Noradrenaline-evoked cation conductance recorded with the nystatin whole-cell method in rabbit portal vein cells

1. Noradrenaline-evoked currents were studied with the perforated patch technique (nystatin in the patch pipette) in freshly dispersed rabbit portal vein cells. 2. With potassium-containing solutions noradrenaline produced an inward current at -50 mV and an outward current (IK(Ca] at 0 mV. In potassium-free conditions noradrenaline elicited two distinct membrane currents which could be differentiated according to their sensitivity to the presence of caffeine in the bathing solution. One current was blocked whereas the second response was unaffected by caffeine. 3. The reversal potential (Er = -2.0 mV) of the caffeine-sensitive current was altered when external chloride was replaced with more permeant anions but Er was not changed when external sodium was replaced by Tris. Therefore the caffeine-sensitive current appears to be a calcium-activated chloride current (ICl(Ca]. 4. Er (+6.0 mV) of the caffeine-insensitive current was not altered by anion substitution but was changed when external sodium was replaced by Tris and barium ions. Thus the caffeine-insensitive conductance is a non-selective cation current (Icat). 5. When external NaCl was replaced by BaCl2, Er was shifted to more positive potentials which suggests that the cation conductance is more permeable to barium than to sodium. Icat was not affected by 10(-6) M-nifedipine. 6. The steady-state current-voltage relationship was linear between -50 and +50 mV for ICl(Ca) but the cation conductance mechanism displayed pronounced inward rectification and little outward current flowed across the membrane at positive potentials. 7. Caffeine (which releases calcium from internal stores) and the calcium ionophore ionomycin, which are expected to increase intracellular calcium concentration, evoked ICl(Ca) but not Icat. Thus ICl(Ca) but not Icat can be activated directly by an increase in intracellular calcium concentration. 8. When calcium was removed from the bathing solution the amplitude of Icat was not altered at early times in Ca(2+)-free conditions but was abolished after 10 min. Icat was readily activated by noradrenaline in the presence of ionomycin, which inactivated ICl(Ca). A permissive role for calcium in the generation of Icat is suggested. 9. The times between the application of noradrenaline and the onset of the chloride and cationic conductances were similar (0.75 and 0.9 s respectively). In contrast the rise time (3.9 s) and half-decay time (11 s) of Icat were much longer than the corresponding values of ICl(Ca) (respectively 1.9 and 2 s). In high-barium solution noradrenaline sometimes evoked a response that lasted for up to 10 min.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of central RAS on one-kidney Grollman hypertension in rats and its mechanism

The effect of central renin-angiotensin system (RAS) on one-kidney Grollman hypertension during the maintaining phase and its mechanism were investigated in rats. The arterial blood pressure (ABP) and the content of angiotensin II (A II) and norepinephrine (NE) in brain regions was measured respectively. 4 weeks after operation the ABP was elevated significantly, and it sustained at high level 8 weeks post-operatively. However, ABP in the control group underwent no significant changes at the same period. The A II and the NE content in the brain regions of the operated group were significantly higher than in those of the age-matched control group. During the maintaining phase of hypertension captopril (150 micrograms/10 microliters) was injected into the lateral cerebroventricle at 0.5 h, 1.0 h and 1.5 h respectively, and ABP and content of A II and NE were determined at the corresponding time. The results showed that the above three parameters decreased consistently at 0.5 h and 1.0 h, and increased gradually at 1.5 h, suggesting that the central RAS might play an important role in the maintaining phase of one-kidney Grollman hypertension in rats.

Lumbar intrathecal administration of naloxone antagonizes analgesia produced by electrical stimulation of the hypothalamic arcuate nucleus in pentobarbital-anesthetized rats

In lightly pentobarbital-anesthetized and acutely-prepared rats, electrical stimulation within the arcuate nucleus of the hypothalamus consistently inhibited the tail-flick responses to noxious heating of the tail. The opioid receptor antagonist, naloxone hydrochloride applied intrathecally at the lumbar level, at dose of 20 micrograms, reversed this inhibition without affecting the baseline pain threshold. The same dose of naloxone, applied to the cervical subarachnoid space, had no effect on the inhibitory modulation by the arcuate nucleus. Naloxone, at doses 2- to 4-fold greater than the intrathecal dose, did not modify the suppression of the tail-flick, when given systemically. With the doses ranging from 5 to 40 micrograms, naloxone showed a dose-dependent blockade of the inhibition produced by stimulation of the arcuate nucleus. These results indicate that an endogenous opioid system is most likely involved in the descending inhibition of spinal nociceptive reflexes, resulting from stimulation of the arcuate nucleus of the hypothalamus.

Design and testing of external fixator bone screws

In external fixation, bone screw loosening still presents a major clinical problem. For this study, the design factors influencing the mechanics of the bone-screw interface were analysed and various experimental screws designed with the intention of maximizing the strength and stiffness of the inserted screw. Push-in, pull-out and bending tests were then carried out on the three experimental screws, and on two commercially available screws in both a synthetic material and in cadaveric bone; photoelastic tests on different screw threadforms were also performed. The results of the push-in and pull-out tests indicate that both the screw threadform and cutting head have a significant effect on the holding strength of the screw. The photoelastic tests show that most of the applied load is distributed over the first few threads closest to the load, and that the area between the thread crests is subjected to high shear stresses.

Characteristics of chloride currents activated by noradrenaline in rabbit ear artery cells

1. Responses to noradrenaline were studied in isolated rabbit ear artery cells with the nystatin method of whole-cell patch-clamp recording. With this technique it was possible to obtain reproducible responses to noradrenaline which was not possible with traditional whole-cell recording. 2. With NaCl as the major constituent of the bathing solution (potassium-free pipette and external solutions) the reversal potential (Er) of the noradrenaline-evoked current was about 0 mV. When external chloride was replaced by thiocyanate, iodide, nitrate and bromide, Er was shifted to more negative potentials which indicates that a chloride conductance increase contributes to the current activated by noradrenaline. 3. When sodium was substituted by Tris, N-methyl-D-glucamine, choline or barium, Er of the noradrenaline-evoked current did not alter. This result suggests that a cation conductance is not implicated in the response to noradrenaline recorded with the nystatin method of whole-cell recording. 4. The chloride current activated by noradrenaline was blocked by the selective alpha 1-adrenoceptor antagonist prazosin but was not affected by the alpha 2-adrenoceptor antagonist yohimbine. 5. When cells were exposed to zero calcium bathing solutions the amplitude of the current elicited by noradrenaline was unaffected when measured within 1-2 min in zero calcium conditions. Continued exposure to 0 Ca + 1 mM-EGTA solution reversibly abolished the chloride current to noradrenaline. 6. In the presence of caffeine, which releases Ca2+ from internal stores and itself induced an inward current (at a holding potential of -50 mV), noradrenaline did not elicit a current. These data suggest that the chloride current evoked by noradrenaline results from an increase in intracellular concentration of calcium derived from internal stores. 7. The chloride channel blocking agents 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS; 0.5 mM) and furosemide (0.5 mM) produced partial reduction of the noradrenaline-evoked chloride current whereas 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulphonic acid (SITS), anthracene-9-carboxylic acid (A-9-C) and picrotoxin were ineffective in concentrations of up to 0.5 mM. However DIDS and furosemide were non-selective blockers as both agents were more effective against the adenosine triphosphate-induced cation current.

Interactions at the nucleic acid binding site of the avian retroviral nucleocapsid protein: studies utilizing the fluorescent probe 4,4'-bis(phenylamino)(1,1'-binaphthalene)-5,5'-disulfonic acid

The structural and functional properties of the nucleocapsid (NC) protein of the avian myeloblastosis virus were examined by steady-state fluorescence and fluorescence anisotropy measurements of the complex between the NC and the extrinsic fluorophore 4,4'-bis(phenylamino)(1,1'-binaphthalene)-5,5'-disulfonic acid (bis-ANS). The intrinsic fluorescence of bis-ANS is enhanced many fold upon forming a complex with the NC. Between 2 and 10 molecules of bis-ANS bind strongly to the NC, with an overall Kd of less than 10(-6) M. The emission of bis-ANS in the complex can also be induced by excitation at 298 nm, indicating that energy is transferred from Trp 80, the sole tryptophan in the NC protein, to bis-ANS. The energy transferred between the Trp 80 and bis-ANS was analyzed to yield a calculated distance of separation between these fluorophores of 28 +/- 3 A; thus, Trp 80 is well removed from the nearest bound bis-ANS. The fluorescence emission of bis-ANS in the NC.bis-ANS complex is efficiently quenched by added salts and by poly(A), suggesting that salt (presumably anions), nucleic acid, and bis-ANS bind to the same, positively charged region on the NC protein. A site size of six nucleotides was determined for nucleic acid binding to the NC protein, with an estimated Kd of less than 10(-6) M. Salt (anion) binding is strong, but nonspecific, with a Kapp of 4 mM, raising the possibility that anion binding to the NC protein might regulate the interaction of the NC with viral RNA inside the host cell.

Analgesic electrical stimulation of the hypothalamic arcuate nucleus: tolerance and its cross-tolerance to 2 Hz or 100 Hz electroacupuncture

Focal electrical stimulation of the arcuate nucleus of the hypothalamus (ARH) for 5 min (1 session) produced a marked elevation of tail flick latency (TFL) to noxious heat in the pentobarbital-anesthetized rat. Repeated stimulation for a total of 11 sessions at 30 min intervals resulted in a gradual decline in the hypoalgesic action, and this tolerance may last for 7 days. Tolerance to the ARH analgesic stimulation reduced the analgesia produced by low (2 Hz) but not high (100 Hz) frequency electroacupuncture (EA); and tolerance to low frequency EA analgesia attenuated the ARH stimulation-produced analgesia without affecting high frequency EA analgesia. Alternatively, rats tolerant to high-frequency EA analgesia were still sensitive to either the ARH or low-frequency EA stimulation. These results suggest that the ARH stimulation and low-frequency EA administration produced analgesia via a common neural mechanism, supporting our hypothesis put forward previously that the ARH plays an important role in mediating low- but not high-frequency EA analgesia.

New Taxanes from Taxus chinensis

Two new taxane diterpenes, decinnamoyltaxagifine ( 3) and decinnanioyltaxagifine acetate ( 2), have been isolated from the leaves and stems of TAXUS CHINENSIS. Their structures were elucidated by spectroscopic methods. Their (13)C-NMR chemical shifts were assigned.

[Effect of disintegrating method on quantity extracted and constituents of essential oil in fructus Aurantii]

The quantity extracted, constituents and their contents in essential oil from Fructus Aurantii were obviously influenced by the method of disintegration. Among stamp breaking in water, crusher-run and miling, stamp breaking in water is the best method. The advantage of this method lies in higher percentage recovery of the oil and smaller loss in volatilization.

[Relationship between mucin histochemical changes at the resection margin and rectal cancer recurrence after anterior resection]

The correlation between the presence of sialomucins at the resection margin identified by high iron diamine-alcian blue (HID-AB) histochemical stain and the subsequent anastomotic recurrence of the tumor was studied retrospectively in 64 rectal cancer patients at the stage of Dukes' B who underwent radical anterior resection. It was found that fourteen of 27 patients (51.9%) with a sialomucins predominant pattern at either resection margin suffered from tumour recurrence compared with 4 of 37 patients (10.8%) with a sulphomucin predominant pattern (P less than 0.001). Abnormal mucus production at the resection margin in patients treated for rectal carcinoma appears to identify those with a higher risk of anastomotic recurrence.

The arcuate nucleus of hypothalamus mediates low but not high frequency electroacupuncture analgesia in rats

Electrolytic, kainic acid or sham lesions were made in the arcuate nucleus of the hypothalamus (ARH) in female Wistar rats to investigate the putative role of the ARH in the organization of low (2 Hz) or high (100 Hz) frequency electroacupuncture (EA) analgesia. Both electrolytic and chemical lesions lead to an almost total suppression of the low frequency EA analgesia as measured 4 and 6 days following the surgical intervention, leaving high frequency EA analgesia unaffected. In sham-operated animals, the antinociceptive effect induced by low or high frequency EA was essentially intact. These data indicate that neurones of the ARH most likely play an important role in mediating low, but not high frequency EA analgesia.

Diencephalon as a cardinal neural structure for mediating 2 Hz- but not 100 Hz-electroacupuncture-induced tail flick reflex suppression

Transections at two levels of the forebrain were undertaken in the rat to investigate the neural structures indispensable for the mediation of tail flick reflex suppression (TFRS) induced by low (2 Hz) and high (100 Hz) frequency electroacupuncture (EA) stimulation. Removal of the telencephalic structures did not affect TFRS produced by high frequency EA, although a mild and temporal attenuation was observed for low frequency EA-induced TFRS. Ablation of the whole forebrain (the telencephalon and diencephalon) resulted in a total abolishment of 2-Hz EA effect as measured 5, 24 and 72 h after surgery. In the meantime there was a moderate attenuation (-32.8%, P less than 0.05) of 100-Hz EA effect, which appeared 5 h after the operation and recovered after 24 h. The results indicate that (1) high frequency EA effect persisted in animals devoid of the whole forebrain structures; (2) an intact diencephalon is indispensable for the neural circuitry controlling low frequency EA-induced TFRS.

[Observation of the outcome of 112 cases with Graves' disease treated by short-term antithyroid drug therapy for one year]

One hundred and twelve new cases with Graves' disease were treated by tapazole for 6 months and followed-up for another 12 months in a prospective study. One hundred and eleven cases completed the whole course of study, and only one failed to be followed up. The results of the follow-up for 12 mon showed that the remission rate was 41.4% (46/111) and relapse rate was 58.6% (65/111). The present study indicated that the remission or relapse was related with serum levels of T3 before treatment, shrank goiter and disappearance of goiter bruit during the treatment as well as thyroid suppression rate, but not related to sex, age, period of illness before therapy or severity of the disease. It is suggested that a 6-mon antithyroid drug therapy instead of traditional long term therapy may be suitable for those patients who are with a reduced goiter and disappeared goiter bruit during the antithyroid therapy, and suppressible thyroid up-take by T3 and whose serum levels of T3 before treatment are not very high. So the patients may benefit from the short-term therapy. They are not only able to obtain a prolonged remission but also save time and money.

[Analgesic and sedative effects of the Chinese drug rhizoma Paridis]

The analgesic and sedative actions of Chinese drug Rhizoma Paridis are reported. All of the 6 experimented species and varieties in common use are obviously effective. Among them Paris polyphylla var. chinensis, P. polyphylla var. yunnanensis are stronger in analgesic action. Sedative action of P. fargesii, P. polyphylla var. chinensis, P. thibetica is also strong. In addition, pariphyllin A and gracillin were also used in the experiment.

Sialomucin at the resection margins and likelihood of local recurrence of carcinoma of the rectum after anterior resection

The histochemical changes in mucin at the postoperative resection margins in carcinoma of the upper and middle third of the rectum after anterior resection were studied retrospectively in 52 patients. The sialomucinous change was most likely to occur at the intestinal margins of those patients who had tumour recurrence (P less than 0.001). The histological types of tumour, degree of differentiation, Dukes' stages and their distance from the anal verge were virtually the same in patients with and without tumour recurrence. By using high iron diamine-alcian blue staining technique we identified patients with higher risk conditions of tumour recurrence after anterior resection. This technique should increase the likelihood of a cure for patients with recurrent carcinoma of the rectum after anterior resection.

Naloxone-reversible analgesia produced by microstimulation of the arcuate nucleus of the hypothalamus in pentobarbital-anesthetized rats

Suppression of the tail flick response to noxious heat and paw withdrawal response to noxious pressure were produced by electrical stimulation of arcuate nucleus of the hypothalamus (ARH) in pentobarbital anesthetized rats. Systemic administration of naloxone (2 mg/kg) greatly antagonized the ARH stimulation-produced inhibition of both algesic reflexes. The thresholds of stimulation for inhibition of two spinal nociceptive reflexes in the lightly anesthetized state were not significantly different from the thresholds of stimulation at the same ARH sites in the awake state in the same animals. These findings provide evidence establishing the (1) usefulness of the anesthetized rat model for investigation of antinociceptive mechanisms; (2) the involvement of endogenous opioid mechanisms in mediating ARH stimulation-produced analgesia.

Chronotropic Effect of the Methanolic Extracts of the Plants of the Paris Species and Steroidal Glycosides Isolated from P. vietnamensis on Spontaneous Beating of Myocardial Cells1

Rhizomes of five identified plants of the PARIS species, Liliaceae, and Rhizoma Paridis which are sold as a crude drug named "Zao Xiu", "Qiyeyizhihua" or other names in nine different markets in China were tested for their effects on cultured cardiomyocytes. In the standard medium, eight methanol extracts out of sixteen at a concentration of 0.2 mg/ml stopped the spontaneous beating of myocardial cell sheets, but these extracts significantly increased the beating rate when the concentration was reduced to one half. In the culture medium with a low calcium concentration, 0.5 mM, the beating rate of the cells decreased to about 60% of that of the control in the standard medium. The addition of five of the extracts to the low calcium medium at a concentration of 0.1 mg/ml caused a stop of cell beating, but the other extracts increased beating rate at least by 10%. Three steroidal glycosides isolated from the rhizomes of P. VIETNAMENSIS (Takht.) H. Li also stimulated cell beating. Among them, diosgenin-3- O-alpha- L-rhamnopyranosyl-(1-->2) - (alpha- L-arabinofuranosyl-(1-->4))- D-glucopranoside (compound 1) was the most effective stimulant for cell beating as well as calcium uptake by the myocardial cells.

Multipoint linkage mapping of the Xq25-q26 region in a family affected by the X-linked lymphoproliferative syndrome

We have performed, in a large Swiss family, a study of linkage between various DNA markers in the Xq24-27 region and the locus for the X-linked lymphoproliferative syndrome (XLP). Our results indicated that the marker DXS37 in Xq25-q26 is genetically linked to the XLP syndrome. The multipoint linkage analysis showed that the disease locus is distal to DXS11, but proximal to the hypoxanthine phosphoribosyl-transferase gene (HPRT).

[Experimental studies on the significance of processing Eucommia ulmoides Oliv]

The inhibitory action on isolated uterus of middle-pregnancy mice have been compared in Eucommia ulmoides and its salt water processed product. It has been found that after processing the inhibitory action on the automatic contraction of isolated uterus, the antagonism on spasmodis contraction caused by acetylcholine are increased, but the antagonism on spasmodis contraction caused by pituitarium posterius and the Pb content are decreased.

Clinical value of N-myc oncogene amplification in 52 patients with neuroblastoma included in recent therapeutic protocols

Southern blot analysis of neuroblastoma (NB) cell DNA from 52 patients (58 samples) allowed the detection of an N-myc amplification on three of the 13 BM samples and three of 13 tumor samples at diagnosis, on two of 17 tumor samples taken after induction therapy, on three of seven BM samples and two of the five local tumor samples taken after relapse. N-myc was amplified in two of the 15 patients with stage I to III NB and in 10 of the 36 patients with stage IV neuroblastoma over 1 year of age. Conclusions from the analysis are as follows: first tumor samples obtained on previously treated patients are histologically modified and detection of N-myc amplification is not accurate; consequently N-myc amplification must be defined at diagnosis. Surgical biopsies of the primary tumors can, however, be delayed and malignant cells obtained by ultrasound-guided punctures or analyzed on bone marrow samples if they represent more than 50% of the total population. Second, any attempt to define the prognostic value must be performed on a group of patients treated with a new aggressive protocol of chemotherapy; it will require a multiparametric analysis including, in particular, results of an extensive clinical staging at diagnosis and histological criteria.

[Comparison of some trace element contents of Eucommia ulmoides Oliv. and its processed products]

A comparison of the contents of 7 trace elements (Zn, Mn, Cu, Fe, Ca, P, Pb) in Eucommia ulmoides and its processed products has been made, and the significance of processing Eucommia ulmoides is discussed in light of the experimental results.

Permeability of the mitochondrial outer membrane to organic cations

Inhibition of mitochondrial respiration by hydrophobic fluorescent dyes (Rhodamine 6G, Safranine O, Pyronine B) is much less potentiated by digitonin-lysis of the outer membrane than that by polyamines or adriamycin. This situation may be explained by impermeability of the anion-selective channels in the outer mitochondrial membrane to large cations and by the ability of hydrophobic (but not polar or amphipathic) ions to directly permeate lipid bilayers.

Characterization of a phospholipase A2 in hamster lung and in vitro and in vivo effects of bleomycin on this enzyme

Phospholipase A2 (PLA2) activity from adult hamster lung was characterized using L-alpha-1-palmitoyl-2-arachidonyl-[arachidonyl-1-14C]-phosphatidylcholine as the substrate. The released [14C]-arachidonic acid was separated by TLC. The enzyme activity increased with increasing incubation time (0-120 minutes), calcium ion concentration (0-25.0 mM) and protein (0-2.0 mg). The optimum pH was 8.0. Deoxycholate had a concentration dependent (0.1 to 0.5 mM) inhibitory effect on the activity. PLA2 specific activity was the highest in mitochondrial fraction. PLA2 activity following incubation with bleomycin was increased in a dose related fashion. In vivo study showed that both PLA2 activity and collagen content in hamster lung were significantly elevated at 14 days followed intratracheal instillation of bleomycin. The activation of PLA2 may play an important role in bleomycin-induced pulmonary toxicity.

High dose-rate afterloading in the treatment of cervical cancer of the uterus

From January 1981 to December 1983, 380 patients with cervical carcinoma of uterus were treated with high dose-rate intracavitary afterloading therapy (Ralstron-20B) and 60Co external irradiation. All of these cases have been followed over 3-5 years. The 5-year survival rates for Stages I, II, III, and IV were 100%, 81.5%, 73.9%, and 0% respectively. The over all 5-year survival rate was 79.1%. The results were comparable to that of conventional radium therapy in 1980 (5-year survival rate for radium group: Stage I 50%, Stage II 72.2%, Stage III 52.2%, Stage IV 0%). The moderate and severe rectal reactions in this series were 11.8% and 1.6% (radium group moderate 6.0%, severe 2.2%), which were higher than that of radium group. The incidence of bladder reaction was 7.1%. The problems of high dose-rate intracavitary therapy and the combination therapy with intracavitary and external irradiation are discussed in this paper.

Inhibition of baroreflex following microinjection of GABA or morphine into the nucleus tractus solitarii in rabbits

The possibility that putative transmitters may influence the aortic nerve stimulation-produced bradycardia and depressor responses was examined in urethane- and chloralose-anesthetized, paralyzed and artificially ventilated rabbits. The ipsilateral microinjection of gamma-aminobutyric acid (GABA, 2-4 micrograms) or morphine hydrochloride (2-4 micrograms) into the nucleus tractus solitarii (NTS) area could partially block the evoked bradycardia and depressor responses produced by stimulation of the aortic nerve without influencing the basal blood pressure and heart rate. This blocking effect of either GABA or morphine was dose-related. Pretreatment with GABA receptor antagonist bicuculline methiodide (0.15-0.20 micrograms) and opiate receptor antagonist naloxone hydrochloride (1-2 micrograms) into the same medullary area completely abolished the effect of GABA and morphine, respectively. Application of bicuculline also greatly antagonized the effect of morphine, but the blocking effect of GABA on the evoked bradycardia and depressor responses still existed following the pretreatment of naloxone. These results indicate that GABAergic and opiate systems present at the NTS exert an inhibitory influence on the evoked baroreflexes and inhibitory effect of the latter may be related to the activation of GABAergic receptor in this nucleus.

Stimulation of the ventrolateral medulla inhibits the baroreceptor input to the nucleus tractus solitarius

Extracellular recording experiments were done in urethane- and chloralose-anaesthetized, paralyzed and artificially ventilated rabbits. Thirty-one baroreceptor-sensitive neurones were identified in the region of the nucleus tractus solitarius (NTS) by their excitatory responses to stimulation of the ipsilateral aortic nerve. A conditioning stimulus delivered to the ipsilateral or contralateral rostral ventrolateral medulla (rVLM) inhibited the excitatory responses of 80.0% (19 out of 24) neurones to an aortic test stimulus as early as 3 ms and extending as long as 400 ms after conditioning. The same inhibitory effect was also observed by application of excitant amino acid DL-homocysteic acid (0.2 M, 100 nl) into the ipsi- or contralateral rVLM area in 6 units. In 5 units inhibited by rVLM stimulation, evoked discharges were inhibited by prolonged electrical stimulation of the superficial peroneal nerve (SP) with low intensity and low frequency (0.1-0.3 mA and 5-10 Hz). These results provide the electrophysiological evidence for the suppressing effect of the rVLM on the excitatory responses of NTS neurones to baroreceptor afferent stimulation.

Interactions between Escherichia coli arginyl-tRNA synthetase and its substrates

Interactions between Escherichia coli arginyl-tRNA synthetase and its substrates were extensively studied and distinctly demonstrated. Various approaches such as equilibrium dialysis, fluorescence titration, and substrate protection against heat inactivation of the enzyme were used for these studies. In the absence of other substrates, the equilibrium dissociation constants for arginine, ATP, and the cognate tRNA were about 70 microM, 0.85 mM, and 0.45 microM, respectively, at pH 7.5, in Tris buffer. The binding of arginine to the enzyme was affected neither by the presence of tRNA nor by the presence of ATP but was considerably enhanced when ATP and tRNA were both present at saturating concentrations. The dissociation constant in this case (about 16 microM) was very close to the Km (12 microM) for arginine during aminoacylation. The binding of ATP (the equilibrium dissociation constant KD approximately 0.85 mM) was not affected by the presence of arginine but was depressed in the presence of tRNA (KD became 3 mM). Arginyl-tRNA showed a dissociation constant of (4-5) X 10(-7) M which was not affected by the presence of a single other substrate. Possible explanations for the high Km for tRNA in the aminoacylation are discussed. Our results indicated pronounced interactions between substrates mediated by the enzyme under catalytic conditions. Periodate oxidation did not alter the tRNA binding to the enzyme. The oxidized tRNA still afforded protection against heat inactivation of the enzyme.