Differential effect of cysteine-to-serine substitutions in metallothionein on cadmium resistance

A set of mutant coding sequences for Chinese hamster metallothionein (MT) 2 in which codons for individual cysteines were replaced by serine codons was cloned into a yeast expression system. MT gene expression was placed under control of a constitutive promoter on a multicopy Escherichia coli-yeast shuttle vector. MTs were expressed in a metal-sensitive host that lacks the endogenous MT gene. The expressed MTs conferred increased metal resistance to the yeast host. A sensitive assay for cadmium resistance was developed in which population doubling times were monitored in rich liquid medium supplemented with a sublethal dose of CdCl2. Measurements on mutants with single cysteine replacements at 12 positions revealed two mutant classes. One class (Cys----Ser at position 5, 13, 19, or 33) did not affect the detoxification capacity of MT. A second class (Cys----Ser at position 7, 15, 26, 29, 44, 48, 50, or 60) conferred to the host markedly less resistance to cadmium. Bridging cysteines were more critical to cadmium resistance. All five bridging cysteine mutants studied (at positions 7, 15, 44, 50, and 60) conferred lower cadmium resistance. In contrast, mutation of four out of seven terminal cysteines (at position 5, 13, 19, or 33) was shown to be inconsequential. Mutations tend to be more detrimental in the alpha domain than in the beta domain in conveying cadmium resistance, suggesting that the contribution of individual cysteine to the detoxification function of MT is site specific.

Glial immunoreactivity for metallothionein in the rat brain

A series of frozen and vibratome coronal sections of the rat brain were examined by immunocytochemistry for the presence of a cysteine-rich metal binding protein, metallothionein (MT). Astrocytes throughout the brain and brainstem stained positively for MT; neurons and oligodendroglia were unstained. Ependymal cells and tanycyte processes in the hypothalamus were also immunoreactive, along with a narrow zone of immunopositivity along the margins of the area postrema. Gomori-positive astrocytes in the hypothalamus, identifiable by toluidine blue staining, metal-containing cytoplasmic granules, represented a subset of MT-positive astrocytes that may be involved in reactions to blood-borne metal compounds that penetrate into circumventricular organs of the brain.

Effects of dietary monounsaturated fatty acids on plasma lipids in humans

In order to test whether monounsaturated fatty acids (MUFA, M) would lower plasma cholesterol and/or triglycerides (TG), 14 young men were studied in a metabolism ward. They were given two experimental diets with the same P/S ratio (P: polyunsaturated; S:saturated fatty acids) but different P + M/S ratios in composition of dietary fatty acids. Six men were in experiment I. In two groups of three, each group was given either the high or the low P + M/S ratio diet for 6 weeks. In experiment II, two groups of four men were subjected to a 3-week cross-over feeding trial (total of 6 weeks). We found in the crossover study that when the dietary P/S ratio was fixed at 1.0, those subjects given a diet with the higher P + M/S ratio of 4.5 showed significantly higher plasma total TG, very low density lipoprotein (VLDL)-TG, low density lipoprotein (LDL)-TG, and LDL-cholesterol than when given a diet with a lower P + M/S ratio of 1.5. Plasma total cholesterol, apoprotein A-I, and apoprotein B-100 were not significantly different between the two dietary periods. Plasma fatty acid patterns showed a significantly higher plasma MUFA level during the period of a higher P + M/S diet. In contrast to some recent reports, our studies indicate that a large amount of dietary MUFA may raise some fractions of plasma lipids in humans.

Assignment of the Nance-Horan syndrome to the distal short arm of the X chromosome

There are three types of X-linked cataracts recorded in Mendelian Inheritance in Man (McKusick 1988): congenital total, with posterior sutural opacities in heterozygotes; congenital, with microcornea or slight microphthalmia; and the cataract-dental syndrome or Nance-Horan (NH) syndrome. To identify a DNA marker close to the gene responsible for the NH syndrome, linkage analysis on 36 members in a three-generation pedigree including seven affected males and nine carrier females was performed using 31 DNA markers. A LOD score of 1.662 at theta = 0.16 was obtained with probe 782 from locus DXS85 on Xp22.2-p22.3. Negative LOD scores were found at six loci on the short arm, one distal to DXS85, five proximal, and six probes spanning the long arm were highly negative. These results make the assignment of the locus for NH to the distal end of the short arm of the X chromosome likely.

Primary sequence of duck metallothionein

Only one metallothionein appears in domesticated duck upon zinc induction. The complete amino acid sequence has been elucidated. This metallothionein has the same sequence as the chicken metallothionein, as determined by chemical sequencing of overlapping peptides produced by selective proteinase digestion and confirmed by mass spectrometry. The observation that animals of divergent origins share a common gene product presents an example of extreme conservation of a stress-inducible protein.

Maternal transmission of mitochondrial DNA in ducks

Maternal transmission of mitochondrial DNA (mtDNA) has been studied in amphibians, insects and mammals, but little is known about mtDNA inheritance in the ovaripirous avian species. In this study, we have constructed the physical maps of mitochondrial genomes from two different genera of ducks (Cairina and Anas) and taken advantage of the availability of their hybrids to demonstrate that mtDNA is maternally inherited.

Complete homology in metallothionein from two genera of ducks and their hybrids

Metallothionein purified from two genera of domesticated duck, Anas platyrhnchos and Cairina muschata, and their hybrid were analyzed and shown to consist of one identical amino acid sequence. Since most vertebrates carry two or more isospecies of this inducible, thiol rich, metal sequestering protein, this finding suggests that duck metallothionein is evolutionary primitive.

Pigeon metallothionein consists of two species

Two isospecies of metallothionein, a cysteine-rich protein that binds metals, exist in all mammals examined, but only one in some invertebrates and lower animals. Lower vertebrates such as fish and birds have one or two metallothionein genes depending upon the organism. In this study, we show by amino acid sequence determinations that two isospecies of metallothionein, 75% homologous to each other, can be induced by zinc to accumulate in pigeon livers. This is in contrast to single isospecies found in chicken and duck. Each of these two sequences consists of 63 amino acids, with all 20 cysteines in positions held invariant in most if not all class I mammalian metallothioneins. One of these two pigeon isometallothioneins is terminated with histidine at the carboxyl end, which is apparently unique to avians. Its sequence differs from that of duck and chicken by only four substitutions and is the predominant isospecies that accumulates upon induction. The other pigeon metallothionein has lysine at its carboxyl terminus and is devoid of arginine. None of these isospecies carries any aromatic amino acid, which is also characteristic of all higher metallothioneins. As this is the first demonstration with sequence data that two isospecies of metallothionein indeed exist in birds, these results suggest that pigeon metallothionein genes evolved from an ancestral form through duplication and mutation upon specification.

Metallothioneins with interdomain hinges expanded by insertion mutagenesis

Specific peptides of varying lengths were inserted between the two metal cluster domains of metallothionein (MT), which normally are spanned by only three amino acids, Lys-Lys-Ser. These interdomain expansions were made to test if such structural alterations would affect MT function. These constructs were engineered by inserting defined oligonucleotides of up to four tandem repeats of dodecanucleotides and hexanucleotides into an Alu-1 endonuclease cleavage site, which separates the two exonic regions of an MT-coding sequence from Chinese hamster ovary cells, MT-2. The native and altered sequences were cloned into a high expression Escherichia coli-yeast shuttle vector and used to transform yeast cells whose endogenous MT genes had been previously deleted. Using metal resistance as a biological marker, all constructs were shown to be functional in rendering the host cells resistant to either copper or cadmium. As the inserts, by nature of their amino acid sequence, could add flexibility to the otherwise compact molecule, the two domains apparently are active independently. The level of activity, however, diminished with the length of the insert. Determinations for copy number of the chimeric plasmids and MT mRNAs in the transformed cells showed that the replicational and transcriptional capacity of the long and short constructs were equivalent.

Intracellular metallothionein concentration and the rate of zinc or cadmium influx and MT mRNA accumulation in a CHO Cdr variant

The regulation of metallothionein (MT) gene expression in a cadmium resistant CHO cell line which overproduces MT was examined in this study. Our results show that MT mRNA levels reach a maximum 24-30 h after a primary zinc exposure and, subsequently, MT mRNA concentrations decline. This decrease in MT mRNA levels can be correlated with the accumulation of metallothionein and decreased rates of zinc and cadmium uptake.

Chronic cadmium intake results in dose-related excretion of metallothionein in urine

Urinary excretion of metallothionein was measured by radioimmunoassay in rats given drinking water containing 5 or 50 mg cadmium/l for up to 2 years. The metallothionein levels corresponded to the concentration of cadmium in the drinking water and increased linearly over the course of the study. These results demonstrate that urinary metallothionein is a sensitive biological indicator of oral cadmium exposure.

Metallothionein accumulation in CHO Cdr cells in response to lead treatment

For CHO Cdr cells the presence of lead acetate in the media in concentrations above 1 mM leads to gradual cell death, as measured by the reduction of [3H]thymidine incorporation into DNA. These cells accumulate an increased amount of newly synthesized metallothionein. Typical 9S metallothionein mRNA could be detected by hybridization using metallothionein cDNA probes, with maximal accumulation occurring after 4-h exposure of cells to 2 mM lead acetate. The intracellular levels of metallothionein protein increase continually with time; metallothionein was identified by its richness in cysteine, chromatographic and electrophoretic behavior and reactiveness to carboxyamidomethylation. When separated by an anion-exchanger, both isospecies MT I and MT II could be observed, as they were identical in every respect tested to those induced by zinc chloride. The induction of metallothionein by lead was not due to an increase in intracellular zinc levels, as zinc uptake was unaffected by the presence of lead acetate in the media.

Excess energy and nitrogen balance at protein intakes above the requirement level in young men

Effects of excess energy on nitrogen balance at above the safe level of protein intake were studied in six young men. They were given test diets with a fixed protein intake of 1.2 g.kg-1.d-1 but with three successive energy (E) levels: at maintenance (1.0 E), 15% above maintenance (1.15 E) and 30% above maintenance (1.3 E), both in ascending (1.0 E, 1.15 E, 1.3 E) and descending (1.3 E, 1.15 E, 1.0 E) sequences. Duration of each dietary period was 10 d. N balance increased from 7.2 to 23.8 to 33.3 mg N.kg-1.d-1 in the ascending series and decreased from 27.8 to 17.6 to 4.8 mg N.kg-1.d-1 in the descending series. The changes in N balance per 100 kcal change in energy intake ranged between 144 and 243 mg with smaller changes at higher energy levels. Biological value and net protein utilization of the dietary protein were positively correlated with energy intake in both the ascending series and the descending series (p less than 0.001).

Age-dependent variation for inducibility of metallothionein genes in mouse liver by cadmium

Cadmium is a toxic metal that induces the expression of metallothionein genes in many tissues and that binds avidly to metallothionein, a soluble transition metal binding protein. The present study examined the temporal pattern and magnitude of accumulation of metallothionein mRNA in liver of C57BL/6J mice of various ages treated with cadmium. In adult female mice, accumulation was dependent on the dosage level of cadmium and related to the concentration of this metal in liver. The accumulation of metallothionein mRNA in liver depended on age at exposure to cadmium. Intraperitoneal administration of 2 mg of cadmium per kg provoked small increases (two- to threefold) in levels of metallothionein mRNA in livers of 7- and 14-day-old mice. In contrast, cadmium treatment of 28- and 56-day-old mice resulted in 12- to 19-fold increases in levels of metallothionein mRNA in liver with maximum increases occurring 3 to 4 hr after treatment. Because similar patterns for the accumulation of cadmium of liver were found in 7-, 28-, and 56-day-old mice, observed age-dependent differences in induction of metallothionein mRNA in liver were probably not due to differences in the accumulation of cadmium in this organ. Taken together, these data suggest that tissue-specific factors controlling the expression of metallothionein genes may account for developmental variation in the inducibility of these genes by cadmium. Ontogenic variation in accumulation of metallothionein mRNA after cadmium treatment may be a factor in developmental variation in the acute lethality of cadmium in C57BL/6J mice.

Ontogenic variation in acute lethality of cadmium in C57BL/6J mice

Susceptibility of C57BL/6J mice to the lethal effects of parenterally administered Cd declined as a function of age at exposure. The 7-day LD50 increased from 1.65 mg Cd/kg body wt in 7-day-old mice to 4.08 mg/kg in adult mice. Survival time following treatment with Cd also increased as a function of age. High constitutive concentrations of metallothionein, a transition metal-binding protein, in livers of young mice did not protect against the lethality of Cd. These results suggest that, in the mouse, the interaction between Cd and this metal-binding protein may be affected by age at exposure to this toxic metal.

Effects of moderate riboflavin deficiency on lipid metabolism in rats

Dietary riboflavin intake of the people in Taiwan has been inadequate, while the fat intake has been increasing remarkably in recent years. Therefore, the effects of a moderate riboflavin deficiency on lipid metabolism in growing young rats fed diets containing 10, 25, or 40 percent calories of fat for 5 weeks were studied. The riboflavin deficiency status of the rats was certified by increased activity coefficients of erythrocyte glutathione reductase. Serum total lipids and cholesterol levels were significantly lower (P less than 0.05) in the medium fat-riboflavin deficient group. In the high fat-riboflavin deficient group, the growth and dietary intake were depressed and the liver weight/100 g body weight increased markedly (P less than 0.001). The liver total lipids, triglycerides, cholesterol and lipid peroxides of the high fat-riboflavin deficient group showed significant increases (P less than 0.025, P less than 0.025, P less than 0.05 and P less than 0.025 respectively), as compared with the pair-fed control groups. However, the increases were not significant in the medium fat and the low fat groups. The present study indicates that a high fat-riboflavin deficient diet would have adverse effects on lipid metabolism.

Transient response of amplified metallothionein genes in CHO cells to induction by alpha interferon

Alpha interferon treatment of CHO cells elicits the rapid synthesis of many gene products, including metallothionein (MT), a protein which avidly binds heavy metals such as zinc, cadmium, and copper. Since MTs appear to have a pleiotropic role in the cell, ranging from metal detoxification to free-radical scavenging, interferon treatment may trigger a generalized defense mechanism. Activation by interferon, however, was transient, with MT mRNA being maximally detectable by a cytodot procedure within the first hour. Subsequent addition of interferon was ineffective until 7 h after the initial treatment. The action of zinc, a potent inducer of MT, however, remained independent of alpha interferon induction. The transient nature of induction by interferon was examined for altered rate of MT mRNA turnover.

Role of metallothionein in detoxification and tolerance to transition metals

Animal tolerance to the transition metals cadmium and zinc is hereditary. The evolution to a high level of resistance can be accelerated through mutation and selective pressure. We have studied inbred strains of mice and mutants of Chinese hamster ovary cells resistant to cadmium to further these understandings. Results with whole animals show that the difference in the rate and level of metallothionein accumulation is at most twofold between sensitive and resistant strains. However, with cadmium resistant CHO mutant cells, there is an over 60 fold increase in metallothionein and its mRNA upon induction. These mutants show over 60 fold amplification in metallothionein genes. These results offer a direct contrast in the correlation between elevation of metal resistance and increases in metallothionein between two genetic systems.

Transition metal-binding proteins from three Chesapeake Bay fish species

Three species of Chesapeake Bay fish were collected, and endogenous levels of metal binding protein (MBP) were determined. In addition, the induction of metal-binding proteins by cadmium was studied. Livers from freshly caught fish were extracted and chromatographed on Sephadex G-75 to resolve MBP in the 5 to 20 kdalton range. All species studied exhibit measurable but varied levels of endogenous MBPs in the molecular weight range investigated, mostly as a copper protein complex. Upon induction with cadmium, the total MBP content increased in both catfish (Ictalurus punctatus) and striped bass (Morone saxatilis), with significant amounts of cadmium bound to the protein. In white perch (Morone americana), induction of MBPs with cadmium could not be demonstrated due to the large amount of constitutive Cu-BP present, although significant quantities of cadmium were bound to MBP. Electrophoresis in polyacrylamide gel was used to further identify these MBPs. Electrochemical analysis of the MBPs by polarography indicated that the wave properties of the fish MBPs resemble that of rat metallothionein. In conclusion, these studies indicate that: MBPs are present in estuarine fish from the Chesapeake Bay; concentrations of MBPs and their inducibility by exogenous cadmium vary with species, and fish MBPs may be related to mammalian metallothionein.