The distribution kinetics of waterborne silver-110m in juvenile rainbow trout

Juvenile rainbow trout (Oncorhynchus mykiss) were subjected to a 2-day radioactive pulse of 110mAg at 11.9 microg/l (as AgNO3), followed by a 19-day post-tracer exposure to non-radioactive Ag(I) (3.8 microg/l). The distribution of 110mAg in the gills, liver, intestine, kidney, brain and remaining carcass was investigated over a 19-day post-tracer period. Initially, the intestine contained the highest proportion of the 110mAg burden (34%), however, by day 8, less than 5% of the total radioactivity remained in this tissue. The majority of the 110mAg eliminated from the intestine appeared to distribute to the liver. Eventually, the 110mAg content in the liver accounted for as much as 65% of the total radioactivity in the fish. Apart from the liver and intestine, only the gills and carcass contained any appreciable amount (>5%) of the total body 110mAg content. Liver and gill samples were fractionated using differential centrifugation techniques to discern the subcellular distribution of 110mAg in these tissues. In the liver, the 110mAg levels in the cytosolic fraction increased from 35% to 72% of the total cellular burden between days 8 and 19, respectively. The radioactive pulse in the gills was predominantly found in a membrane compartment termed the nuclear fraction ( approximately 60% of the total). Little change was observed over time (day 8 to day 19) to the subcellular distribution of Ag in the gills. Using size-exclusion chromatography, most ( approximately 70%) of the 110mAg content in the liver cytosol eluted at a molecular weight characteristic of metallothionein. The cytosolic distribution of 110mAg in gills was quite diffuse, occurring primarily in the heavy molecular weight fractions.     

Effect of cations on structure-linked sedimentability of lysosomal hydrolases

1. A partially purified lysosomal preparation was obtained from mouse liver sucrose homogenates by differential and discontinuous gradient centrifugation. 2. Triton X-100 or repeated freezing and thawing of the lysosomal suspension (subfraction B) allowed comparison of free and activated values for acid phosphohydrolase, beta-glucuronidase and N-acetylglucosaminidase in the presence and absence of ascorbate. 3. The distribution of hydrolase activities between supernatant and pellet after high-speed centrifugation was measured and the percentages of total enzyme found in the supernatant were: acid phosphohydrolase, 40.7; beta-glucuronidase, 51; N-acetylglucosaminidase, 39.4. 4. Differential rates of elution of the three hydrolases from the membrane fraction occurred with increasing Na(+) and K(+) concentrations, whereas complex biphasic elution curves were obtained as a function of bivalent cation concentration with Ca(2+) and Mg(2+). 5. Sucrose-density-gradient centrifugation of frozen-and-thawed subfraction B demonstrated highly significant changes in the protein gradient profile in the presence of a low concentration of bivalent cation, indicating membrane aggregation and enzyme-membrane association. 6. The data provide further evidence for the nature of lysosomal enzyme binding and indicate the presence of different enzyme-membrane bonds conferring structure-linked latency upon individual lysosomal enzymes.     

Sterol biosynthesis in sub-cellular particles of higher plants

Mevalonic acid-2-¹⁴C was administered to cut stems of bean seedlings (Phaseolus vulgaris L.) for time intervals varying from 20 min to 24 hr. The plants were homogenized in a pH 7.8 tris-sucrose buffer and the homogenates separated into chloroplast, mitochondrial, microsomal, and supernatant fractions by means of differential centrifugation. The distribution of radioactivity into non-saponifiable material in each of the fractions was then determined. After short incubation periods labeled squalene was localized in the supernatant fraction. Labeled sterol was limited at all incubation periods to the microsomal and supernatant fractions. The data presented clearly implicate the microsomal and supernatant fractions in sterol biosynthesis in higher plants.     

Isolation and characterization of subcellular membranes from canine stomach smooth muscle

Subcellular membrane fractions were isolated from the circular muscle of the corpus of canine stomach by differential and isopycnic sucrose density gradient centrifugation. Differential centrifugation gave a mitochondrial fraction enriched (fourfold) in cytochrome c oxidase and a microsomal fraction enriched (fourfold) in 5'-nucleotidase and NADPH-cytochrome c reductase over postnuclear supernatant. On the basis of a study using continuous gradient, a discontinuous sucrose density gradient was prepared to yield F1 to F5 fractions. The F3 fraction at the interface of 18-32% (w/w) sucrose was maximally enriched (13-fold) in 5'-nucleotidase. The fraction contained very low levels of cytochrome c oxidase but did contain NADPH-cytochrome c reductase (eightfold enrichment). The F4 fraction, at the interface of 32-40% (w/w) sucrose, was maximally enriched in NADPH-cytochrome c reductase (12-fold) and cytochrome c oxidase (6-fold). The distribution of the azide-insensitive. ATP-dependent Ca2+ uptake correlated very well with that of 5'-nucleotidase but less well with NADPH-cytochrome c reductase and not at all with cytochrome c oxidase. Sodium azide and ruthenium red inhibited the ATP-dependent Ca2+ uptake by the mitochondrial fraction and postnuclear supernatant, but not by the F3 fraction. ATP-dependent Ca2+ uptake by the F3 fraction was inhibited by calcium ionophores A23187 and ionomycin, but not by the sodium ionophore, monensin. These results are consistent with the hypothesis that the plasma membrane plays a major role ih regulating intracellular Ca2+ concentration in canine corpus circular muscle.     

Perparation and comparative analytical study of cat liver microsomes]

Cat liver homogenates have been fractionated by differential centrifugation. Four particulate fractions (1 000 X g, 10 000 X g, and 145 000 X g) and a supernatant have been obtained. The biochemical composition of these fractions has been established from the assay and distribution pattern of 22 enzymatic and chemical constituents including marker enzymes for mitochondria, lysosomes, peroxisomes, plasma membranes, endoplasmic reticulum, Golgi apparatus and cell sap. The microsomal fraction was characterized by a moderate contamination with large cytoplasmic granules and by a low yield in protein and cholesterol. It contained 50 per cent of Golgi complex and about 40 per cent of plasma membranes. Morphological analysis of subcellular fractions was performed and confirmed biochemical results.     

Degradation of somatomedin A by various organ homogenates from rats

Degradative activities of somatomedin A (SMA) have been examined in various tissue homogenates of rat using trichloracetic acid precipitable radioactivity of 125I-SMA. Kidney and testis showed higher specific activities and liver and brain lower activities. They were dependent on SH reagents; 0.5 mM HgCl2 inhibited the degradative activity of liver completely and 1 mM dithiothreitol (DTT) augmented the activity slightly. The activities in liver were separated by differential centrifugation; about 90 per cent of the total activity in the whole homogenate was recovered in the supernatant fraction at 100,00 x g for 60 min, and 10 per cent in the precipitate. The pH profile of each fraction was different; that of the supernatant showed a single peak at pH 7.4 and that of the pellet revealed two peaks at pH 5.9 and 7.4. However, both fractions showed similar SH-dependency.     

Activity and subcellular distribution of protein kinase dependent on adenosine 3':5'-monophosphate in liver from normal and adrenalectomized rats.

We have examined whether glucocorticoids control the activity and (or) the subcellular distribution of protein kinase dependent on cyclic AMP (adenosine 3':5'-monophosphate), since they influence cyclic-AMP-dependent responses to other hormones. Protein kinase activity was determined in rat liver homogenates and subcellular fractions, nuclear, large granular, microsomal and supernatant obtained by differential sedimentation in 0.25 M sucrose. 63% of the tissue protein kinase activity detected in absence of cyclic AMP reside in the particulate fractions. Upon addition of exogenous cyclic AMP, protein kinase activity is stimulated 1.8, 1.2, 1.2 and 4.5-fold in nuclear, large granular, microsomal and supernatant fractions, respectively. Under these conditions, 66% of tissue activity are found in the supernatant fraction. The activity sensitive to exogenous cyclic AMP resolves into a major (84%) cytosoluble and a minor (16%) nucleomicrosomal component. The latter activity resists elution with isotonic saline and is increased in the presence of Triton X-100. Three groups of rats were studied: control and adrenalectomized with or without cortisol treatment. In whole liver homogenates, both protein kinase activity detected in absence of exogenous cyclic AMP and sensitivity of the enzyme to cyclic AMP were comparable in all groups. Moreover, the distribution patterns of proteins kinase activity amoung the fractions were essentially the same in all groups of animals, whether or not particles had been treated with Triton X-100. Finally, in cell-free experiments, glucocorticoids alone or in combination with their intracellular receptor did not modify protein kinase activity of rat liver. Thus the results reported do not support the possibility that glucocorticoids influence cyclic AMP-dependent protein kinase in rat liver. Yet, this study provides data, not available before, on subcellular distribution of this enzyme in rat liver.     

Protein and glycoprotein electrophoretic patterns of enriched fractions of primary and secondary granules from guinea pig polymorphonuclear leukocytes

The postnuclear supernatant fraction of sucrose homogenates of guinea pig polymorphonuclear leukocytes (PMNL) was subjected to differential centrifugation to obtain a total particulate fraction, a particle-free supernatant fraction, highly enriched fractions of primary and secondary granules, and a membrane-rich fraction. The various fractions were solubilized in buffer containing sodium dodecyl sulfate (SDS) and analyzed for protein and glycoproteincomponents by SDS -polyacrylamide gel electrophoresis. The major glycoprotein components of the postnuclear supernatant fraction were found mainly associated with the enriched fraction of secondary granules and, to a lesser extent, with the membrane-rich fraction. No major glycoprotein components were visible in the polypeptide electrophoretic patterns of the primary granule fraction or of the particle-free supernate. Attempts at separation of guinea pig granules by zonal sucrose density gradient centrifugation were only partially successful. Data supporting a species difference in this regard between rabbit and guinea pig PMNL granules are presented.     

Heavy metal intracellular balance and relationship with metallothionein induction in the liver of carp after contamination by silver,cadmium and mercury following or not pretreatment by zinc

Determination of metal levels (copper, zinc, cadmium, silver and mercury) in soluble and insoluble fractions of liver homogenates has been performed after 7 days exposure of carps (Cyprinus carpio) to moderate concentrations of cadmium, silver and mercury in water. Metallothionein (MT) levels have been quantified by a polarographic method before and after the contamination and a subsequent decontamination phase (7 days). The influence of pretreatment by zinc (7 days) has also been evaluated. MT level variations have been interpreted as having regard to inter-related flows of metal between subcellular fractions. Special interest has been focused on heat-stable compound (HSC)-bound heavy metal flows within the cytosol, taking in account that MT is the major component of these ligands. Our data showed differences between the ability of metals to bind cytosolic ligands and HSCs, and their respective potency for MT induction in liver. Regardless of pretreatment, mercury gave the highest increase of liver MT, but the MT level decreased during the decontamination step, especially after pretreatment by zinc. Cadmium and silver gave similar increases, but a significant difference with the control appeared only after the decontamination step with cadmium, while 1 week of contamintion was enough for silver. However, silver binding with MT was achieved only by the end of the decontamination step, while cadmium depicted the highest ratio for HSC-bound toxic metals after the contamination. Our experimental conditions gave the following order of potency for MT induction in liver: mercury silver > cadmium > zinc. Results are discussed comparatively with data obtained with carp gills.     

Subcellular distribution of S-adenosylmethionine decarboxylase in rat liver. Evidence of decarboxylation of S-adenosylmethionine separate from synthesis of spermidine.

The activity of S-adenosylmethionine decarboxylase in rat liver homogenates is localized chiefly in the crude nuclear fraction, probably associated with membrane fragments, with the remainder in the supernatant fraction. This distribution is not paralleled by the activity of the cytoplasmic enzyme, lactate dehydrogenase. The spermidine-synthesizing activity of whole homogenate is recovered entirely in the supermidine-synthesizing activity of whole homogenate is recovered entirely in the supernatant fraction. Measurement of various kinetic parameters in crude fractions provided not positive evidence for isozymes of S-adenosylmethionine decarboxylase. Some species do not possess a sedimentable fraction of S-adenosylmethionine decarboxylase activity in liver. In those species all activity present in the whole homogenate of liver is released into the supernatant fraction.     

The protective role of metallothionein in copper overload: I. Differential distribution of immunoreactive metallothionein in copper-loaded rat liver and kidney

The cytotoxicity of copper is probably determined by its molecular association and subcellular localisation rather than its concentration within tissues. Metallothionein (MT) is a copper binding protein distributed between the particulate and soluble cellular components. The role of MT in conferring protection to the copper-loaded rat has been investigated by comparing the distribution of the immunoreactive protein between the soluble and particulate fractions of liver and kidney during the development of copper tolerance. Young male Wistar rats were fed a high copper (1 g/kg) diet for 16 weeks and killed sequentially during this period; liver and kidneys were retained. Pellet and supernatant preparations from homogenised, pooled samples of liver and kidney were subjected to chromatographic separation. Copper and zinc were analysed in whole tissue, homogenates and eluant fractions and MT identified likewise using an enzyme-linked immunoassay. Copper accumulated for 5 weeks in the liver falling subsequently accompanied by similar changes in MT content. Kidney copper and MT rose to maximum concentrations at 8 weeks and were maintained thereafter. Substantial differences were apparent in the relative distribution of MT between the two organs. MT was the major, predominantly cytosolic, copper-binding protein in the kidney but in the liver immunoreactive MT was pelleted and present in lower concentration than the high molecular weight cuproproteins. It was concluded that whilst MT plays a role in the detoxification and adaptation of rats to copper-loading the regulatory functions of liver and kidney may differ significantly in this respect.     

Subellular distribution of S-adenosylamenthionine decarboxylase in rat liver. Evidence of decarboxylation of S-adenosylmenthionine separate from synthesis of spermidine

The acitivity of S-adenosylmethionine decarboxylase in rat liver homogenates is localized chiefly in the crude nuclear fraction, probably associated with membrane fragments, with the remainder in the supernatant fraction. This distribution is not paralleled by the activity of the cytoplasmic enzyme, lactate dehydrogenase. The spermidine-synthesizing activity of whole homogenate is recovered entirely in the supernatant fraction. Measurement of various kinetic parameters in crude fractions provided no positive evidence for isozymes of S-adenosylmethionine decarboxylase. Some species do not possess a sedimentable fraction of S-adenosylmethionine decarboxylase activity in liver. In those species all activity present in the whole homogenate of liver is released into the supernatant fraction.     

A cytochemical study on the pancreas of the guinea pig. I. Isolation and enzymatic activities of cell fractions

Pancreatic tissue, (guinea pig) homogenized in 0.88 M sucrose, was fractionated by differential centrifugation into a nuclear, zymogen, mitochondrial, microsomal, and final supernatant fraction. The components of the particulate fractions were identified with well known intracellular structures by electron microscopy. The fractions were analyzed for protein-N and RNA, and were assayed for RNase and trypsin-activatable proteolytic (TAPase) activity. The zymogen fraction accounted for 30 to 40 per cent of the total TAPase and RNase activities, and its specific enzymatic activities were 4 to 10 times higher than those of any other cell fraction. The zymogen fraction was cytologically heterogeneous; zymogen granules and mitochondria represented its main components. More homogeneous zymogen fractions, obtained by successive washing or by separation in a discontinuous density-gradient, had specific activities 2 to 4 times greater than the crude zymogen fractions. Chymotrypsinogen was isolated by column chromatography from pancreas homogenates and derived cell fractions. The largest amount was recovered in the zymogen fraction. The final supernatant had properties similar to those of the trypsin inhibitor described by Kunitz and Northrop.     

Studies on guanine deaminase and its inhibitors in rat tissue

1. In kidney, but not in rat whole brain and liver, guanine-deaminase activity was localized almost exclusively in the 15000g supernatant fraction of iso-osmotic sucrose homogenates. However, as in brain and liver, the enzymic activity recovered in the supernatant was higher than that in the whole homogenate. The particulate fractions of kidney, especially the heavy mitochondria, brought about powerful inhibition of the supernatant guanine-deaminase activity. 2. In spleen, as in kidney, guanine-deaminase activity was localized in the 15000g supernatant fraction of iso-osmotic sucrose homogenates. However, the particulate fractions did not inhibit the activity of the supernatant. 3. Guanine-deaminase activity in rat brain was absent from the cerebellum and present only in the cerebral hemispheres. The inhibitor of guanine deaminase was located exclusively in the cerebellum, where it was associated with the particles sedimenting at 5000g from sucrose homogenates. 4. Homogenates of cerebral hemispheres, the separated cortex or the remaining portion of the hemispheres had significantly higher guanine-deaminase activity than homogenates of whole brain. The enzymic activity of the subcellular particulate fractions was nearly the same. 5. Guanine deaminase was purified from the 15000g supernatant of sucrose homogenates of whole brain. The enzyme separated as two distinct fractions, A and B, on DEAE-cellulose columns. 6. The guanine-deaminase activity of the light-mitochondrial fraction of whole brain was fully exposed and solubilized by treatment with Triton X-100, and partially purified. 7. Tested in the form of crude preparations, the inhibitor from kidney did not act on the brain and liver supernatant enzymes and the inhibitor from cerebellum did not act on kidney enzyme, but the inhibitor from liver acted on both brain and kidney enzyme. 8. The inhibitor of guanine deaminase was purified from the heavy mitochondria of whole brain and liver and the 5000g residue of cerebellum, isolated from iso-osmotic homogenates. The inhibitor appeared to be protein in nature and was heat-labile. The inhibition of the enzyme was non-competitive. 9. Kinetic, immunochemical and electrophoretic studies with the preparations purified from brain revealed that the enzyme from light mitochondria was distinct from enzyme B from the supernatant. A distinction between the two forms of supernatant enzyme was less certain. 10. Guanine deaminase isolated from light mitochondria of brain did not react with 8-azaguanine or with the inhibitor isolated from heavy mitochondria.     

Compartmentalization of adenosine 3':5'-monophosphate and adenosine 3':5'-monophosphate-dependent protein kinase in heart tissue.

In rabbit heart homogenates about 50% of the cAMP-dependent protein kinase activity was associated with the low speed particulate fraction. In homogenates of rat or beef heart this fraction represented approximately 30% of the activity. The percentage of the enzyme in the particulate fraction was not appreciably affected either by preparing more dilute homogenates or by aging homogenates for up to 2 h before centrifugation. The particulate enzyme was not solubilized at physiological ionic strength or by the presence of exogenous proteins during homogenization. However, the holoenzyme or regulatory subunit could be solubilized either by Triton X-100, high pH, or trypsin treatment. In hearts of all species studied, the particulate-bound protein kinase was mainly or entirely the type II isozyme, suggesting isozyme compartmentalization. In rabbit hearts perfused in the absence of hormones and homogenized in the presence of 0.25 M NaCl, at least 50% of the cAMP in homogenates was associated with the particulate fraction. Omitting NaCl reduced the amount of particulate-bound cAMP. Most of the particulate-bound cAMP was probably associated with the regulatory subunit in this fraction since approximately 70% of the bound nucleotide was solubilized by addition of homogeneous catalytic subunit to the particulate fraction. The amount of cAMP in the particulate fraction (0.16 nmol/g of tissue) was approximately one-half the amount of the regulatory subunit monomer (0.31 nmol/g of tissue) in this fraction. The calculated amount of catalytic subunit in the particulate fraction was 0.18 nmol/g of tissue. Either epinephrine alone or epinephrine plus 1-methyl-3-isobutylxanthine increased the cAMP content of the particulate and supernatant fractions. The cAMP level was increased more in the supernatant fraction, possibly because the cAMP level became saturating for the regulatory subunit in the particulate fraction. The increase in cAMP was associated with translocation of a large percentage of the catalytic subunit activity from the particulate to the supernatant fraction. The distribution of the regulatory subunit of the enzyme was not significantly affected by this treatment. The catalytic subunit translocation could be mimicked by addition of cAMP to homogenates before centrifugation. The data suggest that the regulatory subunit of the protein kinase, at least that of isozyme II, is bound to particulate material, and theactive catalytic subunit is released by formation of the regulatory subunit-cAMP complex when the tissue cAMP concentration is elevated. A model for compartmentalized hormonal control is presented.     

Biliary excretion of 110mAg and its kinetics in the isolated perfused liver in rats

The biliary excretion of 110mAg in rats after i.v. administration of an aqueous solution of 110mAgNO3 (4.57 micrograms; 16kBq per rat) was studied for a period of 24 hours. The maximum rate of excretion was reached in 30th minute after the metal administration and over 70% of the silver dosed was excreted during 24 hours. Using the method of isolated perfused liver it was observed that 110mAg is rapidly taken up in the liver. During the five minutes period of the perfusion less than 50% of silver administered was found in the perfusion medium. In following minutes the level of the metal in the medium remained approximately constant. It was suggested that the rate of excretion of silver and its high uptake in the liver tissue is in connection with an unusual binding of it in the bile.     

Analog modeling of glucagon-induced autophagy in rat liver. II. Evaluation of iron labeling as a means for identifying telolysosome, autophagosome and autolysosome populations.

To evaluate the potential usefulness of iron labeling as a means for identifying the telolysome, autophagosome and autolysosome populations of rat liver, animals treated with Jectofer (iron-citric acid-sorbitol complex), or with Jectofer followed by glucagon, have been studied with a variety of biochemical and morphological methods. Differential centrifugation studies of liver homogenates revealed that the sedimentation velocity and mechanical fragility of acid phosphatase bearing particles increase with the duration of Jectofer treatment and that iron accumulates in the mitochondrial and nuclear fractions. Rate sedimentation studies confirmed the change in sedimentation velocity, which was shown to be due in part to a marked increase in particle density. Quantitative morphological analysis of liver M + L and N + M + L fractions revealed a nearly complete absence of pericanalicular dense bodies after 6–7 days of Jectofer treatment. In these fractions a new type of particle containing fine electron dense granules was seen. The mean volume of these particles was decreased and their number increased when compared to dense bodies but the general morphology and overall size distribution of the two particle classes were similar. In animals given both Jectofer and glucagon, autophagic vacuole formation was similar to that found in animals receiving only glucagon. However, the increase in osmotic fragility of acid phosphatase bearing particles usually seen after glucagon administration occurred at a significantly slower rate. Examination of paniculate fractions revealed the presence of autophagic vacuoles with (autolysosomes) and without (autophagosomes) fine dense granules. The number of autolysosomes and their relative proportion in the autophagic vacuole population were correlated with an increase in the osmotic fragility of the acid phosphatase bearing particles in the same fraction. Organelle degeneration was observed more frequently in autolysosome profiles. These results support the contention that iron labeling can be used to separate the principal particle populations participating in the autophagic response induced by glucagon.     

Flow cytometric analysis of the effects of high protein diet on isolated rat liver mitochondria

We have investigated changes that occur in mitochondria obtained from the livers of rats that had been maintained on a high protein diet (80% casein instead of 20%) for 6 months. Liver homogenates were separated by centrifugation into a mitochondrial fraction, a nuclear fraction and the supernatant fluid of the nuclear fraction (nuclear wash). Rhodamine-123 was used to selectively stain mitochondria depending upon their membrane potential. The stained organelles were processed through a flow cytometer where the fluorescent stains were excited by the 488 nm wavelength of a laser and the resultant fluorescence signals analysed. After 6 months on a high protein diet, mitochondria displayed an increase in the fluorescence associated with rhodamine-123 uptake in both mitochondrial and nuclear wash fractions, while mitochondrial fluorescence in the nuclear fraction showed a heterogeneous distribution. This was interpreted as an increase in membrane potential in most of the liver mitochondria under these nutritional conditions, with a certain degree of heterogeneity. These functional changes may be correlated with morphological alterations previously reported and show the usefulness of flow cytometry for biochemical analysis of isolated mitochondria.     

A Cytochemical Study on the Pancreas of the Guinea Pig : I. Isolation and Enzymatic Activities of Cell Fractions

Pancreatic tissue, (guinea pig) homogenized in 0.88 M sucrose, was fractionated by differential centrifugation into a nuclear, zymogen, mitochondrial, microsomal, and final supernatant fraction. The components of the particulate fractions were identified with well known intracellular structures by electron microscopy. The fractions were analyzed for protein-N and RNA, and were assayed for RNase and trypsin-activatable proteolytic (TAPase) activity. The zymogen fraction accounted for 30 to 40 per cent of the total TAPase and RNase activities, and its specific enzymatic activities were 4 to 10 times higher than those of any other cell fraction. The zymogen fraction was cytologically heterogeneous; zymogen granules and mitochondria represented its main components. More homogeneous zymogen fractions, obtained by successive washing or by separation in a discontinuous density-gradient, had specific activities 2 to 4 times greater than the crude zymogen fractions. Chymotrypsinogen was isolated by column chromatography from pancreas homogenates and derived cell fractions. The largest amount was recovered in the zymogen fraction. The final supernatant had properties similar to those of the trypsin inhibitor described by Kunitz and Northrop.     

Chelation in metal intoxication. XXVIII: Effect of thiochelators on mercury (II) toxicity: pre- and post treatment

The effect of treatment with alpha-mercapto-beta-(2-furyl)acrylic acid (MFA), N-(N-mercaptopropionyl) glycine (MPG) and N-acetylcysteine (NAC) compared to spironolactone (SPL), a steroid, before and after 203 mercury (II) exposure, on the disposition of Hg and induction of tissue metallothionein (MT), was investigated in rats. The pretreatment with SPL, MFA and MPG enhanced faecal elimination of Hg and reduced its accumulation in liver particularly, the "heat stable fraction" resulting in lowered hepatic MT induction. Neither the renal uptake of Hg nor induction of tissue MT was affected by pre-treatment with the chelating agents; SPL and MFA causing re-distribution of Hg among the renal sub-cellular fractions. The post-Hg exposure treatment with MFA enhanced the faecal and MPG the urinary excretion of Hg. However, both the chelating agents increased the hepatic burden of Hg as reflected in the subcellular fractions and increased MT contents indicating mobilization of Hg from other tissue binding sites. The post-treatment with MPG however, depleted renal Hg as reflected by the sub-cellular distribution, without affecting renal MT levels. The results show that MFA and MPG are more promising preventive than therapeutic agents in Hg intoxication acting as metal chelators.