The isolation of metallothionein and its protective role in cadmium poisoning

Decrease and recovery of serum zinc and bone growth of rats with chronic ingestion of 25 ppm and 50 ppm cadmium

Weanling female Wistar rats ingested 25 ppm or 50 ppm cadmium (Cd) as CdCl2 in water for 330 days. Suppression of body weight gain was observed during the early stages of ingestion. The serum zinc (Zn) concentration from Cd-treated rats was significantly decreased in both groups at 35 days. Cd groups gradually exceeded the control group in body weight gain. At the 330th day of Cd ingestion decreased Zn concentration was observed in the fibula bone, though exchangable serum Zn had almost returned to normal. The decrease of calcium (Ca) and hydroxyproline in fibulae was small. The severe decrease in serum Zn concentration during growing stages did not have a suppressing effect on bone growth. The recovery of body weight gain could be partly explained by the smaller intake of Cd water in adults than in young.

Isolation and properties of cadmium-binding protein induced in rat small intestine by oral administration of cadmium

The chromatographic properties and amino acid composition of Cd-binding protein induced in the rat small intestine by oral administration of Cd were examined. When the intestinal mucosal cytosol from rats treated orally with Cd was chromatographed on Sephadex G-75, Cd in the intestinal mucosa was bound 95.5% to protein of molecular weight about 10,000. A DEAE Sephadex A-25 chromatogram of the Cd-binding protein from Sephadex G-75 showed two major and one minor Cd-containing peaks as hepatic and renal metallothionein. The amino acid composition of the two major Cd peaks resembled that of metallothionein from other materials examined. These results indicate that the low-molecular-weight Cd-binding proteins in the small intestinal mucosa meet the criteria for classification as metallothionein, and suggest that induced intestinal Cd-binding protein may be capable of preventing the transport of orally administered Cd from the intestinal mucosa into the other organs.

Separation and purification of (Cd, Cu, Zn)-metallothionein in carp hepato-pancreas

1. Cd-binding protein was isolated from the hepato-pancreas of carp administered CdCl2 (2 mg/kg). 2. This protein had a high absorption at 254 nm derived from Cd-mercaptide binding, and a low absorption at 280 nm derived from the absence of aromatic amino acids; the authors recognized the presence of two types. 3. The amino acid composition of the proteins was determined. The contents of cysteine residues were 34.24% and 31.90% in MT-I and -II respectively. Tyrosine, phenylalanine, tryptophan, histidine, leucine and arginine residues were absent in both types.

The effects of age, sex, and zinc status on the accumulation of (copper, zinc)- metallothionein in rat kidneys

A study has been made of the distribution of copper in the kidneys of growing rats. Renal copper concentrations increased steadily with age and were greater in female than in male animals. Most of the copper was present as (copper, zinc)-metallothionein and two forms of this protein were isolated and characterized from the kidneys of mature female rats. That copper metabolism in kidneys is subject to hormonal influence was indicated by a reduction in the concentrations of copper and (copper, zinc)-metallothionein in ovariectomized rats and by an increase in their concentrations after the administration of progesterone. Concentrations of renal (copper, zinc)-metallothionein were less in zinc-deficient than zinc-adequate rats during pregnancy and after progesterone administration.

The chemical form of cadmium in microsomal and mitochondrial fractions from rat liver and kidney after long term administration of cadmium chloride

Wistar rats were given drinking water containing 250 ppm cadmium (Cd) for 12 months. After excising the kidney and liver, the organs were subfractionated into nuclear, mitochondrial, microsomal and cytosol fractions, and the chemical forms of Cd in the subcellular fractions were examined. Although approx. 90% of the total Cd was present in the cytosol, in the form of metallothionein, 3-5% was also present in the mitochondrial fraction and 5-7% in the microsomal fraction from both organs. Each membrane fraction was washed 3 times and there was no contamination of metallothionein from the cytosol according to Cd/protein ratios. By Sephadex G-75 gel filtration, after solubilizing the particulate fractions with sodium deoxycholate, approx. 89% of Cd in the microsomal fraction and 94% in the mitochondrial fraction eluted with the same retention time as that of metallothionein in both liver and kidney, while the remainder was found in a high molecular weight protein fraction. The Cd eluted with the high molecular protein fraction might be involved in dysfunctions in subcellular organelles.

Cadmium, copper, and zinc distribution in blood of rats after long-term cadmium administration

Cd, Cu, and Zn were determined in plasma and blood cells of rats given daily sc injections of 0.5 mg Cd per kilogram of body weight for 4, 8, and 15 wk. The distribution of these metals in the plasma was also examined by Sephadex G-75 chromatography. In the whole blood Cd increased continuously and reached 1.5 micrograms/ml at 15 wk. Plasma Cd was less than 2% of the total Cd in blood at first and increased to 7.8% in the last week. A significant portion of plasma Cd was found in the metallothionein fraction at 4 wk. The Cd in this fraction increased to more than 50% of the plasma Cd after the 15-wk exposure. The rest of the plasma Cd was distributed in high-molecular-weight protein fractions. Blood Cu was up to 1.3-1.4 micrograms/ml (about twice the value for controls) from 4 to 8 wk, but diminished to about a half the control value at 15 wk. These changes were greater in the plasma and parallel to the Cu contents of the ceruloplasmin fraction. A small amount of plasma Cu was found in the metallothionein fraction at 4 wk. In this fraction Cu increased to about 8% of the plasma Cu at the last week. Blood Zn remained almost unchanged at first but decreased in the last week. Most of the plasma Zn was recovered from high-molecular-weight protein fractions, but not from the metallothionein fraction. Metallothionein in the plasma contained more Cu than Cd. Plasma concentrations of Cd bound to this protein were 0.008, 0.029, and 0.104 micrograms/ml, and the Cu/Cd molar ratios were 9.9, 3.4, and 1.1 at 4, 8, and 15 wk, respectively.

[Influence of cadmium on calcium metabolism. Effect on cytochrome P450 (author's transl)]

Oral administration of cadmium to female rats for 6 weeks does not reduce the microsomal cytochrome P450 levels in the liver and kidneys, nor the cytochrome P450 content in the renal mitochondria.

Decreased copper content in rat kidney metallothionein and its relation to acute cadmium nephropathy

Repeated s.c. injections of cadmium chloride into rats (3 mg Cd/kg body weight, 4 times a week) caused acute and transitory tubular necrosis after 2 to 3 weeks; recovery was observed after 4 weeks despite continued loading. Kidney copper decreased for the first 3 weeks and was slightly increased after 4 weeks. Distribution profiles of cadmium (Cd), zinc (Zn), copper (Cu) on an SW column revealed that elution profiles of kidney metallothionein changed from typical kidney with high copper content to typical liver metallothioneins with tubular necrosis and restoration.

On the sensitivity of metallothioneins to oxidation during isolation

It is demonstrated that the distribution of metals among the Sephadex G-75 fractions of rat liver and horse kidney supernatant is altered by exposure to oxidizing conditions. In particular, the metals bound to metallothionein are displaced into high-molecular-weight fractions and, to a lesser extent, into the low-molecular-weight forms, under aerobic conditions. In this process, metallothionein zinc is much more labile than cadmium. An appreciable proportion of the thionein is also found in the high-molecular-weight fractions and can be recovered along with the metals by treatment with mercaptoethanol. This result shows that the distributions obtained aerobically with large cadmium content in the high-molecular-weight fractions are an artefact due to metallothionein oxidation and suggests that 'spillage' of metals such as cadmium may be due in large part to oxidative processes rather than saturation effects. Evidence is presented that disulphide-bond formation occurs as thionein becomes bound in the high-molecular-weight region and that chemical reduction is necessary to restore its normal elution behaviour. Mercaptoethanol added to the homogenates maintains the reducing conditions normally found in the cellular milieu and prevents the oxidation of the metallothionein redistribution of the metals during isolation. Under these conditions the rat liver metallothionein isolated from animals exposed to chronic low concentrations of cadmium in drinking water contains appreciable quantities of copper as well as zinc and contains much of the zinc that is present in horse kidney supernatants. Metallothionein can also be extracted from a 40 000g pellet after sonication of the pellet. Thus careful analytical studies of the sites of cadmium deposition in rat liver indicate that greater than 95% is bound to metallothionein.

Low molecular weight cadmium-and copper-binding proteins from rat kidneys

Three isoforms of rat kidney cadmium- and copper-binding proteins [(Cd, Cu)-BP 1, 2, and 3] were isolated. They contained from 75.0 up to 89.0 μg Cd/mg protein, from 7.5 up to 28.0 μg Cu/mg, and from 1.5 up to 12.0 μg Zn/mg protein. Apparent molecular weights of all three isoforms were of about 10,000. Their amino acid compositions were similar to that of rat metallothionein, with cysteine amounting to 25.8-32.7% of all amino acids.

Cadmium metabolism and toxicity in rats after long-term subcutaneous administration

Male Sprague-Dawley rats were given sc injections of Cd at 0.5 mg/kg body weight, 6 d/wk, for 22 wk. Concentrations in the liver, kidney, spleen, heart, testis, and blood were determined every week for 8 wk and at the end of 10, 12, 15, and 22 wk. Daily excretion of Cd and total protein in urine were determined every week in another series of rats given the same dose for up to 25 wk. Hepatic and renal Cd increased linearly for the first several weeks of Cd injection. The Cd concentration in the kidney leveled off at 156 microgram/g wet tissue after 7 wk, whereas hepatic Cd continued to increase for a few more weeks, reached its maximum level (330 microgram/g) at 10 wk, and then declined. Blood Cd showed a steady increase expressed by a logarithmic curve for the first several weeks and a rapid rise in response to the decline of hepatic Cd. Urinary excretion of Cd increased linearly but slightly for several weeks from the beginning of injections. In this period daily excretion of Cd remained less than 1% of the daily Cd dose. From 6 wk the Cd excretion increased rapidly and reached a plateau of about 10 microgram/d (several percent of the daily dose) with a simultaneous increase in urinary excretion of total protein. Urinary excretion of Cd showed a second sharp increase after 10 wk and reached a higher plateau level of 95 microgram/d (about 63% of the daily dose). From these findings the response of the exposed animals could be divided into three stages. The first stage was characterized by a steady increase in hepatic and renal Cd and low-level excretion in the urine. This stage was regarded as a latent period of Cd poisoning. The second stage, which developed between 5 and 7 wk, was characterized by leveling off of Cd accumulation in the kidney and increased excretion of Cd and total protein in the urine. This was an initial toxic stage represented by renal lesions. The third stage was characterized by the second sharp increase in urinary Cd excretion and an elevated level of blood Cd after 10 wk. These responses were related to a decrease in the hepatic capacity for Cd retention as a result of toxic effects of Cd on the liver.

Influence of cadmium and copper on the distribution pattern of plutonium in the rat

The results of pilot experiments on metal interactions in vivo are presented. In rats pretreated with a single injection of Cd, substantially less Pu accumulated in the liver, while significantly more was found after pretreatment with Cu. In bone, somewhat more Pu was detected after Cd and less after Cu administration. Both metals increased the retention of Pu in the kidneys and spleen. The changes were similar with 238Pu and 239Pu and were still present 1 wk after Pu administration; in the case of Cu they were dependent on the amount administered. All the changes described concern solely the tissue distribution pattern of Pu; the total amount of Pu retained in the organs studied remained the same. The possible relation of Pu uptake to induced synthesis of Cd-thionein and Cu-chelatin is discussed.

Depression of metallothionein in fetal rat liver following maternal cadmium exposure

Pregnant rats were injected subcutaneously on day 10 of pregnancy with 0.0, 0.25, 0.50, 1.0 or 3.0 mg cadmium/kg and sacrificed at term (day 21). There were no fetal or maternal deaths following the cadmium exposure with the exception of the e.0 mg/kg level where a 54% fetal mortality rate was observed. At doses of 1.0 mg/kg or less, non-specific parameters of fetal toxicity including body weight, crown-rump length, and liver weight were not significantly different from control values. The same was true of term placental weight as well as maternal weight gain over the 48-h treatment period. Following gel-filtration of hepatic cytosols from control fetuses, over 70% of the endogenous cytosolic zinc was associated with a peak previously described as metallothionein. It was found that cadmium exposure at sub-lethal doses caused a depression in both total cytosolic zinc and metallothionein-bound zinc levels in the fetus, whereas both these parameters increased in the maternal liver. In vitro cadmium saturation prior to gel filtration revealed that the cadmium-binding capacity of the metallothionein peak was significantly reduced at all dosage levels in the fetus but increased in maternal liver. These findings suggest that maternal administration of cadmium can depress fetal metallothionein levels and cytosolic zinc levels at doses which do not produce overt fetal toxicity. This reduction in fetal metallothionein is in sharp contrast with the well established finding of induction of metallothionein seen in the adult following exposure to cadmium.

Isolation and partial characterization of a cadmium-binding protein from the American oyster (Crassostrea virginica)

American oysters (Crassostrea virginica) were exposed to 0.1 ppm cadmium for 0--15 days in a flowing seawater system and then placed into clean flowing seawater for 24 h prior to sacrifice. Whole oysters were homogenized and a cadmium-binding protein isolated and purified by a process of centrifugation, heat-treatment, Sephadex G-75 chromatography, DEAE cellulose chromatography and disc gel electrophoresis. A highly anionic protein which is not present in control oysters was found to be present in cadmium-exposed animals after 3 days of treatment and to increase in concentration at succeeding time points. The protein does not extensively bind zinc or copper. Amino acid analysis of the purified protein disclosed an amino acid composition characterized by a high percentage of dicarboxylic amino acids and relatively little cysteine.

Nutritional and environmental factors affecting metallothionein levels

A number of elements including zinc, cadmium, mercury and copper will enhance the synthesis of MT, but only zinc and cadmium are deposited with this protein at higher levels than with other proteins in the liver when these are given orally. However, mercury and copper are deposited with MT in the kidney regardless of whether given orally or by injection. As the level of zinc is increased in the diet a corresponding increase of zinc concentration occurs only in the MT fraction of tissue cytosols. Zinc disappears from the MT fraction to the supplementation levels within 3 to 4 days when rats which have been fed a high zinc diet are changed to a zinc deficient diet. The evidence indicates that MT is involved with zinc metabolism. The half-life of MT is longer when cadmium is used to induce its synthesis than when zinc is used. Dietary sulfur deficiency limits the synthesis of MT. Various stresses, in which strenuous exercise, cold environment, and CC14 injection were most effective, and infections were found to enhance MT synthesis, suggesting a role for it in the defense mechanism.

Effect of chronic low-level cadmium intoxication on the Haversian remodeling system in dogs

This experiment was carried out in order to determine whether the chronic administration of low doses of cadmium resulted in an alteration of the haversian bone remodeling system in dogs. Two pairs of littermate beagles were administered 25 ppm cadmium chloride in their drinking water for 6 months. Four beagles matched for age and sex from the same colony served as controls. By means of fluorescent labeling, we measured haversian bone remodeling parameters according to the techniques described by Frost. Statistical analysis of the results showed significant changes at the 0.01 level in: activation frequency, appositional rates, and number of osteoid seams. At the 0.05 level, significant differences were found in the number of resorption spaces and the bone formation rate. In the absence of other evidence indicative of an alteration in the internal milieu of the dogs, it is concluded that a direct toxic action of cadmium on the mechanisms of activation of cells responsible for the creation and formation of new haversian systems cannot be excluded.

Metal-binding studies of metallothioneins in Menkes kinky hair disease

Two species of metallothioneins were isolated from both normal and Menkes kinky hair disease (MKHD) patient livers. Atomic absorption determination of metals indicated that the patient liver metallothioneins had lower copper and cadmium content than normals. Isotope exchange studies, carried out by incubating native metallothioneins with copper-64 or cadmium-109 demonstrated a decreased affinity for copper and an increased affinity for cadmium in both MKHD metallothioneins. An hypothesis is proposed in which metallothionein functions as an intracellular copper carrier and is responsible for the transport of copper between the cells and the surrounding. Change in the copper affinity of the metallothioneins was suggested to be the major abnormality in MKHD.

Induction of cadmium-thionein in isolated rat liver cells

The uptake of cadmium by isolated liver cells was linearly related to the cadmium concentration to which the cells were exposed in the medium. Cadmium-treated cells synthesized proteins de novo with the characteristics of cadmium-thionein induced in the liver of cadmium-treated animals. Thionein from liver cells incorporated cadmium and [35S]cysteine, had a Ve/Vo (Sephadex G-50) of 1.8-1.9, and was separated into two subfractions by DEAE-cellulose ion-exchange chromatography. Cycloheximide and actinomycin D when added after a cadmium exposure prevented the synthesis of thionein. However, addition of actinomycin D after synthesis had started only decreased the total amount of thionein synthesized. The concentration of cadmium to which the cells were exposed affected the amount of cadmium-thionein synthesized in 6h. The maximum response occurred when cells were exposed to 0.5 microgram of cadmium/ml; at higher metal concentrations the total amount of cadmium-thionein synthesized declined. The system described in the present paper can be used to study the mode of metal toxicity and the mechanism of cadmium-thionein synthesis.

Biomedical responses of rats to chronic exposure to dietary cadmium fed in ad libitum and equalized regimes

Forty 100 g male rats were fed, in groups of eight, either 0, 5, or 25 ppm cadmium in a purified diet for 14 wk. Three groups were fed each of the levels of cadmium on an ad libitum basis. Two other groups were fed either 0 or 5 ppm cadmium in amounts that were equalized to that consumed by the 25 ppm group fed ad libitum. Cadmium ingestion decreased daily diet consumption, weight gain, and terminal body weight. These parameters were not significantly different in rats whose diet consumption was equalized. Packed cell volume and serum iron as well as serum zinc were decreased in the rats fed 25 ppm cadmium. These effects were not related to diet intake. No major differences were observed in serum ceruloplasmin, glucose, protein, leucine aminopeptidase activity, or copper in any of the groups. Blood urea nitrogen and renal leucine aminopeptidase activity were decreased by cadmium ingestion in the rats fed ad libitum only. In contrast, serum alkaline phosphatase activity was elevated by cadmium in the equalized-intake groups only. Cadmium and zinc concentrations were elevated and the iron concentration was decreased in the kidney, liver, and intestinal mucosa of the cadmium-fed rats irrespective of level of diet consumption. The increased uptake of cadmium in these tissues was coincident with the increased content of the cadmium-binding protein, metallothionein, in the cytosol fraction. The results indicate that some parameters of chronic cadmium toxicity are associated with diet consumption whereas others are not.

Cadmium levels in hair and other tissues during continuous cadmium intake

Rats that received cadmium 300 ppm in drinking water (average daily cadmium intake = 4.5 mg/rat) for 12 wk attained peak cadmium levels of 112, 34, and 19 mug/g in hair, liver, and kidney, respectively, at week 4. Rats that ingested cadmium 200 ppm (average daily cadmium intake = 3.6 mg/rat) for 13 wk attained peak cadmium levels of 29 mug/g in kidney at week 7, and 94 and 27 mug/g in hair and liver, respectively, at week 9. Despite continuous exposure to the heavy metal, tissue cadmium concentrations declined to steady-state levels of 24-33 mug/g in hair and 10-17 mug/g in liver and kidney. Histopathologic effects were not observed in liver or kidney. In contrast to cadmium in hair, blood cadmium levels, which remained consistently low (less than 0.04 mug/ml) throughout the study, did not correlate with changes in cadmium levels in liver and kidney.