melanoma cells were exposed to two different concentrations of the FBPI prior to their inoculation into animals. In the second, the mice were treated intraperitoneally with FBPI at a dose of 100 mg/kg body weight once daily for 10 days, the treatment being started soon after the systemic injection of the tumour cells. The study revealed that both modes of FBPI treatment could effectively block lung cell metastasis by the melanoma cells and that FBPI has plasmin blocking activity. Since urokinase type plasminogen activator and plasmin are known to play significant roles in tumour cell metastasis, the dose-dependent inhibitory effect of FBPI with antiplasmin activity on tumour cell metastasis suggests that its antimetastatogenic action is probably mediated through its plasmin inhibitory action.

Synthesis and biological evaluation of novel macrocyclic paclitaxel analogues.

[reaction: see text] This work describes the synthesis of two novel macrocyclic taxoid constructs by ring-closing olefin metathesis (RCM) and their biological evaluation. Computational studies examine conformational profiles of 1 and 2 for their fit to the beta-tubulin binding site determined by electron crystallography. The results support the hypothesis that paclitaxel binds to microtubules in a "T" conformation.

Physique anxiety and exercise in middle-aged adults.

The present study examined the relationships among body composition, exercise participation, and physique anxiety in a sample of middle-aged, formerly sedentary males and females. Study variables were assessed prior to and following participation in a 20-week aerobic exercise program. Subjects reduced their weight, body fat, and body circumferences over the course of the program, as well as physique anxiety. Multivariate and hierarchical regression analyses revealed females and those subjects between 45 and 54 years to be significantly more physique-anxious than their older counterparts. Being female and failing to reduce hip circumference were significantly related to postprogram physique anxiety. Although those who exercised more frequently tended to be less physique-anxious in correlational analyses, exercise frequency was a nonsignificant predictor when controlling for body composition. Thus, it appears that body image is a concern across the female age spectrum rather than the sole province of undergraduate females.

Using validation techniques to improve communication with cognitively impaired older adults.

1. Nurses working with confused older adults in clinical settings need improved communication techniques. 2. Reality orientation is commonly used in nursing homes with disoriented persons; experienced nurses, however, often find it meets with withdrawal, vegetation or increasing hostility. 3. Validation strategies are communication techniques which are specifically matched to a particular stage of confusion. 4. When validation techniques are appropriately matched to confusional stage the effectiveness of the intervention can be significantly improved.

Steroid use in rhinoplasty: an objective assessment of postoperative edema.

The objective of this study was to measure the effect of a single, preoperative 10 mg dose of dexamethasone on postoperative edema associated with rhinoplasty. This was a randomized, double-blind prospective study conducted in a military academic tertiary referral center. Twenty men, aged 18 to 45 years, were enrolled in the study over 28 months. All 20 men underwent rhinoplasty with osteotomy. Preoperative magnetic resonance imaging scans were obtained on the morning of surgery and postoperative scans were obtained within 48 hours. Postoperative edema was quantified as the difference in soft tissue thickness (mm) between the pre- and postoperative scans. Contrary to our expectations, the rhinoplasty patients who received dexamethasone had increased postoperative edema (p < 0.02) when compared to patients not receiving dexamethasone. This is the first objective, double-blind study that shows an increase in postoperative edema after rhinoplasty with a single preoperative dose of dexamethasone.

Rural mental health and aging: implication for case management.

Older adults are underrepresented as recipients of mental health services, despite evidence that preventive interventions are effective and complement physical health interventions. Barriers to using these services include the stigma associated with mental health problems, lack of trained professionals and general barriers of availability and accessibility in rural service delivery. Case managers can play an important role in breaking down some of these barriers and assisting older adults to access t-ese services. Several outreach and educational models have been developed that are replicable and available to case managers.

The effects of colchicine analogues on the reaction of tubulin with iodo[14C]acetamide and N,N'-ethylenebis(iodoacetamide).

We have previously found (Ludueña, R. F., and Roach, M. C. (1981b) Biochemistry 20, 4444-4450) that colchicine and podophyllotoxin inhibit the alkylation of tubulin by iodo[14C]acetamide and the formation of an intrachain cross-link in the beta-tubulin subunit by N,N'-ethylenebis(iodoacetamide) (EBI). It was not clear whether these effects were due to conformational changes in tubulin induced by drugs or to direct steric blockage of the sulfhydryl groups involved. In an effort to characterize further these phenomena, we have examined the effects of single-ring and bicyclic analogues of colchicine on the reaction of tubulin with iodo[14C]acetamide and EBI. We have found that neither the A-ring analogues, 3,4,5-trimethoxybenzyl alcohol, 3,4,5-trimethoxybenzaldehyde, 2,3,4-trimethoxybenzaldehyde, and benzaldehyde, nor the C-ring analogues, tropolone and tropolone methyl ether, inhibited alkylation. In contrast, colchicine, podophyllotoxin, and nocodazole and the bicyclic analogues, 5-(2',3',4'-trimethoxyphenyl)-2-methoxytropone and combretastatin, inhibited tubulin alkylation. Since the presence of a bond joining the A and C rings seems to be the determining factor in the suppression of alkylation, it is likely that inhibition by colchicine of the reaction with iodo[14C] acetamide is due largely to a conformational change induced by colchicine. A different pattern was obtained when the effects on cross-link formation by EBI were examined. Here, all the A-ring analogues, the bicyclic analogues, and colchicine, podophyllotoxin, and nocodazole all inhibited formation of the cross-link, whereas the C-ring analogue tropolone methyl ether did not inhibit cross-link formation. Since compounds whose effect on alkylation is markedly different have the same effect on cross-link formation, it is possible that this effect is a steric one and that perhaps the A-ring of colchicine binds to tubulin very close to one of the sulfhydryls involved in the intrachain cross-link formed by EBI in beta-tubulin.

Mechanism of tubulin-colchicine recognition: a kinetic study of the binding of the colchicine analogues colchicide and isocolchicine.

Colchicide (IDE) is a colchicine (COL) analogue in which the C-10 methoxy group is replaced by a hydrogen atom. Its binding to tubulin is accompanied by a quenching of the protein fluorescence. The fluorescence decrease shows a monoexponential time dependence. The observed rate constant increases in a non-linear way with the total concentration of IDE, allowing the determination of a binding constant for an initial binding site (K1=5300+/-300 M-1) and the rate constant for the subsequent isomerization (k2=0.071+/-0.002 s-1) at 25 degrees C. The rate constant, k-2, for the reversed isomerization can be determined by displacement experiments. Despite the minor alteration of the C-ring substituent, the kinetic and thermodynamic parameters of binding are substantially different from those of COL itself, for both steps. In isocolchicine (ISO) the carbonyl oxygen atom and the methoxy groups of the C-ring have been interchanged. Its binding to tubulin only results in small fluorescence and absorbance changes. Therefore competition experiments with MTC [2-methoxy-5-(2',3',4'-trimethoxyphenyl)-2,4, 6-cycloheptatrien-1-one] were performed. ISO competes rapidly and with low affinity with MTC. Fluorimetric titrations of tubulin with MDL (MDL 27048 or trans-1-(2,5 dimethoxyphenyl)-3-[4-(dimethylamino)phenyl]-2-methyl-2-propen-1 -one) in the presence and absence of ISO give evidence for the existence of a second, slow-reacting low-affinity site for ISO that is not accessible to MTC or MDL. The relevance of these results for the recognition of COL is analysed.

Exercise in middle-aged adults: self-efficacy and self-presentational outcomes.

BACKGROUND: Whereas self-efficacy expectations have been identified as important determinants of exercise participation patterns, little empirical work that examines efficacy expectations as outcomes of exercise participation or their theoretical relationship to other psychological outcomes associated with exercise has been conducted. In the context of middle-aged males and females, the present study attempted to integrate social cognitive and impression management perspectives with respect to anxiety associated with exercise. METHODS: Formerly sedentary subjects participated in a 5-month exercise program with assessments of physique anxiety, efficacy, outcome expectations, and anthropometric variables prior to and following the program. RESULTS: Both acute bouts and long-term participation in exercise resulted in significant increases in self-efficacy. In turn, these changes in efficacy and initial positive outcome expectations were significant predictors of reductions in physique anxiety, even when controlling for the influence of gender and reductions in body fat, weight, and circumferences. CONCLUSIONS: The findings are discussed in terms of the implications for structure and content of exercise environments and the utility of the proposed theoretical integration. Strategies for enhancing beliefs regarding health and fitness outcomes associated with exercise rather than appearance outcomes may be required to maximize reductions in negative body image.

Exercise and self-esteem in middle-aged adults: multidimensional relationships and physical fitness and self-efficacy influences.

In the context of sedentary middle-aged adults, the present study examined the relationships among domain-specific and global levels of self-esteem over the course of a 20-week exercise program. Additionally, the roles played by physical fitness, body composition, self-efficacy, and exercise participation as possible contributors to changes in physical self-worth were examined. Significant improvements in self-esteem at all levels were discovered with global esteem, physical self-worth, and perceptions of physical condition and attractive body increasing. Tests of the hierarchical structure of self-esteem showed greater improvements in physical condition and physical self-worth than global esteem and the relationships between global esteem and subdomain levels were shown to be mediated by physical self-worth. Hierarchical regression analyses showed changes in ratings of importance to have little impact on changes in physical self-worth. Both changes in efficacy and aerobic capacity were demonstrated to account for modest but significant variation in physical self-esteem. Results are discussed in terms of contemporary models of self-esteem, potential mediators of exercise effects on esteem, and the need to measure the constructs of interest appropriately.

The mechanism of tubulin-colchicine recognition--a kinetic study of the binding of a bicyclic colchicine analogue with a minor modification of the A ring.

2-Methoxy-5-(2',3',4'-trimethoxy)-2,4,6-cycloheptatrien-1-one (MTC) is a colchicine analogue that lacks the B ring. 2-Methoxy-5-(2',4'-dimethoxyphenyl)-2,4,6-cycloheptatrien-1-one (MD) is an A-ring analogue of MTC, in which one methoxy group is replaced by a hydrogen atom. This paper describes the kinetic features of MDC binding to tubulin, and compares its behaviour with MTC to analyse the effect of the A-ring modification on the recognition process by tubulin. Binding is accompanied by a strong enhancement of MDC fluorescence and quenching of protein fluorescence. The kinetic and thermodynamic parameters were obtained from fluorescence stopped-flow measurements. The kinetics are described by a single exponential, indicating that this drug does not discriminate between the different tubulin isotypes. The observed pseudo-first-order rate constant of the fluorescence increase upon binding increases in a non-linear way, indicating that this ligand binds with a similar overall mechanism as colchicine and MTC, consisting of a fast initial binding of low affinity followed by a slower isomerisation step leading to full affinity. The K1 and k2 values for MDC at 25 degrees C were 540 +/- 65 M(-1) and 70 +/- 6 s(-1) respectively. From the temperature dependence, a reaction enthalpy change (deltaH(o)1) of the initial binding of 49 +/- 11 kJ/mol(-1) and an activation energy for the second step of 28 +/- 9 kJ/mol(-1) were calculated. Displacement experiments of bound MDC by MTC allowed the determination of a rate constant of reverse isomerisation of 0.60 +/- 0.07 s(-1) at 25 degrees C and the activation energy of 81 +/- 6 kJ/mol(-1). The overall binding constant was (6.3 +/- 0.2) x 10(4) M(-1) at 25 degrees C. Combination of these results with the kinetic parameters for association gives a full characterisation of the enthalpy pathway for the binding of MDC. The pathway of MDC is shown to differ considerably from that of MTC binding. Since its structural difference is located in ring A, this result indicates the use of ring A in the first step. The kinetics of the binding of MDC in the presence of some A-ring colchicine analogues (podophyllotoxin, 3',4',5'-trimethoxyacetophenone and N-acetylmescaline) and a C-ring analogue (tropolone methyl ether) suggest that the A and C rings are involved in the binding of MDC.

Binding to tubulin of the colchicine analog 2-methoxy-5-(2', 3', 4'-trimethoxyphenyl)tropone. Thermodynamic and kinetic aspects.

The thermodynamics and kinetics of the binding to tubulin of the colchicine analog 2-methoxy-5-(2', 3', 4'-trimethoxyphenyl) tropone (termed AC because it lacks the B-ring of colchicine) have been characterized by fluorescence techniques. The fluorescence of AC is weak in aqueous solution and is enhanced 250-fold upon binding to tubulin. The following thermodynamic values were obtained for the interaction at 37 degrees C: K = 3.5 X 10(5) M-1; delta G0 = -7.9 kcal/mol; delta H0 = -6.8 kcal/mol; delta S0 = 3.6 entropy units. The AC-tubulin complex is 1-2 kcal/mol less stable than the colchicine-tubulin complex. The change in fluorescence of AC was employed to measure the kinetics of the association process, and quenching of protein fluorescence was used to measure both association and dissociation. The association process, like that of colchicine, could be resolved into a major fast phase and a minor slow phase. The apparent second order rate constant for the fast phase was found to be 5.2 X 10(4) M-1 S-1 at 37 degrees C, and the activation energy was 13 kcal/mol. This activation energy is 7-11 kcal/mol less than that for the binding of colchicine to tubulin. The difference in activation energies can most easily be rationalized by a mechanism involving a tubulin-induced conformational change in the ligand ( Detrich , H. W., III, Williams, R. C., Jr., Macdonald, T. L., Wilson, L., and Puett , D. (1981) Biochemistry 20, 5999-6005). Such a change would be expected to have a small activation energy in AC because it possesses a freely rotating single bond in place of the B-ring of colchicine.

Baccatin III induces assembly of purified tubulin into long microtubules.

Baccatin III is widely considered to be an inactive derivative of Taxol. We have reexamined its effect on in vitro assembly of tubulin under a variety of conditions. We found baccatin III to be active in all circumstances in which Taxol is active: it assembled GTP-tubulin, GDP-tubulin, and microtubule protein into normal microtubules and stabilized these polymers against cold-induced disassembly. The effect of baccatin III on in vitro microtubule assembly was quantitatively assessed through determination of critical concentrations, which can be used to obtain the apparent equilibrium constants for the addition of tubulin subunits to growing microtubules. The apparent equilibrium constants for the growth reaction for baccatin III-induced GTP-tubulin and GDP-tubulin assembly measured at 37 degrees C were 4.2-4.6-fold less than those measured for Taxol-induced GTP-tubulin and GDP-tubulin assembly. These data indicate that the entire Taxol side chain contributes only about -1 kcal/mol to the apparent standard free energy of microtubule growth at 37 degrees C regardless of the nature of the E site nucleotide. These data also support the idea that the majority of the interactions between Taxol and tubulin that affect this equilibrium occur between the baccatin portion of the molecule and the binding site. We have also observed a structural difference in microtubules formed using baccatin III and Taxol. Baccatin III-induced microtubules were routinely much longer than those assembled by Taxol, even when very high concentrations of baccatin III were employed. One interpretation of these data is that baccatin III and Taxol differ in their abilities to nucleate GTP-tubulin. This difference in activity may have bearing on the large disparity in cytotoxicity of the two molecules.

The syntheses of 16a'-homo-leurosidine and 16a'-homo-vinblastine. Generation of atropisomers.

The synthesis of 16a'-homo-leurosidine was achieved through enantioselective generation of a ring D'-seco-precursor 33 (without requirement of a chiral auxiliary). Its cyclization provided the N(b')-quaternary salt 35 with a configuration corresponding to the atropisomeric form 8a rather than 8b of the target product. On debenzylation, the amine 8a was obtained and found not to isomerize thermally to the anticipated atropisomer 8b (in contrast to its lower homologue, with its formation of natural leurosidine). However, on protonation, a 1:1 mixture of atropisomers of 16a'-homo-leurosidine was obtained. A synthesis of 16a'-homo-vinblastine provided two atropisomers 5a and 5b for the free base at equilibrium (1:2.3 at room temperature in CDCl(3)), with a shift to the major conformer 5b with increasing solvent acidity or decreasing temperature. The synthesis was achieved through a stereoselective inversion of the tertiary hydroxyl function in the enantioselectively generated C-20' progenitor 39.

Synthesis and microtubule binding of fluorescent paclitaxel derivatives.

The preparation of two new fluorescent derivatives of paclitaxel in which the fluorophore is bonded to paclitaxel at the C-10 position is reported. Both analogues, 10-deacetyl-10-(m-aminobenzoyl)paclitaxel (1, BTax) and 10-deacetyl-10-[7-(diethylamino) coumarin-3-carbonyl]paclitaxel (2, CTax) retain good activity as promoters of in vitro tubulin assembly. Microtubule binding enhances the emission intensity of both probes.

Equilibrium studies of a fluorescent paclitaxel derivative binding to microtubules.

A fluorescent derivative of paclitaxel, 3'-N-m-aminobenzamido-3'-N-debenzamidopaclitaxel (N-AB-PT), has been prepared in order to probe paclitaxel-microtubule interactions. Fluorescence spectroscopy was used to quantitatively assess the association of N-AB-PT with microtubules. N-AB-PT was found equipotent with paclitaxel in promoting microtubule polymerization. Paclitaxel and N-AB-PT underwent rapid exchange with each other on microtubules assembled from GTP-, GDP-, and GMPCPP-tubulin. The equilibrium binding parameters for N-AB-PT to microtubules assembled from GTP-tubulin were derived through fluorescence titration. N-AB-PT bound to two types of sites on microtubules (K(d1) = 61 +/- 7.0 nM and K(d2) = 3.3 +/- 0.54 microM). The stoichiometry of each site was less than one ligand per tubulin dimer in the microtubule (n(1) = 0.81 +/- 0.03 and n(2) = 0.44 +/- 0.02). The binding experiments were repeated after exchanging the GTP for GDP or for GMPCPP. It was found that N-AB-PT bound to a single site on microtubules assembled from GDP-tubulin with a dissociation constant of 2.5 +/- 0.29 microM, and that N-AB-PT bound to a single site on microtubules assembled from GMPCPP-tubulin with a dissociation constant of 15 +/- 4.0 nM. It therefore appears that microtubules contain two types of binding sites for paclitaxel and that the binding site affinity for paclitaxel depends on the nucleotide content of tubulin. It has been established that paclitaxel binding does not inhibit GTP hydrolysis and microtubules assembled from GTP-tubulin in the presence of paclitaxel contain almost exclusively GDP at the E-site. We propose that although all the subunits of the microtubule at steady state are the same "GDP-tubulin-paclitaxel", they are formed through two paths: paclitaxel binding to a tubulin subunit before its E-site GTP hydrolysis is of high affinity, and paclitaxel binding to a tubulin subunit containing hydrolyzed GDP at its E-site is of low affinity.