Effects of cisplatin and thiosulfate upon auditory brainstem responses of guinea pigs

Two side effects which limit the use of cisplatin in cancer chemotherapy are severe nephrotoxicity and ototoxicity. The concurrent administration of sodium thiosulfate with cisplatin reportedly protects from cisplatin nephrotoxicity, however, protection from ototoxicity has not been documented. The purpose of this study was to examine the efficacy of using thiosulfate to ameliorate the ototoxic effects of cisplatin. Toward this end, the effects of cisplatin alone, cisplatin administered concurrently with sodium thiosulfate (CIS/THIO), and sodium thiosulfate alone on the auditory brainstem response (ABR) of guinea pigs were compared. ABR waveforms, comparing latencies, amplitudes and response thresholds, were monitored before, immediately after, and 30 days post treatment. Sodium thiosulfate administered with cisplatin (CIS/THIO) consistently protected animals from hearing loss and surprisingly yielded significant increases in amplitude when compared to baseline and saline controls. However, ABRs of CIS/THIO animals returned toward baseline values after 30 days.

The differential functional stability of various forms of bovine liver rhodanese

Bovine liver rhodanese (thiosulfate:cyanide sulfurtransferase, EC 2.8.1.1) was prepared in dilute solutions and subjected to conditions that led to a time-dependent loss of enzyme activity. The rate of this activity loss was found to be dependent upon the sulfur substitution state of the enzyme, and the presence or absence of the substrates, thiosulfate and cyanide. In the absence of excess substrates, free enzyme (E), and the covalent intermediate form of the enzyme bearing a divalent sulfur atom in the active site (ES), are of approximately equal functional stability. In comparison, E, in the presence of excess cyanide, was markedly more labile, while ES, supported by 10-50 mM thiosulfate, showed no significant loss of activity under any of the conditions tested. All the enzyme solutions were shown to be losing assayable protein from solution. However, it was demonstrated that, for rhodanese in the E form, the amount of protein lost was insufficient to account for the activity lost, and a marked decline in specific activity was observed. Enzyme in the ES form, whether supported by additional thiosulfate or not, did not decline in the specific activity, though comparable protein loss did occur from these solutions. Intrinsic fluorescence measurements of rhodanese in the ES form, before and after removal of the persulfide sulfur through the addition of cyanide, indicated that loss of enzymic activity was not accompanied by loss of the bound sulfur atom. Therefore, the stabilizing effect observed with thiosulfate could not be explained simply by its ability to maintain enzyme in the sulfur-substituted state. Since the concentration of thiosulfate employed in these experiments was insufficient to maintain all the enzyme in ES.S2O3 form, thiosulfate was acting as a chemical reagent rather than a substrate in stabilizing enzyme activity.

Alpha-adrenoceptor occupancy by N,N-dimethyl-2-bromo-2-phenethylamine hydrobromide (DMPEA) in rat vas deferens

The effects of N,N-dimethyl-2-bromo-2-phenethylamine hydrobromide (DMPEA) on alpha 1- and alpha 2-adrenoceptors were examined in the isolated rat vas deferens. The active species of DMPEA was the corresponding ethyleniminium ion, which forms in the biophase, since pretreatment of tissues with sodium thiosulphate completely prevented the DMPEA-induced inhibition at alpha 1- and alpha 2-adrenoceptors. DMPEA was approximately 42-fold more potent in inhibiting alpha 2-adrenoceptors than it was in inhibiting alpha 1-adrenoceptors. The antagonism of both receptor types was reversible since washing of the tissues after incubation with DMPEA brought the agonist dose-response curve back to the control value. At alpha 1-adrenoceptors, DMPEA displaced the noradrenaline dose-response curve to the right and concomitantly depressed the maximum response, effects which are consistent with a non-competitive mechanism of action. At alpha 2-adrenoceptors, DMPEA caused a parallel shift of the clonidine or noradrenaline dose-response curve to the right in field-stimulated prostatic portions of the rat vas deferens. The antagonism appeared to be competitive at low concentrations, whereas the shift of the clonidine dose-response curve at higher concentrations became overproportional to the DMPEA concentration. A combination of DMPEA with idazoxan produced a less-than-additive shift of the dose-response curve for clonidine, indicating that these antagonists do not bind to the same site.

Mitomycin C toxicity and pharmacokinetics in mice given sulfur nucleophiles

The sulfur nucleophiles, sodium thiosulfate (Na2S2O3) and N-acetylcysteine (NAC) given in maximally tolerated doses did not reduce the hematologic toxicity of high dose mitomycin C (MMC) in normal mice. In addition, neither sulfur nucleophile significantly altered the antileukemic activity of MMC. Pharmacokinetic studies of MMC in normal mice, demonstrated rapid plasma elimination (T1/2 beta = 0.53 hrs) and substantial drug distribution to the bone marrow which was enhanced by NAC. These results demonstrate a lack of MMC antidotal activity for Na2S2O3 and NAC.

Inhibition of methyl isocyanate toxicity in mice by starvation and dexamethasone but not by sodium thiosulfate, atropine, and ethanol

Effects of starvation (24 and 48 h), dexamethasone, sodium thiosulfate, atropine, and ethanol on the toxicity of methyl isocyanate (MIC) vapor, which escaped during the Bhopal accident of December 3, 1984, were studied in male Swiss-Webster mice. Toxicity to MIC appeared to be biphasic; majority of animals died between 1 and 2 d or between 7 and 21 d after exposure to 40 ppm MIC. Starvation (24 or 48 h) or an injection of 2 mg dexamethasone/kg prior to exposure inhibited the toxicity of MIC, especially during the first 6-7 d; administrations of sodium thiosulfate, alcohol, and atropine before or of dexamethasone after the exposure to MIC were ineffective. Starvation increased serum corticosterone levels. The antidotal effects of both starvation and dexamethasone might be due to suppression of the inflammatory response to MIC.

Inactivation of human immunodeficiency virus by Betadine

Human immunodeficiency virus (HIV), the etiological agent of the acquired immunodeficiency syndrome (AIDS), was treated with either Betadine (povidone-iodine) Solution or Betadine Surgical Scrub. HIV inactivation was analyzed using the viral reverse transcriptase assay or by observing the cytopathic effect produced in HIV-infected, H-9, T-cell cultures. The minimum effective Betadine dose was 0.25% for complete inactivation of HIV that was treated for various time intervals (immediate vortex to ten minutes). The titer of HIV stocks used in these experiments (10(5) TCID50 per mL) was greater than amounts generally detected in clinical specimens. Our results provide a rationale for the use of povidone-iodine as a topical antiseptic against HIV in the clinic or laboratory.

Inhibition of beta-propiolactone-induced mutagenesis and neoplasia by sodium thiosulfate

Studies have been initiated to find compounds that can trap direct-acting carcinogens within the stomach. Sodium thiosulfate (STS) is a potent nucleophile and in initial experiments was found to inhibit mutagenesis resulting from exposure of Salmonella typhimurium strain TA100 to the direct-acting carcinogens beta-propiolactone and styrene oxide. In in vitro experiments STS was shown to maintain its nucleophilicity in the acid pH range. It reacted with beta-propiolactone as rapidly at pH 2 as at pH 7.4. Thus STS has the prerequisite attributes to inhibit the carcinogenic effects of electrophiles in the stomach. Experiments were performed in which STS was administered by p.o. intubation to female A/J mice 5 min before p.o. administration of beta-propiolactone. Under these conditions, inhibition of formation of the forestomach tumors occurred. The data obtained suggest that use of nucleophiles to protect against direct-acting carcinogens is a potential strategy for chemoprevention.

Application of a hepatocyte-erythrocyte coincubation system to studies of cyanide antidotal mechanisms

A coincubation system composed of hepatocytes in primary monolayer culture and erythrocytes suspended in the culture medium was developed and used as a model for investigations of mechanisms of cyanide antidote action at the cellular level. Hepatocyte ATP was used as the cytotoxicity indicator. Treatment of rat hepatocytes in the coincubation system with KCN (1.0 mM) for 10 min at 37 degrees C selectively reduced hepatocyte ATP levels to 33 +/- 15% of control (no KCN added) levels. 4-dimethylaminophenol (DMAP), cobalt(II) chloride, sodium nitrite, sodium thiosulfate, or a combination of the last two antidotes added to the KCN-containing medium significantly reversed ATP depression and the response was concentration dependent. The relative effectiveness, on a molar basis, was estimated to be DMAP greater than CoCl2 much greater than NaNO2 congruent to Na2S2O3. NaNO2 and DMAP induced methemoglobin formation in the absence of cyanide and cyanmethemoglobin formation in its presence; erythrocytes were required in the medium for effectiveness. CoCl2 produced neither cyanmethemoglobin nor thiocyanate in appreciable quantities nor required erythrocytes for antagonism. Na2S2O3 converted cyanide to thiocyanate and reversed ATP depression without erythrocytes in the medium. The addition of erythrocytes increased these rates significantly and to a greater extent than albumin. The overall results are consistent with previously proposed modes of action for these antidotes. However, the enhancement in cyanide metabolism and ATP recovery with Na2S2O3 and erythrocytes in the system was unexpected and raises the possibility that erythrocytes may contribute to cyanide disposition and antagonism in vivo when this antidote is administered.

Sodium nitroprusside: pharmacological aspects of its interaction with hydroxocobalamin and thiosulphate

Hydroxocobalamin (HOCb), when mixed with sodium nitroprusside (SNP) in a 10:1 or 1:1 molar ratio and injected (i.v.) into the anaesthetized rat, prolonged the depressor response to SNP by 25-50%, but did not affect the degree of blood pressure lowering. Both the 'onset' and 'offset' components of the response were prolonged. Injecting [14C]SNP along with a 10-fold molar excess of HOCb resulted in a 2- to 3-fold elevation of plasma radioactivity which was maintained during the first 10 min of a 40 min experimental period. These effects of HOCb on the pharmacodynamics and pharmacokinetics of SNP are probably due to complex formation between the two compounds. Sodium thiosulphate (ST) added to SNP (12:1 molar ratio) had no effect on the depressor response to SNP. This mixing of ST and SNP had a less-marked influence on the plasma [14C] SNP-derived radioactivity than occurred with HOCb. There was no initial elevation of radioactivity, but the levels were raised by 50-60% at 4, 6 and 10 min. Since the depressor response to SNP was unaffected by ST, it is presumed that the higher concentrations of radioactivity were due to inactive degradation products rather than the active species itself.

Chemical modification of bovine liver rhodanese with tetrathionate: differential effects on the sulfur-free and sulfur-containing catalytic intermediates

Sulfhydryl groups of bovine liver rhodanese (thiosulfate: cyanide sulfurtransferase, EC 2.8.1.1) were modified by treatment with tetrathionate. There was a linear relationship between loss of enzyme activity and the amount of tetrathionate used. At a ratio of one tetrathionate per mole of rhodanese, 100% of enzyme activity was lost in the sulfur-free E-form as compared with a 70% loss for the sulfur-containing ES-form of the enzyme. Addition of up to a 100-fold molar excess of tetrathionate to ES gave no further inactivation. Addition of cyanide to the maximally inactivated ES-tetrathionate complex gave complete loss of activity. Kinetic studies of maximally inactivated ES and partially inactivated E gave Km (Ks) values that were essentially the same as native enzyme, indicating that the active enzyme, in all cases, bound thiosulfate similarly. Reactivation was faster with the ES-form than with the E-form. The substrate, thiosulfate, could reactivate the enzyme up to 70% in 1 h with ES as compared to 24 h with E. Tetrathionate modification of rhodanese could be correlated with the changes in intrinsic fluorescence and with the binding of the active site reporter 2-anilinonaphthalene-8-sulfonic acid (2,8-ANS). Circular dichroism spectra of the protein suggested increased ordered secondary structure in the protein after reaction with tetrathionate. Cadmium chloride and phenylarsine oxide totally inactivated the enzyme at levels usually associated with their effect on enzymes containing vicinal sulfhydryl groups. Further, cadmium inhibition could be reversed by EDTA. Tetrathionate modification of rhodanese may proceed through the formation of sulfenylthiosulfate intermediates at sulfhydryl groups, close to but not identical with the active-site sulfhydryl group, which then can react further with the active-site sulfhydryl group to form disulfide bridges.

Pathochemistry, pathogenesis and enzyme replacement in multiple-sulfatase deficiency

Multiple-sulfatase deficiency (MSD) is now considered to be heterogeneous and could be classified into three or four clinical phenotypes according to the onset of the disease: neonatal, late infantile, juvenile and possibly adult type. Neonatal-type MSD shows severe clinical involvement and practically no arylsulfatase A, B and C activities in cultured skin fibroblasts. Furthermore, arylsulfatase A activity in neonatal-type MSD was not enhanced by the addition of thiosulfate. Therefore, it is distinct from late infantile-type MSD. The degradation of 14C-sulfatide can occur in MSD-cultured skin fibroblasts and was much higher than in late infantile-type MLD. The addition of thiol protease such as leupeptin to cultured MSD skin fibroblasts enhanced arylsulfatase A activity as well as the degradation of 14C-sulfatide. This suggests that the decreased activities of arylsulfatase A is due to an acceleration of the enzyme degradation. Enzyme replacement by the addition of arylsulfatases of different sources (human liver, brain, fungus) was carried out in cultured MSD skin fibroblasts. Human enzymes of arylsulfatase A and B were mostly taken up by MSD cells rather than those of fungus origin. By the exposure to leukocytes to cultured skin fibroblasts, MSD cells corrected arylsulfatase A and B activities.

Early microcirculatory stasis in acute gastric mucosal injury in the rat and prevention by 16,16-dimethyl prostaglandin E2 or sodium thiosulfate

We used in vivo microscopy and laser-Doppler velocimetry to examine the effects on the gastric mucosal microcirculation and in gastric mucosal blood flow of agents that induce acute gastric mucosal damage. In vivo microscopic observation of superficial mucosal capillaries revealed vascular stasis within a mean of 54, 81, or 61 s after 100% ethanol, 0.6 N HCl, or 0.2 N NaOH, with the subsequent development of hemorrhagic mucosal lesions. Mucosal blood flow estimated by laser-Doppler velocimetry decreased by 30% at 5 min after luminal application of 100% ethanol, and decreased further to about 40% of basal levels by 15 min. The decreased mucosal blood flow 15 min after application of 50% ethanol correlated with the extent of hemorrhagic mucosal lesions. Examination of the submucosal vessels that supply and drain the mucosa showed moderate dilation of small arterioles 1, 3, and 6 min after exposure to 100% ethanol but there were no consistent changes in venules. Mild vasoconstriction of small- and medium-sized venules could be detected 6, 10, and 15 min after NaOH but not after exposure to HCl. Pretreatment with 16,16-dimethyl prostaglandin E2 or sodium thiosulfate before exposure of the mucosa to ethanol prevented capillary stasis, maintained mucosal blood flow, and prevented the development of hemorrhagic gastric mucosal lesions. Topical mucosal application of 16,16-dimethyl prostaglandin E2 decreased, whereas topical exposure to sodium thiosulfate increased gastric mucosal blood flow, indicating that change in blood flow per se is an unlikely mediator of protection.

Protection of antiproliferative effect of cis-diamminedichloroplatinum (II) by sodium thiosulfate

Utilizing the phytohemagglutinin (PHA) stimulation assay of human peripheral blood mononuclear cells (PBM), the protective effect of sodium thiosulfate (STS) on the antiproliferative action of cis-diamminedichloroplatinum (II) (DDP) against human cells was investigated. DDP alone significantly inhibited the proliferation of PBM over the concentration range of 10(-7) to 5 X 10(-5) M. The antiproliferative effect of DDP was significantly blocked when STS was added to the stimulation culture at the time of exposure to DDP at molar STS/DDP ratios of more than 500. However, STS at molar ratios of less than 100 induced minimal protection. Then, STS was added at various times after DDP exposure with a molar STS/DDP ratio of 1000. The protection was effective within 10 min after exposure to DDP at a concentration of 5 X 10(-5) M, whereas it was not effective beyond 30 min after the exposure. The results indicate that effective protection against DDP cytotoxicity in human cells can be achieved by the concurrent presence of STS with molar STS/DDP ratios of more than 500, but not when a molar STS/DDP ratio of less than 100 is used.

The effect of sodium thiosulfate on subcellular localization of platinum in rat kidney after treatment with cisplatin

Female Sprague-Dawley rats were given 10 mg/kg of i.v. cis-diammine-dichloroplatinum(II) (cisplatin) simultaneously with i.v. sodium thiosulfate (STS) at a 200-fold molar excess to cisplatin, then subcellular (nuclei, mitochondria, microsomes, cytosol) distribution of platinum within rat kidney cells was determined 15 min, 1 h, 8 h and 24 h after cisplatin injection. Blood urea nitrogen levels were measured in rats treated in the same manner described above. STS was found to block cisplatin-induced nephrotoxicity. However, differences in platinum concentrations in total homogenate or each subcellular fraction between STS-treated rats and controls were not significant enough to fully account for the drastic protective effect of STS against cisplatin nephrotoxicity.

Stereospecific effect of naloxone hydrochloride on cyanide intoxication

Cyanide intoxication in mice can be antagonized by the opiate antagonist, (-)naloxone HCl, alone or in combination with sodium thiosulfate and/or sodium nitrite. Potency ratios, derived from LD50 values, were compared in groups of mice pretreated with sodium nitrite (sc, 100 mg/kg), sodium thiosulfate (ip, 1 g/kg), and (-)naloxone HCl (sc, 10 mg/kg) either alone or in various combinations. These results indicate that naloxone HCl provides a significant protection against the lethal effects of potassium cyanide. The protective effect of sodium thiosulfate, but not sodium nitrite, was enhanced with (-)naloxone HCl. The combined administration of sodium nitrite and sodium thiosulfate was further enhanced with (-)naloxone HCl. This protective effect of naloxone HCl against the lethal effect of cyanide appears to be restricted to the (-)stereoisomer, as the (+)stereoisomer, the inactive opiate antagonist, is also inactive in protecting against the lethal effects of cyanide. The mechanism of antagonism is discussed.

Plaque enhancement effect of sodium thiosulfate for foot-and-mouth disease viruses

A plaque enhancement effect by the addition of sodium thiosulfate for foot-and-mouth disease viruses was demonstrated when this salt was incorporated in agar and in agarose overlay media. Most of the mechanism is obscure, however, as one of the effects is that sodium thiosulfate seems to interact in a reversible manner against the plaque inhibitor action of neutral red in cellular cytoplasm. A plaque inhibitor contained in agar could be removed in some degree by the addition of this salt.

Liver damage does not increase the sensitivity of mice to cyanide given acutely

The major detoxification pathway for cyanide (CN) in many species is a biotransformation to the less toxic thiocyanate (SCN). Hepatic thiosulfate: cyanide sulfurtransferase (rhodanese) is the principal enzyme demonstrating in vitro catalytic activity. Despite the assumed importance of the hepatic enzyme for CN detoxification in vivo, the effects of liver damage (surgical or chemical) on cyanide lethality in animals have not been examined previously. Male CD-1 mice were pretreated with carbon tetrachloride (CCl4, 1 ml/kg, i.p.) 24 h prior to the administration of sodium cyanide (NaCN). In other experiments CCl4 was given in the same doses at both 48 h and 24 h prior to NaCN. Hepatotoxicity was documented by elevated serum glutamicpyruvic transaminase (SGPT) activity, by histologic evaluation of the extent of cellular necrosis, by electron microscopy of the mitochondrial fraction, and by the increased duration of zoxazolamine-induced paralysis. Lethality was not changed by CCl4 pretreatments when NaCN was given alone in doses of 4 or 6 mg/kg or at a dose of 10.7 mg/kg following sodium thiosulfate (Na2S203, 1 g/kg, i.p.). A small but statistically significant protective effect was exhibited by CCl4 when NaCN was given at a dose of 16 mg/kg following the administration of Na2S203. Rhodanese activity as measured in mitochondrial preparations fractionated from the livers of mice pretreated with CCl4 was not different from that in animals given the corn oil vehicle even though electron micrographs showed extensive mitochondrial damage. No difference in CN lethality was evident between sham-operated mice and partially (2/3) hepatectomized mice at 24 h post-surgery. An intact healthy liver does not appear to be essential for cyanide detoxification in mice whether or not thiosulfate is also given. Because rhodanese activity was slightly but significantly higher in mitochondria lysed by Triton X-100 than in intact mitochondria, the mitochondrial membrane may constitute a barrier to Na2S203.

The efficacy of alpha-ketoglutaric acid in the antagonism of cyanide intoxication

It has been reported that compounds containing carbonyl groups can readily react with cyanide. Pyruvic acid, an alpha-ketocarboxylic acid, has been shown to antagonize the lethal effects of cyanide. It is suggested that its mechanism of action rests in its ability to react with or "bind" cyanide. In this study, alpha-ketoglutaric acid, also an alpha-ketocarboxylic acid, was evaluated for its ability to counteract the lethal effects of cyanide. alpha-Ketoglutaric acid increased the LD50 value of cyanide (6.7 mg/kg) by a factor of five, a value statistically equivalent to that ascertained in mice pretreated with sodium thiosulfate and sodium nitrite. The combination of alpha-ketoglutaric acid and sodium thiosulfate increased the LD50 value of cyanide to 101 mg/kg. Addition of sodium nitrite to the alpha-ketoglutaric acid/sodium thiosulfate regimen increased the LD50 value of cyanide to 119 mg/kg. Unlike sodium nitrite, no induction of methemoglobin formation was observed with alpha-ketoglutaric acid pretreatment. It is apparent from these studies that the administration of alpha-ketoglutaric acid in conjunction with sodium thiosulfate resulted in fewer animal deaths than sodium nitrite and sodium thiosulfate without the dangerous formation of methemoglobin.

Inactivation of cis-diamminedichloroplatinum (II) in blood and protection of its toxicity by sodium thiosulfate in rabbits

The mode of inactivation of cis-diamminedichloroplatinum(II) (DDP) in the bloodstream and protection from its toxicity by sodium thiosulfate (STS) were investigated in rabbits. Plasma ultrafiltrate in rabbits given 5 mg/kg DDP IV and various excess molar ratios of STS IV were assayed for the active platinum levels with a new microbiological assay system using an E. coli strain. The active platinum species in the plasma were inactivated completely by co-administration of a 400-fold excess of STS IV. The rabbits were almost completely protected against both BUN increase and body weight loss normally caused by DDP when 400-fold doses of STS were given. Diuretic effects were also observed. Our data provide evidence for the basis of optimum use of STS to protect against DDP toxicity.

Phosphate uptake by primary renal proximal tubule cell cultures grown in hormonally defined medium

The uptake of labeled inorganic phosphate into primary rabbit kidney proximal tubule cells has been examined. Phosphate was accumulated into the primary proximal tubule cells against a concentration gradient. This accumulation was sensitive to inhibition by metabolic inhibitors. The dependence of phosphate uptake on the extracellular phosphate concentration was examined. Similarities were observed between primary proximal tubule cells and the LLC-PK1 cell line in these regards. These phosphate uptake data were then plotted on a Lineweaver-Burke plot. A nonlinear plot was obtained, which suggested that phosphate uptake occurs by means of a Na+ dependent, carrier mediated process, as well as by another Na+ independent mechanism. The pH dependence of phosphate uptake was also examined. Unlike previous observations with LLC-PK1 cells, optimal phosphate uptake occurred at pH 6.5. However, this difference between the two cell culture systems may possibly be explained by differences in uptake conditions. The dependence of phosphate uptake on the extracellular NaCl concentration was examined at three different pH values. The rate of phosphate uptake at pH 7.0 was observed to saturate at a lower NaCl concentration than at either pH 6.0 or pH 6.5. Furthermore, the optimal rate of phosphate uptake at pH 7.0 was observed to be higher than at the other two pH values studied when the NaCl concentration was below 120 mM. However, when the NaCl concentration was raised to 150 mM, optimal phosphate was observed to occur at pH 6.5 rather than at pH 7.0. These observations may be explained if the pH affects not only the rate of phosphate uptake but also the affinity of the phosphate uptake system for sodium. Phosphate uptake was also observed to be sensitive to several agents, Na2 X SO4 and NaSCN, which affect the membrane potential. As observed with phosphate uptake by LLC-PK1 (and renal brush border membrane vesicles), phosphate uptake was highly sensitive to inhibition by the phosphate analogue arsenate. Novel observations were that the phosphate analogue vanadate and its cellular metabolite vanadyl stimulated the initial rate of phosphate uptake.

Intraperitoneal chemotherapy: the use of concurrent systemic neutralizing agents

There is a very great pharmacologic advantage to the intraperitoneal administration of many agents active against human ovarian carcinoma. In principle, the use of a systemic neutralizing agent can further improve the therapeutic index of this approach. In the case of the cell cycle phase-specific antimetabolite methotrexate, systemic administration of leucovorin permits very long duration exposure to methotrexate. In the case of cisplatin, systemically administered thiosulfate appears to provide relatively specific protection for the kidneys, and this in turn permits a doubling of the plasma exposure to unneutralized cisplatin. These concepts are now ready for phase II trial.

Active absorption of selenate by rat ileum

Intestinal absorption of selenate and selenite in rats was investigated in vitro by using everted sacs of the duodenum, jejunum and ileum. Only the ileal sacs incubated with selenate accumulated selenium in the serosal fluid during the incubation. Selenium transport across the ileum did not occur against a concentration gradient when selenite instead of selenate was present in the incubation medium. Sulfate and thiosulfate significantly inhibited ileal selenate transport. Decreasing the activity of the Na+,K+-ATPase, by the addition of ouabain, resulted in a significant reduction of concentrative selenate transport by the ileum. Furthermore L-leucine, but not D-glucose and D-galactose, significantly reduced selenium absorption by ileal sacs incubated with selenate. It is concluded from these results that selenate is transported actively by the ileal mucosa and that a common transport mechanism for selenate and sulfate exists. The Na+-gradient across the intestinal brush border membrane seems to be capable of energizing active ileal selenate transport.

The binding of a 2-chloroethylamine derivative of oxotremorine (BM 123) to muscarinic receptors in the rat cerebral cortex

The interaction of a mustard analogue of oxotremorine, N-[4-(2-chloroethylmethylamino)-2-butynyl]-2-pyrrolidone (BM 123), with muscarinic receptors in the rat cerebral cortex was investigated using 3H-ligand-binding methods. When cortical homogenates were preincubated with BM 123 (1.0 mM), washed extensively, and then assayed for the binding of the specific muscarinic antagonist, [3H](-)-N-methylscopolamine, a decrease in binding capacity was noted without an accompanying change in affinity. The rate at which BM 123 alkylated muscarinic receptors was sensitive to temperature, with little or no receptor alkylation occurring at 0 degree. Thus, it was possible to estimate the affinity of BM 123 and its transformation products for muscarinic receptors by measuring their ability to competitively inhibit 3H-ligand binding to cortical homogenates at 0 degree. When measured by competitive inhibition of [3H]oxotremorine-M and [3H](-)-N-methylscopolamine binding, the concentrations of the aziridinium ion of BM 123 required to displace 50% of specific 3H-ligand binding were 3.5 nM and 4.5 microM, respectively. In contrast, the parent 2-chloroethylamine and its alcoholic hydrolysis product were much less active. The kinetics of the alkylation of muscarinic receptors by BM 123 were consistent with a model in which the aziridinium ion rapidly forms reversible complexes with superhigh high and low affinity sites which slowly convert to covalent complexes. The rate of alkylation of the superhigh affinity site was slowest whereas the converse was true for the low affinity site. It was possible to alkylate the high and low affinity sites selectively with BM 123 by taking advantage of kinetic differences in the rates of alkylation of these two sites. Atropine, oxotremorine, and oxotremorine-M antagonized the rate of alkylation of muscarinic receptors in a manner that was consistent with competitive inhibition.

Reaction of the glucose carrier of erythrocytes with sodium tetrathionate: evidence for inward-facing and outward-facing carrier conformations

Sodium tetrathionate reacts with the glucose carrier of human erythrocytes at a rate which is greatly altered in the presence of competitive inhibitors of glucose transport. Inhibitors bound to the carrier on the outer surface of the membrane, either at the substrate site (maltose) or at the external inhibition site (phloretin and phlorizin), more than double the reaction rate. Inhibitors bound at the internal inhibition site (cytochalasin B and androstenedione), protect the system against tetrathionate. After treatment with tetrathionate, the maximum transport rate falls to less than one-third, and the properties of the binding sites are modified in unexpected ways. The affinity of externally bound inhibitors rises: phloretin is bound up to seven times more strongly and phlorizin and maltose twice as strongly. The affinity of cytochalasin B, bound at the internal inhibition site, falls to half while that of androstenedione is little changed. The affinity of external glucose falls slightly. Androstenedione prevents both the fall in transport activity and the increase in phloretin affinity produced by tetrathionate. An inhibitor of anion transport has no effect on the reaction. The observations support the following conclusions: Tetrathionate produces its effects on the glucose transport system by reacting with the carrier on the outer surface of the membrane. The carrier assumes distinct inward-facing and outward-facing conformations, and tetrathionate reacts with only the outward-facing form. The thiol group with which tetrathionate is presumed to react is not present in either the substrate site or the internal or external inhibitor site.(ABSTRACT TRUNCATED AT 250 WORDS)

AF64A: an active site directed irreversible inhibitor of choline acetyltransferase

Ethylcholine mustard aziridinium ion (AF64A, MEChMAz) has been proposed as a cholinergic neuron-specific neurotoxin. We report that in further studies on its mechanism of action incubation of the cholinergic neuroblastoma X glioma cell line, NG-108-15, with 100 microM AF64A resulted in a rapid decrease in cellular choline acetyltransferase (ChAT) activity which preceded cytotoxicity. Thus, a 60-85% decrease in ChAT activity was measured within 5 h of AF64A exposure, whereas cell lysis (measured as the release of the cytosolic enzyme lactate dehydrogenase into the medium) did not become apparent until 18 h of AF64A exposure. This led us to examine the effects of AF64A on partially purified ChAT. We report a concentration- and time-dependent inhibition of partially purified ChAT by AF64A that could not be reversed by dialysis but could be prevented by coincubation of the enzyme and AF64A with choline but not with acetyl-coenzyme A. We present kinetic evidence that choline and AF64A compete for the same site on the enzyme. In addition, thiosulfate, which inactivates the aziridinium ion, eliminated AF64A's capacity to inhibit the enzyme. AF64A also irreversibly inhibited partially purified choline kinase and acetylcholinesterase but not lactate dehydrogenase, alcohol dehydrogenase, carboxypeptidase A, or chymotrypsinogen, enzymes that do not use choline as a substrate or product. Thus, the data suggest that AF64A acts as an irreversible active site directed inhibitor of ChAT and possibly other enzymes recognizing choline.

Acetone potentiation of acute acetonitrile toxicity in rats

The purpose of these studies was to investigate the nature and mechanism of a toxicologic interaction between acetonitrile and acetone. Results of oral dose-response studies utilizing a 1:1 (w/w) mixture of acetonitrile and acetone, or varying doses of acetonitrile administered together with a constant dose of acetone, indicated that acetone potentiated acute acetonitrile toxicity three- to fourfold in rats. The onset of severe toxicity (manifested by tremors and convulsions) was delayed in the groups dosed with both solvents compared to the groups that received acetonitrile or acetone alone. Blood cyanide (a metabolite of acetonitrile) and serum acetonitrile and acetone concentrations were measured after oral administration of 25% aqueous solutions of acetonitrile, acetone, or acetonitrile plus acetone. Concentrations of cyanide in the blood of rats given acetonitrile plus acetone remained near baseline, in contrast to the high concentrations found in rats dosed with acetonitrile alone. At 34-36 h, high blood cyanide concentrations were found in rats dosed with both of the solvents. This delayed onset of elevation of blood cyanide coincided with the occurrence of clinical signs and with the disappearance of serum acetone. In further pharmacokinetic studies, blood cyanide concentrations were measured after similar dosage regimens of acetone and acetonitrile. Peak cyanide concentrations were found to be significantly greater in rats dosed with both solvents than in rats given only acetonitrile. Administration of either sodium thiosulfate or a second dose of acetone prevented the toxicity associated with exposure to both solvents. These results suggest that the effects of acetone on acetonitrile toxicity are due to a biphasic effect on the metabolism of acetonitrile to cyanide, that is, an initial inhibition followed by a stimulation of this metabolism upon acetone elimination.

Plasma free cyanide and blood total cyanide: a rapid completely automated microdistillation assay

Techniques are presented which provide direct measurement of both free cyanide (CN-) in plasma and total CN- in whole blood. Loss of total CN- from blood is prevented by conversion to cyanmethemoglobin. Both free and total CN- are assayed by a completely automated method providing readout 17 minutes after sampling. No prior isolation technique is required and sensitivity is adjustable to cover a broad range of CN- concentrations from 1 to 4000 uM. Precision of blood CN- values from 2 to 2500 uM is within +/- 2.3%. No interference results from thiocyanate or thiosulfate at a concentration of approximately 1 mM.

In vivo and in vitro release of cyanide from neurotoxic aminonitriles

Cyanide release from neurotoxic aminonitriles was measured following in vitro incubation with both microsomes and liver slices. Investigation of cyanide released as urinary thiocyanate following ip aminonitrile administration to rats was also measured. The yield of cyanide in the in vivo study, as measured by the mole percent of administered dose, was greatest from dimethylaminonitrile (DMAA), followed by trimethylaminopropionitrile (TMAPN), dimethylaminopropionitrile (DMAPN), 3,3'-iminodipropionitrile (IDPN), dimethylaminobutyronitrile (DMABN), and monomethylaminopropionitrile (MMAPN). Urinary excretion of thiocyanate accounted for 48.9% of the administered DMAA, 11.6% of TMAPN, 8.0% of DMAPN, 6.8% of IDPN, 3.1% of DMABN, and 1.8% of MMAPN. Incubation of aminonitriles and related compounds with microsomes or liver slices from rats yielded measurable quantities of cyanide from all the compounds tested except for DMABN, TMABN, and succiononitrile. Quantitative evaluation of the yield of formaldehyde by demethylation following microsomal incubation was also determined. The signs of acute toxicity in rats after ip administration of KCN were similar only to those in rats administered DMAA.

A subset of non-histone nuclear proteins reversibly stabilized by the sulfhydryl cross-linking reagent tetrathionate. Polypeptides of the internal nuclear matrix

When rat liver nuclei are isolated in the presence of the irreversible sulfhydryl-blocking reagent iodoacetamide, digested with DNase I and RNase A, and extracted with 1.6 M NaCl, nuclear envelope (NE) spheres depleted of intranuclear material, as analysed by thin-section electron microscopy, are obtained. Two-dimensional isoelectric focusing (IEF)/SDS-PAGE and non-equilibrium pH gradient electrophoresis (NEPHGE)/SDS-PAGE reveal that the predominant polypeptides are lamins A, B and C. Nuclei isolated in the absence of sulfhydryl blocking reagents yield salt- and nuclease-resistant structures which contain sparse but demonstrable intranuclear material. A number of non-histone polypeptides are seen in addition to the lamins. Nuclei treated with the sulfhydryl cross-linking reagent sodium tetrathionate (NaTT) yield, after exposure to nucleases and 1.6 M NaCl, nuclear matrix-like structures containing an extensive intranuclear network and components of the nucleolus in addition to the NE. Increased amounts of the non-lamin, non-histone polypeptides are recovered with these structures. Subsequent treatment of these NaTT-cross-linked structures with reducing agents in 1.0 M NaCl selectively solubilizes the intranuclear components but leaves the nuclear envelope apparently intact. The lamins remain sedimentable and are virtually absent from the soluble (intranuclear) material. Instead, the major solubilized polypeptides are (a) 68 and 63 kD polypeptides which migrate in the vicinity of lamins B and C, respectively, but are distinguishable from the lamins by immunoblotting and by uni-dimensional peptide mapping; (b) a series of basic 60-70 kD polypeptides (pI greater than 8.0) which are not recognized by anti-lamin antisera; (c) an acidic (pI 5.3) 38 kD polypeptide; and (d) a number of high molecular mass (greater than 100 kD) polypeptides. These observations not only suggest a convenient method for fractionating matrix structures from rat liver nuclei into biochemically and morphologically discrete components, but also identify a subset of major non-lamin, non-histone nuclear polypeptides (comprising approx. 20% of the total nuclear protein) whose intermolecular interactions can be reversibly stabilized apparently by intermolecular disulfide bond formation by NaTT.

[Pharmacokinetics of cisplatin and the effect of sodium thiosulfate]

The pharmacokinetics of cisplatin (cis-dichlorodiammineplatinum (II), CDDP) have been studied in 11 patients. Plasma and urine CDDP concentrations were determined by flameless atomic absorption spectrophotometry. The treatment schedule was administered either by a 3-hour and 4-hour i.v. infusion or a 1.5-hour i.a. infusion. In the 3-hour i.v. infusion, the peak level of plasma total CDDP was 4.09 micrograms/ml after the end of infusion and non-protein-bound CDDP was 0.78 microgram/ml. Plasma total CDDP level declined with two peaks, the half-life of the alpha phase being 3.3 hours and that of the beta phase being 5.29 days. The half-life of non-protein-bound CDDP was 1.6 hours. CDDP might have remained for a long time, because CDDP values were obtained 3 weeks after infusion and urinary recovery of CDDP showed low values. Its urinary recovery was 40.3% of the dose infusion in the first 24 hours and 48.0% in 96 hours. The protein-binding of CDDP was inhibited by sodium thiosulfate(STS) and renal urinary excretion was enhanced. Our data suggest that STS can be applied as a neutralizer of CDDP.

A role for sulfite in inducing surface changes in Neisseria gonorrhoeae

Addition of pyruvate to growth medium failed to induce the changes in Neisseria gonorrhoeae that have been reported previously. Addition of 3.8 mM sulfite or 1.3 mM sulfite plus 14 mM pyruvate restored the medium's reactivity in a test for cysteine and its ability to induce changes in N. gonorrhoeae. The induced changes that were restored were (i) increased colonial opacity and roughness, (ii) increased sensitivity to killing by normal human serum, and (iii) electrophoretic changes that may represent changes in lipopolysaccharide. Further characterization of the electrophoretic changes showed that the bands were resistant to treatment with proteinase K, that they were not affected by EDTA and urea, and that they were not dependent upon the stage of growth.

Cerebral cytochrome a,a3 inhibition by cyanide in bloodless rats

Brain cytochrome a,a3 inhibition is presumed to be the site of lethal histotoxic hypoxia in cyanide poisoning perhaps because of the relative inability of the brain to metabolize cyanide. However, only limited data are available about cyanide toxic effects and possible antagonism in the in vivo brain. In this study, in situ, multiple wavelength, spectrophotometric monitoring of brain cytochrome a,a3 was used to observe oxidation-reduction (redox) responses of cerebral cytochrome a,a3 to intravenous potassium cyanide administration. Bloodless rats prepared by perfluorochemical emulsion (FC-43) exchange transfusion allowed monitoring of cyanide-cytochrome a,a3 interaction without spectral interference by hemoglobin. We found that cyanide-induced transient increases in cytochrome a,a3 reduction level and subsequent redox recovery kinetics were similar in bloodless and normal blood circulated rats. Electroencephalographic activity was maintained until a 50% increase in the reduction level of cytochrome a,a3 was induced with cyanide. Pre-treatment with the cyanide antagonist sodium thiosulfate also protected brain cytochrome a,a3 from cyanide-mediated redox state changes by approximately 4-fold both in normal blood circulated controls and during FC-43 circulation. These latter results indicate that sodium thiosulfate, presumably acting at tissue sites of rhodanese activity, can prevent cerebral cytochrome a,a3 reduction by cyanide even in the virtual absence of blood or circulating proteins.

Sodium thiosulfate inhibits cis-diamminedichloroplatinum (II) activity

To elucidate the mechanisms of inactivation of cis-diamminedichloroplatinum(II) (DDP) toxicity by its antidote sodium thiosulfate (STS), we studied the effects of STS on plasma concentrations of platinum (Pt) in vivo, binding of Pt to serum protein in vitro, and uptake of Pt by bacterial cells (E. coli, WP 2 uvrA strain) or cultured mouse tumor cells (FM3A) in vitro. STS did not significantly affect either plasma levels of total Pt or non-protein-bound Pt in vivo, but did inhibit binding of Pt to serum protein and cellular uptake of Pt in vitro. These results suggest that when DDP is given in combination with STS in vivo, the binding to macromolecules and entry of DDP into the cells are prevented due to formation of the Pt-thiosulfate complex in the extracellular fluid.

Se-Aryl selenenylthiosulphates and S-aryl sulphenylthiosulphates as thiol-blocking reagents

Se-Aryl selenenylthiosulphates and S-aryl sulphenylthiosulphates inhibit papain at pH 5.8 much more rapidly than do the corresponding Se-aryl selenosulphates and S-aryl thiosulphates, and also more rapidly than do Se-alkyl selenosulphates. Se-p-Nitrophenyl selenenylthiosulphate and S-p-nitrophenyl sulphenylthiosulphate inactivate papain most rapidly, but the inactivation is slowly and spontaneously reversible. Inactivation by Se-o-nitrophenyl selenenylthiosulphate and S-o-nitrophenyl sulphenylthiosulphate, although less rapid than that by the para isomers, is essentially irreversible.

Effects of oxygen on the antagonism of cyanide intoxication: cytochrome oxidase, in vitro

Since oxygen was reported to be an effective cyanide antagonist in vivo, particularly in the presence of the classic antidotal combination of sodium nitrite and sodium thiosulfate, in vitro studies were initiated in an attempt to investigate the mechanism of oxygen-mediated cyanide antagonism. The effect of oxygen on cyanide-inhibited cytochrome oxidase with and without cyanide antagonist(s) was investigated in a purified membraneous enzyme system prepared from rat liver mitochondria. Cyanide produced a concentration dependent inhibition of cytochrome oxidase, and 100% oxygen did not alter the inhibition produced by KCN either in the presence or absence of sodium thiosulfate. However, the addition of sodium thiosulfate and rhodanese to the assay reactivated the cyanide-inhibited cytochrome oxidase. Kinetic analysis indicated rhodanese competes with cytochrome oxidase for cyanide, and oxygen had no effect on this coupled reaction. In conclusion, the in vivo antidotal properties of oxygen cannot be attributed to oxygen-mediated reactivation of cyanide-inhibited cytochrome oxidase or an oxygen-mediated acceleration of rhodanese detoxification.

[Resistance of Aujeszky disease viruses, present in the blood of pigs, to potassium thiosulphate and hydrogen chloride]

A trial was conducted to study the sensitivity of the Aujeszky disease virus, present in the blood of pigs, to the effect of two preservatives: potassium thiosulphate ( K2S2O5 ) and hydrogen chloride (HCl). The sensitivity of the virus was determined on rabbits infected with pig blood containing Aujeszky disease virus with different additions of the two preservatives. As found, 0.5% of K2S2O5 and 3% of HCl are concentrations fully sufficient for the inactivation of all the viruses present in the blood. Blood preserved in this way can be used for feeding purposes in pig stocks without the hazard of introducing Aujeszky disease in healthy stocks via the blood included in feed rations.

Kinetics of sodium thiosulfate, a cisplatin neutralizer

Sodium thiosulfate kinetics were studied in eight subjects, six of whom were given the drug as a cisplatin neutralizer. Thiosulfate at a dose of 12 gm/m2 was injected by continuous intravenous infusion over 6 hr. Under these conditions, steady-state plasma concentrations were not achieved and apparent volume of distribution could not be calculated. The drug was eliminated from plasma by first-order kinetics, and the data best fit a one-compartment kinetic model with a t1/2 (mean +/- SD) of 80 +/- 38 min. Total body thiosulfate clearance was 190 +/- 76 ml/min/m2 and renal clearance was 50 +/- 11 ml/min/m2. The plasma elimination t1/2 and renal thiosulfate clearance correlated poorly with clearance of endogenous creatinine. Only 28.5% +/- 9.4% of the thiosulfate was recovered unchanged in the urine. Urinary excretion was rapid, with approximately 95% of recoverable drug eliminated within 4 hr after termination of the infusion. No toxic effects of thiosulfate were observed. These data provide the basis for the rational development of dose schedules when sodium thiosulfate is used as a cisplatin neutralizer.

Rhodanese from Cercopithecus aethiops (vervet monkey) liver. II. Aspects of enzyme kinetics and mechanism of action

Initial velocity kinetic studies were undertaken and certain kinetic parameters ( KmSSO2 -3 = 3.1 X 10(-3) M and Vmax = 153.85 U/ml/min.) were determined and the mechanism identified as a ping-pong (double displacement) mechanism. Competitive inhibition of rhodanese by both substrates, viz. thiosulphate and cyanide, provides additional evidence of Ping-Pong Bi-Bi mechanism for this transferase.

[Characteristics of the sulfate requirement of propionic acid bacteria]

The kinetics of sulfate assimilation by Propionibacterium shermanii was found to be peculiar. The assimilation and excretion of sulfate into the medium had an oscillatory character. Sulfate was shown to pass into the cell by active transport. Sulfate transport is described by the Michaelis--Menten kinetics. Thiosulfate and sulfite inhibit sulfate assimilation. Cysteine does not entirely inhibit sulfate assimilation by the cells. The system of sulfate transport was repressed by cysteine to a small extent. The intracellular pool of inorganic sulfate changed in the process of culture growth.

Effect of chlorpromazine on cyanide intoxication

Previous reports from our laboratory indicated that prophylactic protection against cyanide intoxication in mice can be enhanced by administration of chlorpromazine when it is given with sodium thiosulfate. The mechanism of potentiation of sodium thiosulfate by chlorpromazine was studied alone and in combination with sodium nitrite. Although chlorpromazine was found to induce a hypothermic response, the mechanism of enhancement of the antagonism of cyanide by chlorpromazine does not correlate with the hypothermia produced. Various other possible mechanisms were investigated, such as rate of methemoglobin formation, enzymatic activity of rhodanese and cytochrome oxidase, and alpha-adrenergic blockade. The alpha-adrenergic blocking properties of chlorpromazine may provide a basis for its antidotal effect, since this protective effect can be reversed with an alpha-agonist, methoxamine.

Tetrathionate-blocked creatine kinase as a substrate for human plasma 'creatine kinase conversion factor'

'Creatine kinase conversion factor' has been partially purified from whole plasma. The preparation has the ability to convert slowly migrating, native CK-MM into a faster electrophoretic form through a form of intermediate mobility. These changes are accompanied by a loss in CK activity. The rates of both electrophoretic conversion and loss of enzyme activity are reduced or prevented by the presence of 2-mercaptoethanol, EDTA, or by formation of the creatine-MgADP-nitrate dead-end complex. When the thiol group essential for enzyme activity found at the CK active site is first blocked by treatment with potassium tetrathionate, full activity may be retained under conditions that fully convert CK to the faster electrophoretic forms. The conversion factor activity was not inhibited by a pre-incubation with the protease inhibitor, PMSF and the converted form of CK showed no evidence of proteolytic cleavage. The molecular basis of the chemical alteration caused by the conversion is discussed.

The use of intrinsic protein fluorescence to quantitate enzyme-bound persulfide and to measure equilibria between intermediates in rhodanese catalysis

The intrinsic fluorescence of the enzyme rhodanese is quenched by as much as 30% when sulfur is transferred to the free enzyme form, E, giving the sulfur-substituted enzyme, ES. This fluorescence change (lambda ex = 295 nm and lambda em = 335 nm) has been used to quantitate the E and ES forms which are isolatable, obligatory intermediates in rhodanese catalysis. Fluorescence titration was performed using cyanide to irreversibly remove sulfur from ES. The results show a stoichiometry corresponding to 1 bound sulfur/molecule of the ES form of rhodanese (Mr = 33,000). The fluorescence changes were used to measure the concentrations of E and ES when these were in reversible equilibria induced by interactions with the substrates S2O3(2-) and SO3(2-). These results were compared with an equilibrium constant derived from published kinetic studies for the reaction (formula; see text) The very close agreement between the physical and kinetic methods indicate that there are no significant concentrations of intermediates other than E and ES. Overall, the results are compatible with the formation of a persulfide intermediate in rhodanese catalysis and are consistent with conclusions from x-ray crystallography and absorption spectroscopy. In addition, these procedures offer a facile method to measure equilibria between catalytic intermediates in the rhodanese reaction using functionally relevant concentrations.

Effects of thiosulfate on cyanide pharmacokinetics in dogs

One method to treat cyanide poisoning involves the administration of a combination of sodium thiosulfate and sodium nitrite. Sodium thiosulfate is believed to exert its antidotal effect by serving as a sulfur donor, thereby increasing the rate of rhodanese catalyzed biotransformation of cyanide to thiocyanate. To gain insight into the mechanism of action of thiosulfate on cyanide toxicity, a pharmacokinetic analysis of cyanide distribution and metabolism with and without sodium thiosulfate was conducted in mongrel dogs. A compartmental model for thiocyanate, the major metabolite of cyanide, was developed from plasma concentrations determined at various times after iv administration of thiocyanate; sodium thiosulfate did not alter thiocyanate-model parameters. The model for thiocyanate was coupled to a model for cyanide, and model based equations were fitted to the blood levels of both cyanide and thiocyanate that were measured after iv administration of cyanide. This kinetic analysis showed that thiosulfate increased the rate of conversion of cyanide to thiocyanate over 30-fold. The mechanism of thiosulfate protection appeared to be due to extremely rapid formation of thiocyanate in the central compartment, which thereby limited the amount of cyanide distribution to sites of toxicity.

Sulphate transport in Candida utilis

Sulphate uptake by Candida utilis follows Michaelis-Menten type kinetics characterized by a Km of 1.43 mM for sulphate. The process is unidirectional, pH, temperature and energy dependent. Molybdate, selenate, thiosulphate, chromate and sulphite are competitive inhibitors. Dithionite is a mixed-type inhibitor of sulphate uptake. If cells are pre-incubated with sulphate, sulphite, thiosulphate, dithionite or sulphide, sulphate uptake is severely blocked. Inhibition by endogenous sulphate, sulphite and thiosulphate was specific for sulphate uptake. Thus, incorporation of extracellular sulphate seems to be under the control of a heterogeneous pool of sulphur compounds. These results are discussed in connection with the regulation of sulphur amino acid biosynthesis in C. utilis.

Electron-spin-resonance studies of the NADH-dependent nitrite reductase from Escherichia coli K12

The NADH-dependent nitrite reductase of Escherichia coli, which contains sirohaem, flavin, non-haem iron and labile sulphide, was examined by low-temperature e.s.r. spectroscopy. The enzyme, stored in the presence of nitrite and ascorbate, gave the spectrum of a nitrosyl derivative, with hyperfine splitting due to the nitrosyl nitrogen. On removal of these reagents, a series of signals centred around g = 6 was observed, typical of high-spin ferric haem. Cyanide converted this into a low-spin form. On reduction of the enzyme with NADH, an axial spectrum at g = 1.92, 2.01 was observed. The temperature-dependence of this signal is indicative of a [2Fe-2S] iron-sulphur cluster. The midpoint potential of this cluster was estimated to be -230 +/- 15 mV by two independent methods. Reduction of the enzyme with dithionite yielded further signals, which are at present unidentified, at g = 2.1-2.28. No signals were observed that could be assigned to a [4Fe-4S] cluster, such as is found in other sulphite reductases and nitrite reductases that contain sirohaem.