An evaluation of methods to decrease the availability of inorganic sulphate for sulphate conjugation in the rat in vivo

Hepatic clearance concepts and misconceptions: Why the well-stirred model is still used even though it is not physiologic reality?

The liver is the most important drug metabolizing organ, endowed with a plethora of metabolizing enzymes and transporters to facilitate drug entry and removal via metabolism and/or biliary excretion. For this reason, much focus surrounds the development of clearance concepts, which are based on normalizing the rate of removal to the input or arterial concentration. By so doing, some authors have recently claimed that it implies one specific model of hepatic elimination, namely, the widely used well-stirred or venous equilibration model (WSM). This commentary challenges this claim and aims to provide a comprehensive discussion of not only the WSM but other currently applied hepatic clearance models - the parallel tube model (PTM), the dispersion model (DM), the zonal liver model (ZLM), and the heterogeneous capillary transit time model of Goresky and co-workers (GM). The WSM, PTM, and DM differ in the patterns of internal blood flow, assuming bulk, plug, and dispersive flows, respectively, which render different degrees of mixing within the liver that are characterized by the magnitudes of the dispersion number (D_N), resulting in different implications concerning the (unbound) substrate concentration in liver (Cu_H). Early models assumed perfusion rate-limited distribution, which have since been modified to include membrane-limited transport. The recent developments associated with the misconceptions and the sensitivity of the models are hereby addressed. Since the WSM has been and will likely remain widely used, the pros and cons of this model relative to physiological reality are further discussed.

Effect of ethanol on lipid metabolism

Hepatic lipid metabolism is a series of complex processes that control influx and efflux of not only hepatic lipid pools, but also organismal pools. Lipid homeostasis is usually tightly controlled by expression, substrate supply, oxidation and secretion that keep hepatic lipid pools relatively constant. However, perturbations of any of these processes can lead to lipid accumulation in the liver. Although it is thought that these responses are hepatic arms of the 'thrifty genome', they are maladaptive in the context of chronic fatty liver diseases. Ethanol is likely unique among toxins, in that it perturbs almost all aspects of hepatic lipid metabolism. This complex response is due in part to the large metabolic demand placed on the organ by alcohol metabolism, but also appears to involve more nuanced changes in expression and substrate supply. The net effect is that steatosis is a rapid response to alcohol abuse. Although transient steatosis is largely an inert pathology, the chronicity of alcohol-related liver disease seems to require steatosis. Better and more specific understanding of the mechanisms by which alcohol causes steatosis may therefore translate into targeted therapies to treat alcohol-related liver disease and/or prevent its progression.

Sulphate and osteoarthritis: decrease of serum sulphate levels by an additional 3-h fast and a 3-h glucose tolerance test after an overnight fast

BACKGROUND

Low sulphate levels in blood may contribute to osteoarthritis by decreasing cartilage chondroitin sulphation.

OBJECTIVE

To measure serum levels of sulphate during 3 h of fasting or glucose ingestion after overnight fasts to determine how much sulphate lowering may occur during this period.

METHODS

Sera from 14 patients with osteoarthritis who fasted overnight were obtained every 15-30 min during 3 h of continued fasting and during 3 h after ingestion of 75 g of glucose. Sulphate was assayed by high-performance liquid chromatography with a Metrohm-Peak 761 Compact IC and simultaneously assayed for glucose by high-performance liquid chromatography with a Metrohm-Peak 817 Bioscan.

RESULTS

Continuation of overnight fasting for 3 h resulted in a near-linear 3-h decrease in levels for all 14 patients ranging from 3% to 20% with a mean drop of 9.3%, whereas the 3-h decrease after glucose ingestion ranged from 10% to 33% with a mean drop of 18.9%.

CONCLUSION

A 3-h continuation of fasting caused a marked reduction in serum sulphate levels, whereas ingestion of 75 g of glucose in the absence of protein resulted in doubling the reduction. This suggests that fasting and ingestion of protein-free calories may produce periods of chondroitin undersulphation that could affect osteoarthritis.

Fasting serum sulfate levels before and after development of osteoarthritis in participants of the veterans administration normative aging longitudinal study do not differ from levels in participants in whom osteoarthritis did not develop

OBJECTIVE

To determine whether the development of osteoarthritis (OA) in men over a 33-year period is related to lower sulfate levels in stored serum collected during that time interval.

METHODS

Stored serum samples from participants in the Veterans Administration Normative Aging Study were assayed for sulfate by ion-exchange chromatography. Samples had been obtained every 3-5 years during part or all of a 33-year portion of the study. Sulfate levels were determined in serum from all participants who underwent knee replacement surgery and had evidence of radiographic hand OA, from some of the participants who had evidence of radiographic hand OA but had not undergone knee replacement surgery, from all participants who underwent knee replacement surgery but had no evidence of radiographic hand OA, and from age-matched participants who had no evidence of OA by history, physical examination, or hand radiography.

RESULTS

Serum sulfate levels in participants, with or without radiographic hand OA and/or knee replacements, who were ages 34-72 years at the first examination, ranged from 0.21 mM to 0.51 mM over the course of a maximum of 33 years. Both the overall mean and median sulfate levels rose from 0.32 mM at age 40-50 years to 0.38 mM at age 70-80 years, and the overall mean and median for all ages was 0.36 mM. There were no significant differences in sulfate levels between subjects in any of the 4 groups.

CONCLUSION

There was no evidence of a relationship between these serum sulfate levels and the development of OA. However, all samples were collected after overnight fasting, and no participant was younger than age 34 years at the initiation of the study. It remains to be determined whether differences in the time of ingestion of daily dietary protein providing sulfate are related to the development of OA, or whether sulfate levels measured at an earlier age could be a factor.

Physiological roles and regulation of mammalian sulfate transporters

All cells require inorganic sulfate for normal function. Sulfate is among the most important macronutrients in cells and is the fourth most abundant anion in human plasma (300 microM). Sulfate is the major sulfur source in many organisms, and because it is a hydrophilic anion that cannot passively cross the lipid bilayer of cell membranes, all cells require a mechanism for sulfate influx and efflux to ensure an optimal supply of sulfate in the body. The class of proteins involved in moving sulfate into or out of cells is called sulfate transporters. To date, numerous sulfate transporters have been identified in tissues and cells from many origins. These include the renal sulfate transporters NaSi-1 and sat-1, the ubiquitously expressed diastrophic dysplasia sulfate transporter DTDST, the intestinal sulfate transporter DRA that is linked to congenital chloride diarrhea, and the erythrocyte anion exchanger AE1. These transporters have only been isolated in the last 10-15 years, and their physiological roles and contributions to body sulfate homeostasis are just now beginning to be determined. This review focuses on the structural and functional properties of mammalian sulfate transporters and highlights some of regulatory mechanisms that control their expression in vivo, under normal physiological and pathophysiological states.

Proteoglycan and collagen biochemical variations during fluoroquinolone-induced chondrotoxicity in mice

Although fluoroquinolone antibacterials have a broad therapeutic use, with a relatively low incidence of severe side effects, they have been reported to induce lesions in the cartilage of growing animals by a mechanism that remains unclear. This study was undertaken to determine the potentially deleterious effect of a high dose of pefloxacin (400 mg/kg of body weight) on two main constituents of cartilage in mice, i.e., proteoglycans and collagen. Variations in levels of proteoglycan anabolism measured by in vivo [(35)S]sulfate incorporation into cartilage and oxidative modifications of collagen assessed by detection of carbonyl derivatives were monitored after administration of pefloxacin. Treatment of mice with 1 day of pefloxacin treatment significantly decreased the rate of biosynthesis of proteoglycan for the first 24 h. However, no difference was observed after 48 h. The decrease in proteoglycan synthesis was accompanied by a marked drop in serum sulfate concentration and a concomitant increase in urinary sulfate excretion. The decrease in proteoglycan synthesis, also observed ex vivo, may suggest a direct effect of pefloxacin on this process, rather than it being a consequence of a low concentration of sulfate. On the other hand, treatment with pefloxacin for 10 days induced oxidative damage to collagen. In conclusion, this study demonstrates, for the first time, that pefloxacin administration to mice leads to modifications in the metabolism and integrity of extracellular proteins, such as collagen and proteoglycans, which may account for the side effects observed. These results offer new insights to explain quinolone-induced disorders in growing articular cartilage.

Metabolic acidosis regulates rat renal Na-Si cotransport activity

Recently, we cloned a cDNA (NaSi-1) localized to rat renal proximal tubules and encoding the brush-border membrane (BBM) Na gradient-dependent inorganic sulfate (Si) transport protein (Na-Si cotransporter). The purpose of the present study was to determine the effect of metabolic acidosis (MA) on Na-Si cotransport activity and NaSi-1 protein and mRNA expression. In rats with MA for 24 h (but not 6 or 12 h), there was a significant increase in the fractional excretion of Si, which was associated with a 2.4-fold decrease in BBM Na-Si cotransport activity. The decrease in Na-Si cotransport correlated with a 2.8-fold decrease in BBM NaSi-1 protein abundance and a 2.2-fold decrease in cortical NaSi-1 mRNA abundance. The inhibitory effect of MA on BBM Na-Si cotransport was also sustained in rats with chronic (10 days) MA. In addition, in Xenopus laevis oocytes injected with mRNA from kidney cortex, there was a significant reduction in the induced Na-Si cotransport in rats with MA compared with control rats, suggesting that MA causes a decrease in the abundance of functional mRNA encoding the NaSi-1 cotransporter. These findings indicate that MA reduces Si reabsorption by causing decreases in BBM Na-Si cotransport activity and that decreases in the expression of NaSi-1 protein and mRNA abundance, at least in part, play an important role in the inhibition of Na-Si cotransport activity during MA.

Chronic K depletion inhibits renal brush border membrane Na/sulfate cotransport

BACKGROUND

The purpose of this study was to determine if dietary potassium (K) deficiency regulates renal proximal tubular sodium gradient-dependent sulfate transport (Na/Si cotransport) in the rat and, furthermore, determine if the regulation takes place at the level of the recently cloned Na/Si cotransport system (NaSi-1). Methods and Results. Rats treated chronically (seven days) with a K-deficient diet had a significant decrease in serum Si levels and an increase in fractional excretion of ultrafilterable Si, which paralleled a significant decrease in brush border membrane (BBM) Na/Si cotransport activity. The decrease in BBM Na/Si cotransport activity was associated with decreases in BBM NaSi-1 protein and renal cortical NaSi-1 mRNA abundance. In addition, in Xenopus oocytes injected with mRNA from kidney cortex slices of K-deficient rats, there was a significant reduction in the induced Na/Si cotransport, whereas there was no alteration in l-leucine uptake, suggesting that in K-deficient rats, there is a specific decrease in functional mRNA encoding the NaSi-1 mRNA.

CONCLUSION

These findings indicate that chronic K deficiency leads to a reduction in serum Si levels and an increase fractional excretion of Si, and reduces Si reabsorption by down-regulating the expression of the proximal tubular Na/Si-1 cotransporter protein and mRNA.

Biology and function of the reversible sulfation pathway catalysed by human sulfotransferases and sulfatases

Sulfation and sulfate conjugate hydrolysis play an important role in metabolism, and are catalysed by members of the sulfotransferase and sulfatase enzyme super-families. In general, sulfation is a deactivating, detoxication pathway, but for some chemicals the sulfate conjugates are much more reactive than the parent compound. The range of compounds which are sulfated is enormous, yet we still understand relatively little of the function of this pathway. This review summarises current knowledge of the sulfation system and the enzymes involved, and illustrates how heterologous expression of sulfotransferases (SULTs) and sulfatases is aiding our appreciation of the properties of these important proteins. The role of sulfation in the bioactivation of procarcinogens and promutagens is discussed, and new data on the inhibition of the sulfotransferase(s) involved by common dietary components such as tea and coffee are presented. The genetic and environmental factors which are known to influence the activity and expression of human SULTs and sulfatases are also reviewed.

Effects of acute caffeine ingestion and menopause on sulfate homeostasis in women

Inorganic sulfate is a physiological anion which is utilized in the metabolism of both endogenous compounds and xenobiotics. Its homeostasis is maintained predominantly by facilitated reabsorptive processes in the kidneys. The objectives of the present investigation were to evaluate the effects of menopausal status and caffeine ingestion on the serum concentrations and clearance of inorganic sulfate. Thirty-nine women who were classified as premenopausal, postmenopausal with or without estrogen treatment, and postmenopausal with osteoporosis participated in the study. The women were studied on two separate occasions following the ingestion of a decaffeinated beverage to which 6 mg caffeine/kg lean body mass or no caffeine was added. All women were habitual caffeine users (mean ingestion of 588 mg caffeine per day) but abstained from all caffeine sources for 2 weeks prior to the control study day. Postmenopausal women with estrogen supplementation exhibited significantly lower sulfate serum concentrations (0.24 +/- 0.02 mM vs. 0.32 +/- 0.04 mM in premenopausal women, mean +/- SD, p < 0.05) and a decreased renal reabsorption of sulfate for the control (no caffeine) period. There was no difference in serum sulfate or sulfate reabsorption in estrogen supplemented postmenopausal women, compared with women not taking estrogen. Postmenopausal women with osteoporosis had significantly lower creatinine and sulfate clearances than postmenopausal women with estrogen supplementation which may be related to their older age, or factors related to the disease process. The 6 mg/kg dose of caffeine caused a diuresis, but no change in GFR, as indicated by urine volume and creatinine clearance values, respectively. Caffeine administration resulted in an increase in the sulfate excretion rate; there was no change in sulfate serum concentrations. The results of this investigation indicate that menopause results in decreased sulfate serum concentrations that may be the consequence of a decreased renal reabsorption of sulfate. Secondly, this investigation demonstrated that caffeine ingestion increases the urinary excretion of sulfate, an effect that may be related to the diuretic effect of caffeine or due to a caffeine-induced alteration in the renal reabsorption of sulfate.

The diastrophic dysplasia gene encodes a novel sulfate transporter: positional cloning by fine-structure linkage disequilibrium mapping

Diastrophic dysplasia (DTD) is a well-characterized autosomal recessive osteochondrodysplasia with clinical features including dwarfism, spinal deformation, and specific joint abnormalities. The disease occurs in most populations, but is particularly prevalent in Finland owing to an apparent founder effect. DTD maps to distal chromosome 5q and, based on linkage disequilibrium studies in the Finnish population, we had previously predicted that the DTD gene should lie about 64 kb away from the CSF1R locus. Here, we report the positional cloning of the DTD gene by fine-structure linkage disequilibrium mapping. The gene lies in the predicted location, approximately 70 kb proximal to CSF1R, and encodes a novel sulfate transporter. Impaired function of its product is likely to lead to undersulfation of proteoglycans in cartilage matrix and thereby to cause the clinical phenotype of the disease. These results demonstrate the power of linkage disequilibrium mapping in isolated populations for positional cloning.

Nutritionally and chemically induced impairment of sulfate activation and sulfation of xenobiotics in vivo

Sulfation requires 3'-phosphoadenosine 5'-phosphosulfate (PAPS) as the sulfate donor. In the search for methods to inhibit sulfation reactions via impairment of PAPS synthesis, two experimental conditions have been tested in rats. A low-sulfur diet, which does not deplete hepatic glutathione, reduced inorganic sulfate but not PAPS levels in the liver and moderately decreased sulfation of acetaminophen. Administration of molybdate, which is an alternative substrate for intestinal and renal sulfate transport as well as for ATP-sulfurylase, depleted both sulfate and PAPS in liver and markedly inhibited sulfation of acetaminophen. Therefore, administration of molybdate may be used as an experimental tool to study the role of sulfation in the fate and effect of xenobiotics.

Isolation, identification, and analysis of 4-acetylaminophenol-glucuronide in body fluids of dialyzed renal patients; a molecular mass marker for peritoneal diffusive transport

A noncharacteristic solute, appearing in gradient elution liquid chromatography (HPLC) profiles of body fluids of dialyzed renal patients, was isolated and identified by preparative HPLC, beta-glucuronidase induced enzymatic peak shift, and mass spectrometry. The compound was shown to be p-acetylaminophenol ('paracetamol')-glucuronide (PG). Serum and peritoneal dialysate PG concentrations were determined in a number of patients. Cuprophan in vivo dialyzer clearances were calculated. Peritoneal membrane mass transfer coefficients (MTC) of PG were calculated and compared with those of molecular mass markers for peritoneal diffusive mass transport studies (urea, creatinine, uric acid, and inulin). By extrapolation of an MTC versus molecular mass calibration line for urea, creatinine, and uric acid it is shown that PG behaves as expected from its molecular mass. We suggest that PG (Mr = 327) is suitable as a molecular mass marker for the molecular mass range between Mr 200 and 500. It may also be used as a marker for diffusive solute transport in hemodialysis treatment. The HPLC gradient elution technique used here appears to be suitable for the simultaneous analysis of the molecular mass markers creatinine, uric acid, and paracetamolglucuronide.

The effect of chronic paracetamol administration to rats on the glycosaminoglycan content of patellar cartilage

Male Wistar rats were treated with paracetamol (200 mg/kg twice a day) for 2, 3, 4 and 9 weeks. During the first four weeks of paracetamol administration the serum sulfate concentration was significantly decreased. However, during the fourth until the ninth week, the serum sulfate concentration was only diminished to a small and insignificant extent. The paracetamol administration did not lead to serious liver or renal toxicity, as determined by alanine aminotransferase and creatinine levels in the serum of the rats. The paracetamol-induced serum sulfate depletion, observed during the first four weeks of the experiment, led to a significantly lower glycosaminoglycan content of the patellar cartilage of the rats after three and four weeks paracetamol treatment. When after the fourth week the serum sulfate concentration rose to nearly normal levels also the glycosaminoglycan content in the rat patellar cartilage reached control levels. These data indicate that the serum sulfate depletion might be the causative factor for the observed reduction in glycosaminoglycan content of rat patellar cartilage.

Decrease of inorganic blood sulfate following treatment with selected antirheumatic drugs: potential consequence for articular cartilage

The elimination kinetics of inorganic blood sulfate in mice was followed for four hours after a single, oral administration of an antirheumatic drug. Sodium salicylate, aspirin, diflunisal and benorylate, all in a dose of 1.25 mmol/kg, reduced the sulfate level to the less than half that of control. This phenomenon was also demonstrated by phenylbutazone, oxyphenbutazone (both 1 mmol/kg), chloroquine diphosphate (0.6 mmol/kg) and tiaprofenic acid (0.02-0.35 mmol/kg). Niflumic acid (1.08 mmol/kg), piroxicam (0.03 mmol/kg), indomethacin (6.10(-3) mmol/kg), diclofenac (5.10(-3) mmol/kg), ketoprofen (0.2 mmol/kg), naproxen (0.08 mmol/kg) and ibuprofen (0.24 mmol/kg) possessed no sulfate lowering properties. The potential relevance of the use of sulfate lowering drugs for articular cartilage integrity is discussed in the light of what is already known about this subject.

Inhibition of glycosaminoglycan synthesis in anatomically intact rat patellar cartilage by paracetamol-induced serum sulfate depletion

We have studied the effect of low sulfate concentrations on the glycosaminoglycan synthesis in rat patellar cartilage in vivo as well as in vitro. The oral administration of 200 mg/kg paracetamol to male Wistar rats resulted in a significant reduction of the serum sulfate concentration. Reduced serum sulfate availability resulted in a 34% decrease of glycosaminoglycan synthesis in patellar cartilage. This is due to sulfate depletion since paracetamol had no direct effects on glycosaminoglycan synthesis and a slight but significant inhibitory effect on the catabolism of radiolabeled glycosaminoglycans in vitro. The glycosaminoglycans synthesized at low sulfate concentrations in vivo were similar to the glycosaminoglycans synthesized at physiological sulfate concentrations. Studying the effect of sulfate availability in vitro on glycosaminoglycan synthesis in patellar cartilage we found that incubation of rat patellae in medium containing less than 0.5 mM inorganic sulfate led to a decreased sulfate incorporation. The use of potential sulfate decreasing drugs can lead to inhibition of glycosaminoglycan synthesis. This argues for a reconsideration of the use of these drugs in patients with already dysfunctioning cartilage metabolism as in rheumatoid arthritis and osteoarthrosis.

The inhibition by chlorate of the sulphation of polyethyleneglycol in the isolated perfused guinea pig liver

1. The sulphation of polyethyleneglycol 200 by the isolated perfused guinea pig liver is inhibited to about 60% by 10 mM ClO3- in the plasma of the perfusate when the concentration of SO4(2-) therein is 1.18 mM. 2. The inhibition is almost complete when the concentration of SO4(2-) is about 0.1 mM, a level which can be achieved by using a modified Ringer-bicarbonate solution, devoid of sulphate, to prepare the perfusate. 3. Chlorate, presumably through its action on ATP-sulphurylase, may therefore be a useful inhibitor of sulphation in the isolated perfused liver when the activity of the sulphurylase is rate-limiting. 4. The rate of bile production in the presence of chlorate is no different from that in its absence showing that, in the time scale of the perfusion, chlorate is not a general liver poison. 5. When the synthesis of PAPS is not rate-limiting, as in the sulphation of oestrone metabolites by rat liver, chlorate has no effect on the rate of sulphation.

Competition between two enzymes for substrate removal in liver: modulating effects due to substrate recruitment of hepatocyte activity

Modulating effects of competing pathways, exemplified by sulfation (high affinity-low capacity) and glucuronidation (low affinity-high capacity), on drug disappearance and metabolite formation were investigated in a simulation study. The phenomenon of substrate recruitment of hepatocyte activity in drug removal and metabolite formation was shown with respect to inlet substrate concentration, and drug processing from inlet to outlet by enzyme systems localized differentially along the sinusoidal flow path in liver. Three enzymic distribution models: (A) sulfation and glucuronidation evenly distributed in liver, (b) sulfation occurring exclusively in the first half of the liver and glucuronidation in the second half, and (C) glucuronidation solely in the first half and sulfation in the second half, were described. The influence of Km and Vmax of the competing pathway, including enzyme induction (increase in Vmax), on any given pathway was also explored. Competing pathways exert their effects on other given pathways by modulating intrahepatic drug concentration from the inlet to outlet of the liver. When a competing pathway is similarly distributed or is at an anterior location to another pathway, the former pathway effectively reduces intrahepatic drug concentrations which reach downstream hepatocytes for recruitment of activity. For example, when glucuronidation activity is anterior to sulfation activity (defined with respect to flow direction), sulfation is without an effect on glucuronidation, but glucuronidation exerts a maximal influence over sulfation rates (Model C). When glucuronidation is in direct competition with sulfation (Model A) or is posteriorly distributed to sulfation (Model B), saturation of the high-affinity sulfation pathway leads to greater fluxes of substrate available downstream for glucuronidation. This results in an apparent compensatory increase in glucuronidation with reduced sulfation capacity, which occurs at input concentrations greater than the Km for sulfation but less than the Km for glucuronidation. This compensation pattern is more prominent for highly extracted compounds where both sulfation and glucuronidation are effective pathways in drug removal, and where large intrahepatic drug concentration gradients are expected. Since the physiologic description of intrahepatic drug concentration is often described by a concentration gradient from the inlet to outlet of the liver, the logarithmic average concentration has been used to estimate the mean liver concentration in the determination of kinetic constants for enzymic reactions.(ABSTRACT TRUNCATED AT 400 WORDS)

Determination of small quantities of sulfate (0-12 nmol) in serum, urine, and cartilage of the mouse

The colorimetric benzidine method of K. S. Dodgson and B. Spencer (1953, Biochem. J. 55, 436-440) for the measurement of inorganic sulfate can be scaled down about 100 times by using disposable 96-well microplates instead of individual cuvettes. Ten-microliter samples of serum and urine, derived from mice, can be analyzed in a simple, rapid, and reliable way without sacrificing the animals. Without prior isolation of sulfated glycosaminoglycans, ester sulfate in mouse patellar cartilage is liberated quantitatively as inorganic sulfate upon acid hydrolysis in 3 M HCl for 16 h at 80 degrees C. To this end the articular cartilage layer of the patella must be separated in toto from the underlying bone. Subsequent hydrolysis in polypropylene tubes gives accurate results. In contrast, hydrolysis in borosilicate glass vials is useless, since nanomoles of sulfate added cannot be recovered adequately. The thin patellar cartilage layer obtained from 10-week-old male mice contains about 5 nmol of sulfate, an amount easily measured with the developed microplate benzidine method.

Salicylate-induced depletion of endogenous inorganic sulfate. Potential role in the suppression of sulfated glycosaminoglycan synthesis in murine articular cartilage

Sodium salicylate has been shown to suppress glycosaminoglycan (GAG) synthesis by articular hyaline cartilage in vitro. We investigated the in vivo effect of sodium salicylate on murine patellar cartilage, using incorporation of intraperitoneally administered 35S-sulfate as a measure of sulfated GAG synthesis. Our results indicated that a single dose of sodium salicylate (200 mg/kg) inhibited in vivo sulfated GAG synthesis by 56%, compared with controls, and had no effect on sulfated GAG breakdown. A striking finding was that sodium sulfate treatment reduced the serum concentration of inorganic sulfate from 1.1 mM to approximately 0.3 mM, and that this serum reduction was associated with a twofold increase in urinary excretion of sulfate. Using anatomically intact patellar cartilage, in vitro studies clearly showed that, in concentrations reached in vivo (greater than or equal to 1 mM), salicylate suppressed murine chondrocyte GAG synthesis. However, in the presence of serum, the effects of 1 mM salicylate were abolished. We also found that sulfated GAG synthesis was clearly inhibited at low concentrations of sulfate (less than 0.5 mM). Our data indicate that sodium salicylate can suppress articular chondrocyte sulfated GAG synthesis in vivo, and that this effect may particularly be due to a drug-induced reduction of endogenous sulfate.

Sulfation in isolated kidney tubule fragments of rats. Dependence on inorganic sulfate

Uptake and conjugation of sulfate was studied in isolated kidney tubule fragments of rats: Sulfate is rapidly taken up, and maximal cellular concentrations are attained after 5-10 min; intracellular steady-state levels depend on the sulfate concentrations of the medium and attain a maximal value at 2 mM. At 1 mM sulfate in the medium the ratio of intracellular/extracellular concentrations of the inorganic anion amount to about 0.25. Formation of 7-hydroxycoumarin sulfate increases almost linearly up to an extracellular sulfate concentration of 500 microM. Thereafter, rates of sulfation increase more slowly and maximal sulfation rates are attained at 2 mM sulfate. The data indicate that sulfation of 7-hydroxycoumarin proceeds at almost maximal rates in kidney tubule fragments at physiological serum sulfate concentrations.

D-cysteine as a selective precursor for inorganic sulfate in the rat in vivo. Effect of D-cysteine on the sulfation of harmol

D-Cysteine, the unphysiological isomer of the sulfur containing amino acid L-cysteine, is not utilized for protein synthesis, glutathione synthesis or taurine production; it was tested as a selective precursor for inorganic sulfate, required for sulfation of xenobiotics. Both cysteine isomers were injected intravenously in the rat, in order to investigate their sulfoxidation to inorganic sulfate. The rates of sulfoxidation were very similar, so that stereospecificity for the amino acid seemed not to play a role. When the rats were fed a low-protein diet (LP-diet; containing only 8% casein as source of amino acids) the serum sulfate concentration decreased to about 20% of control. Under these circumstances the rate of sulfoxidation of both isomers was decreased to the same extent. In order to confirm that both cysteine isomers were equally efficient in providing inorganic sulfate for sulfation of xenobiotics, a constant infusion of harmol (a substrate for sulfation) was given to rats fed the LP-diet. Administration of L- or D-cysteine yielded similar increases in sulfation of harmol under these conditions. These results show that D-cysteine can be used to selectively enhance sulfate availability.

Factors influencing sulfation in isolated rat hepatocytes

Sulfation of harmol by isolated hepatocytes was dependent on an exogenous source of sulfate. Inorganic sulfate ion stimulated sulfation by over ten fold. Analysis of the stimulation of harmol sulfation by sulfate indicated a Km of 239 microM and a Vmax of 1.1 mumoles harmol sulfate/min/10(6) cells. Cysteine also stimulated the rate of harmol sulfation but was less effective than sulfate ion. Lithium chloride inhibited harmol sulfation. Sulfation was unaffected by several metabolic alterations which inhibited harmol glucuronidation. Fasting for 24 hours, and incubation with ethanol or linoleic acid, did not influence the rate of sulfation but inhibited glucuronidation by 50 percent.

Acetaminophen-induced hepatic glycogen depletion and hyperglycemia in mice

Two hours following administration of a hepatotoxic dose of acetaminophen (500 mg/kg, i.p.) to mice, liver sections stained with periodic acid Schiff reagent showed centrilobular hepatic glycogen depletion. A chemical assay revealed that following acetaminophen administration (500 mg/kg) hepatic glycogen was depleted by 65% at 1 hr and 80% at 2 hr, whereas glutathione was depleted by 65% at 0.5 hr and 80% at 1.5 hr. Maximal glycogen depletion (85% at 2.5 hr correlated with maximal hyperglycemia (267 mg/100 ml at 2.5 hr). At 4.0 hr following acetaminophen administration, blood glucose levels were not significantly different from saline-treated animals; however, glycogen levels were still maximally depleted. A comparison of the dose-response curves for hepatic glycogen depletion and glutathione depletion showed that acetaminophen (50-500 mg/kg at 2.5 hr) depleted both glycogen and glutathione by similar percentages at each dose. Since acetaminophen (100 mg/kg at 2.5 hr) depleted glutathione and glycogen by approximately 30%, evidence for hepatotoxicity was examined at this dose to determine the potential importance of hepatic necrosis in glycogen depletion. Twenty-four hours following administration of acetaminophen (100 mg/kg) to mice, histological evidence of hepatic necrosis was not detected and serum glutamate pyruvate transaminase (SGPT) levels were not significantly different from saline-treated mice. The potential role of glycogen depletion in altering the acetaminophen-induced hepatotoxicity was examined subsequently. When mice were fasted overnight, hepatic glutathione and glycogen were decreased by 40 and 75%, respectively, and fasted animals showed a dramatic increase in susceptibility to acetaminophen-induced hepatotoxicity as measured by increased SGPT levels. Availability of glucose in the drinking water (5%) overnight resulted in glycogen levels similar to those in fed animals, whereas hepatic glutathione levels were not significantly different from those of fasted animals. Fasted animals and animals given glucose water overnight were equally susceptible to acetaminophen-induced hepatotoxicity, as quantitated by increases in SGPT levels 24 hr after drug administration. The potential role of a reactive metabolite in glycogen depletion was investigated by treating mice with N-acetylcysteine to increase detoxification of the reactive metabolite. N-Acetylcysteine treatment of mice prevented acetaminophen-induced glycogen depletion.

The effectiveness of different sulfate precursors in supporting extrahepatic sulfate conjugation

The effectiveness of different sulfur-containing compounds in supplying inorganic sulfate for sulfate conjugation was studied in isolated cells from rat small intestine, kidney and lung. With cells isolated from the small intestine and kidney, inorganic sulfate was by far the most effective source for intracellular active sulfate as judged by the ability to support sulfate conjugation of 7-hydroxycoumarin. Kidney cells could also use cysteine, N-acetylcysteine and glutathione as a sulfate source, whereas isolated small intestinal cells did not seem to break down and use these sulfur-containing compounds. With isolated lung cells cysteine was the most efficient sulfate precursor. Of the other precursors N-acetylcysteine and inorganic sulfate were used for sulfate conjugation to some extent.

Biochemical and pharmacological characteristics of conjugated catecholamines in the rat brain

Mass-fragmentographic methods are described that enable the simultaneous measurement of total, free, and conjugated catecholamines in brain tissues. These methods were used to assess the distribution, kinetics, and pharmacological characteristics of total, free, and conjugated catecholamines in the hypothalamus, caudate nucleus, hippocampus, and septum. Conjugated norepinephrine (NE) represents approximately 20% of total NE in the hypothalamus, septum, and hippocampus, whereas the percentage is approximately 50% in the caudate nucleus. The percentages of conjugated dopamine (DA) in these brain areas are consistently less than those of NE (approximately 13%). Although in the hypothalamus the steady-state concentrations of total, free, and conjugated NE are over four times higher than those of the corresponding total, free, and conjugated DA, the turnover rates of this DA are comparable with those of the corresponding NE. Further, the ratios of conjugated NE or DA turnover rates to those of the total amines are higher than the corresponding ratios of their steady-state concentrations. Treatments with pargyline (75 mg/kg, i.p.; rats killed 30 and 60 min later) failed to change the contents of conjugated catecholamines in the hypothalamus and the caudate nucleus significantly. Pharmacological manipulation with a number of prototypic drugs revealed that although the assay of conjugated catecholamines might shed additional light on the effects of drugs on central catecholamines, the assessment of total or free amines are on the whole equally informative. In conclusion, a detailed assessment of brain conjugated catecholamines is reported. The information provided, fills a gap in our knowledge that has up to now not been adequately addressed.

Presence, distribution and pharmacology of conjugated catecholamines in the rat spinal cord

A method is described for the assay of conjugated catecholamines in the brain and spinal cord. Employing this method, the distribution of conjugated norepinephrine (NE) and dopamine (DA) was evaluated in the 5 main regions of the rat spinal cord and compared with those in the hypothalamus. In the spinal cord 26-35% of total catecholamines are conjugated. The percent of NE conjugated in the hypothalamus NE is about 17%, while that of DA is around 12%. Acute treatments with amphetamine, desmethylimipramine and haloperidol failed to change the concentrations of both total and conjugated NE and DA in the cervical spinal cord. Only amphetamine increased hypothalamic conjugated NE. It is concluded that evaluation of central conjugated catecholamines may provide an additional dimension to our ability to critically assess the mechanisms of action of drugs on central catecholamines.

The dependence of the rate of sulphate conjugation on the plasma concentration of inorganic sulphate in the rat in vivo

The dependence of the rate of sulphation of harmol [7-hydroxy-1-methyl-9H-pyrido(3,4-b)indole] on the plasma concn of inorganic sulphate was estimated in the rat in vivo. This was done in rats that were fed a low protein diet for 4 days to deplete the pool of inorganic sulphate, which decreased the plasma concn of sulphate from 0.9 mM normally to about 0.15 mM. Infusion of sodium sulphate at a rate that was stepwise increased, yielded a range of constant plasma sulphate levels below and above the physiological concn in plasma. By a constant infusion of harmol (6 mumoles/min/kg body wt), a steady state in the excretion of harmol sulphate could be reached at various plasma concns of sulphate. The apparent Km for inorganic sulphate was approx 0.3 mM. The relevance of these data in the evaluation of the effects of changes in the availability of inorganic sulphate on sulphate conjugation of xenobiotics is discussed.