Stability of tubulin polymers formed with dideoxyguanosine nucleotides in the presence and absence of microtubule-associated proteins

We have examined the effects of dilution, Ca2+, reduced temperature, and triphosphate depletion on microtubules formed from purified tubulin, heat-treated microtubule-associated proteins (MAPs), and either GTP, 2',3'-dideoxyguanosine 5'-diphosphate (ddGDP), or 2',3'-dideoxyguanosine 5'-triphosphate (ddGTP). The stability of the polymer formed with tubulin plus ddGTP without MAPs was also examined. In all cases dilution resulted in rapid depolymerization of polymer until a new turbidity plateau was established. These experiments yielded estimates of the critical concentration of tubulin of 0.09 mg/ml with GTP plus MAPs, 0.04 mg/ml with either ddGDP or ddGTP plus MAPs, and 0.07 mg/ml with ddGTP minus MAPs. Addition of CaCl2 to polymer resulted in depolymerization of microtubules formed with either GTP or ddGDP plus MAPs; but both with and without MAPs the polymer formed with ddGTP was stable to Ca2+. The polymer formed with ddGTP minus MAPs was the most cold-labile, major depolymerization occurring at 25 degrees C. With MAPs, microtubules were progressively less cold-labile when formed with GTP, ddGDP, or ddGTP. Depolymerization with GTP was virtually complete at 15 degrees C, with ddGDP at 5 degrees C, and with ddGTP at 0 degrees C. Rapid triphosphate depletion was achieved with phosphofructokinase. GTP-formed tubules were rapidly and completely depolymerized at all GTP concentrations after the enzyme was added to the reaction mixture. Both with and without MAPs polymer formed with ddGTP was progressively more stable upon enzyme addition the higher the initial ddGTP concentration. At specific ddGTP concentrations, however, less depolymerization was observed following enzyme addition if MAPs were present. Microtubules formed with ddGDP plus MAPs were unaffected by phosphofructokinase addition. This comparison of the properties of microtubules formed with MAPs and either ddGDP or ddGTP demonstrates that their stability is enhanced rather than reduced following nucleotide hydrolysis. The greater stability of microtubules formed with ddGTP plus MAPs than of the polymer formed with ddGTP minus MAPs similarly implies substantial enhancement of microtubule stability by the MAPs.

Inhibition of tubulin polymerization with ribose-modified analogs of GDP and GTP. Reduced inhibition with microtubule-associated proteins and magnesium

Inhibitory effects of ribose-modified GDP and GTP analogs on tubulin polymerization were examined to explore nucleotide structural requirements at the exchangeable GTP binding site. With microtubule-associated proteins and Mg2+, GTP-supported polymerization was only modestly inhibited by GDP, and still weaker inhibitory activity was found with two analogs, dGDP and 9-beta-D-arabinofuranosylguanine-5'-diphosphate (araGDP). Omission of Mg2+ significantly enhanced the inhibitory effects of GDP, dGDP and araGDP and resulted in weak inhibition of the reaction by several other GDP analogs. The relative inhibitory activity of the GDP analogs had no discernible relationship to the relative activity of cognate GTP analogs in supporting microtubule-associated protein-dependent polymerization. One GTP analog, 2',3'-dideoxyguanosine 5'-triphosphate (ddGTP), supports polymerization both with and without microtubule-associated proteins. The inhibitory activity of GDP and GDP analogs in ddGTP-supported polymerization was much greater in the absence of microtubule-associated proteins than in their presence; and both reactions were more readily inhibited than was microtubule-associated protein-dependent, GTP-supported polymerization. Microtubule-associated protein-independent, ddGTP-supported polymerization was also potently inhibited by GTP and a number of GTP analogs. GTP was in fact twice as inhibitory as GDP. The relative inhibitory activity of the GTP analogs was comparable to the relative inhibitory activity of the cognate GDP analogs and very different from their relative activity in supporting polymerization.

Microtubule assembly with the guanosine 5'-diphosphate analogue 2',3'-dideoxyguanosine 5'-diphosphate

The GDP analogue 2',2'-dideoxyguanosine 5'-diphosphate (ddGDP) supports efficient tubulin polymerization. Microtubule-associated protein (MAP) dependent microtubule assembly occurs in 0.1 M 2-(N-morpholino)-ethanesulfonate, and sheets of parallel protofilaments are formed in 1.0 M glutamate without MAPs. The nucleotide is bound to tubulin in the course of polymerization, presumably in the exchangeable GTP site. The ddGDP is not hydrolyzed, however, and is completely stable in the reaction mixture. Nor was the nonexchangeable GTP bound to tubulin hydrolyzed in ddGDP-supported polymerization: equivalent amounts of GTP remained associated with polymerized tubulin after polymerization with either ddGDP or GTP. Higher concentrations of ddGDP than GTP were required under all conditions. Nevertheless, under optimum conditions for the ddGDP-supported reaction, polymerization began with a shorter lag period and both the rate and extent of polymerization were greater with ddGDP than with GTP. The MAP-dependent reaction with ddGDP is temperature dependent, cold reversible, and inhibited by calcium and antimitotic drugs. It differs from the GTP-supported reaction in being most vigorous at minimal Mg2+ concentrations and exquisitely sensitive to GDP inhibition.

Tubulin-dependent biochemical assay for the antineoplastic agent taxol and application to measurement of the drug in serum

A biochemical assay for taxol with sensitivity to 0.1 microM has been developed. Taxol-dependent formation of tubulin polymers occurs at 37 degrees C in 1.0 M glutamate in the absence of GTP. These polymers are cold-stable and hydrolyze GTP at 0 degrees C, whereas tubulin alone will not hydrolyze the nucleotide in the cold. This assay has been used to follow rabbit serum levels of taxol injected iv. Although the drug appears to be almost totally protein-bound, it is nevertheless rapidly cleared from serum. The apparent alpha-phase and beta-phase half-lives after iv bolus administration in one rabbit are 2.7 and 42 mins, respectively.

Effects of organic acids on tubulin polymerization and associated guanosine 5'-triphosphate hydrolysis

We have examined the effects of a number of organic anions, which stabilize tubulin, on tubulin polymerization, associated GTP hydrolysis, and polymer morphology. While microtubule-associated proteins, as well as glycerol, induced formation of typical microtubules in a reaction coupled to GTP hydrolysis at an initial 1:1 stoichiometry, the organic anions had varying effects. Only 2-(N-morpholino)ethanesulfonate induced formation of structures with the morphology of microtubules. With glutamate, fructose 1,6-bisphosphate, piperazine-N-N'-bis(2-ethanesulfonate), glutarate, and glucose 1-phosphate, the predominant structures formed were sheets of parallel protofilaments rather than microtubules. Creatine phosphate induced the formation of clusters of rings. GTP hydrolysis was closely coupled to polymerization only with glutamate. With creatine phosphate, there was minimal GTP hydrolysis. With all other organic anions, GTP hydrolysis substantially exceeded polymerization at all time points, with the onset of hydrolysis significantly preceding the onset of turbidity development. Nevertheless, the rate of GTP hydrolysis was a sigmoidal function of tubulin concentration under all conditions examined, suggesting that tubulin-tubulin interactions are required for hydrolysis. All anion-induced reactions were temperature dependent and cold reversible, but only the creatine phosphate induced reaction was not inhibited by GDP, CA2+, or colchicine and did not require GTP.

Effects of inhibitors of tubulin polymerization on GTP hydrolysis

The effects of a number of antimitotic drugs on the GTPase activity of tubulin were examined. The previously reported stimulation with colchicine and inhibition with podophyllotoxin and vinblastine wee confirmed. Maytansine, which competes with vinblastine in binding to tubulin, was comparable to the latter in inhibiting GTP hydrolysis. Nocodazole, which competes with colchicine in binding to tubulin, was significantly superior to colchicine in enhancing GTP hydrolysis. This superiority arose from the more rapid bindng of nocodazole to tubulin, as the two drugs had comparable activity when drug and tubulin were preincubated prior to the addition of GTP. Both colchicine and podophyllotoxin contain a trimethoxybenzene ring, while the closest structural analogy of nocodazole to colchicine includes the trimethoxybenzene ring. To explore this apparent paradox, we examined a number of simpler colchicine analogs for their effects on tubulin-dependent GTP hydrolysis. While tropolone was without effect, 3,4,5-trimethoxybenzaldehyde and 2,3,4-trimethoxybenzaldehyde stimulated the reaction. We therefore conclude that the trimethoxybenzene ring of colchicine is primarily responsible for the drug's stimulation of the GTPase activity of tubulin and that the inhibitory effect of podophyllotoxin must derive from the latter's tetrahydronaphthol moiety.

Stabilization of the colchicine-binding activity of tubulin by organic acids

A number of carboxylic acids and organic phosphates were found to be highly effective in stabilizing the colchicine-binding activity of calf brain tubulin. The most active were glutamate, glutarate, delta-aminovalerate, glucose 1-phosphate, glucose 6-phosphate, fructose 1,6-(bis)phosphate, creatine phosphate and 6-phosphogluconate Maximum effects occurred at high concentrations. Combinations of agents were also examined, and the most effective mixture for stabilizing tubulin found thus far was the combination of 1.0 M glutamate, 100 mM glucose 1-phosphate, 1 mM GTP and 0.5 mg/ml of albumin. No loss of activity occurred over 48 h at 37 degrees C with tubulin was present at a concentration of 100 microgram/ml.

Interaction of tubulin with ribose-modified analogs of GTP and GDP: evidence for two mutually exclusive exchangeable nucleotide binding sites

Interactions of tubulin with a number of guanine nucleotides modified at the 2' and 3' ribose hydroxyls were examined. Deoxy analogs of GTP were equal or superior to GTP in supporting tubulin polymerization, but analogs bearing either methyl or phosphate groups on the hydroxyls had significantly reduced ability to support polymerization. These substituted GTP analogs were hydrolyzed at the 5'-gamma-phosphate position, although less rapidly than GTP, at rates exceeding those of polymerization. GTP hydrolysis, however, was closely coupled to polymerization. Moreover, the partially active GTP analogs were not effective inhibitors of GTP-dependent polymerization. These data indicate that the substituted GTP analogs have reduced affinity for tubulin at the exchangeable site because of steric factors. No deoxy or substituted GDP analog was as effective as GDP itself in inhibiting GTP-supported tubulin polymerization. Furthermore, there was no apparent relationship between the ability of nucleoside 5'-triphosphates to support polymerization and that of nucleoside 5'-diphosphates to inhibit the reaction. These findings suggest that GTP and GDP may actually bind to different, mutually exclusive sites rather than to a single exchangeable site.

Binding of maytansine to tubulin: competition with other mitotic inhibitors

The effect of maytansine on the binding of [3H]vinblastine and [3H]colchicine to tubulin was examined by Sephadex gel filtration column chromatography. When varying concentrations of maytansine were employed, competition between vinblastine and maytansine was observed at a vinblastine to maytansine ratio of 1:10 and 1:100, while a colchicine to maytansine ratio of 1:100 did not affect the binding of colchicine to tubulin. These results confirm earlier findings with a DEAE-cellulose disc paper assay and a tubulin polymerization assay that maytansine shares a common binding site with vinblastine and support the view that tubulin has at least two drug binding sites.

Antigenic analyses of two axenized strains of Entamoeba histolytica by two-dimensional immunoelectrophoresis

Antigens in extracts of two strains of Entamoeba histolytica (HT-31 and HK-9) were studied by two-dimensional immunoelectrophoresis (2D-IEP) and 32 and 20 different antigenic components were detected, respectively. Cross-reactions between heterologous systems showed a large number of shared antigens between the two strains. Extracts were fractionated by Sephadex G-200 chromatography. Of the fractions obtained, fraction II of both strains was found to have the most reactive antigenic components. Precipitin patterns of fraction II from HT-31 and HK-9 with sera from patients showed similar 2D-IEP profiles and suggested that in this fraction antigens common to both strains exist which elicit antibody in individuals with E. histolytica infections.

Reliability of goniometric measurements

This study determined the intratester and intertester variability and reliability of goniometric measurements taken by four physical therapists on upper and lower extremity motions of normal male subjects. The same subjects were measured once weekly for four weeks by testers with varied experience in goniometry. Data were analyzed by analyses of variance with repeated measures. Intratester variation for all measurements was less than intertester variation. Further, intertester variation was less for the three upper extremity motions than for those of the lower extremity. These findings indicate the necessity for using the same tester when effects of treatment are evaluated. When the same tester measures the same movement, increases in joint motion of at least three to four degrees determine improvement for either the upper or lower extremity. When more than one tester, however, measures the same movement, increases in joint motion should exceed five degrees for the upper extremity and six degrees for the lower extremity to determine improvement.

Comparison of some methods for the detection of antigen-antibody reactions in amoebiasis

The indirect hemagglutination test (IHA) and counterimmuno-electrophoresis (CIE) were used to test for antibodies to Entamoeba histolytica in human sera using antigens from axenic cultures of the HK9 strain. Correlation between the test was excellent with most sera from patients with amoebiasis demonstrating precipitin lines by CIE at IHA titers considered diagnostically positive, larger than or equal to 1:128. Precipitin reactions with positive sera were also compared by CIE, immunoelectrophoresis (IEP) and two-dimensional electrophoresis is (2D-IEP). The greatest number of antigen-antibody components were demonstrated by 2D-IEP. Further studies utilizing 2D-IEP should be of value in the antigenic analysis of E. histolytica.