Hepatic metallothionein as a source of zinc and cysteine during the first year of life

Premature infants fed mothers' milk to 6 months corrected age demonstrate adequate growth and zinc status in the first year

The objective of this investigation was to describe zinc status to 12 months corrected age in premature infants fed their mother's milk in relation to nutritional management in hospital and post-hospital discharge. Twenty-five premature infants fed their mother's milk in hospital were randomized to receive either a multi-nutrient fortifier (MNF), providing protein, calcium, phosphorus and zinc (MM + MNF) or calcium and phosphorus alone (MM + CaGP). Twelve preterm infants fed a preterm formula (PTF) served as a comparison group. At 35 weeks post-menstrual age zinc retention was determined using the stable isotope tracer 70Zn. After hospital discharge infants in MM + MNF and MM + CaGP were designated to a mother's milk-feeding group to 6 months corrected age (Post-MM) or formula feeding group (Post-FF) based on parental choice of feeding practice. Anthropometry was performed at term, three, 6 and 12 months corrected age. At 6 and 12 months corrected age a hair sample was obtained to determine hair zinc concentrations. Preterm infants receiving supplemental zinc in hospital, as MNF, had significantly greater zinc retention in hospital compared to MM + CaGP but not greater hair zinc concentrations at 6 or 12 months corrected age. Despite significantly lower zinc intakes to 6 months corrected age, Post-MM had significantly greater hair zinc concentrations at 6 months compared to PTF (median[25-75th percentile]: 146[106-190] versus 85[54-91] microg/g, P < 0.05). Hair zinc in Post-FF (124[77-163] microg/g) was lower than Post-MM, but this was not significant (P = 0.09). Only in Post-MM were hair zinc concentrations above the median of reference values from term born infants at 12 months corrected age. Mean values of weight, length and head circumference of the preterm infants in Post-MM, Post-FF and PTF groups were between the 3rd and 97th percentiles derived from WHO reference growth standards for mother's milk-fed term infants. Our findings suggest that supplemental zinc either in hospital or post-hospital discharge does not appear to be required for preterm infants fed their mother's milk.

Age-dependent changes in metallothionein levels in liver and kidney of the Japanese

Samples of liver, renal cortex, and medulla were obtained from 55 forensic autopsies (0- to 95-yr-old Japanese). Metallothionein (MT) was determined by the Ag-hem or Cd-hem method. Zinc (Zn), copper (Cu), and cadmium (Cd) were determined by atomic absorption spectrophotometry. The mean levels of MT were 250 micrograms/g in the liver, and 394 micrograms/g (cortex) and 191 micrograms/g (medulla) in the kidney. Age-dependent changes were observed in both the liver and kidney. In the liver, MT level decreased during infancy and increased thereafter with age. Similar age-dependent changes in the levels of Zn and Cu were observed. In the kidney cortex, MT level increased with age, although no correlation was found after middle age. The levels of Cd and Zn also increased with age until middle age; however, they decreased thereafter. These results suggest that age-dependent changes in renal MT levels are associated with accumulation of Cd.

Relationships between iron and zinc metabolism: predictive value of digestive absorption on tissue storage

The responses of animals to intake of a trace element could vary if it is ingested with a single test meal or due to chronic intake. The metabolic relationships between zinc (Zn) and iron (Fe) were assessed in the young animal by comparing their digestive absorption studied at the beginning of the study with their tissue storage after two months of being fed on experimental diet. Diets supplied adequate intakes of Fe (45 and 300mg/kg diet) and Zn (14 and 45 mg/kg). A significant effect of Fe supply (p < 0.0001) but not of Zn was displayed on Fe absorption; both Fe and Zn diet concentrations influenced Zn absorption (p < 0.01, p < 0.0001). Fe and Zn organ contents significantly correlated with the amount absorbed during the metabolic balance (p < 0.0001). There was a positive correlation between liver, bone, and muscle Fe and Fe absorption (mg/d)(p < 0.0001), and Fe absorption and bone and muscle Zn (p < 0.04) and a negative one with liver Zn (p < 0.0001); a positive correlation was displayed between Zn absorption (mg/d) and Zn organ content (p < 0.0001). There was no correlation between Zn absorption and Fe tissue content (p > 0.05). This study suggests that interactions occur at every step of Fe and Zn metabolism; Fe is more efficient in altering Zn storage than the reverse. The organism seems to be unable to diminish the consequences of an unbalanced diet and digestive absorption. Care must be taken to give the young growing balanced diets.

Detecting subclinical deficiency of essential trace elements in children with special reference to zinc and selenium

OBJECTIVE

This article reviews situations in which we should be alert to the risk of trace element deficiencies, and the difficulties of assessing trace element status, as illustrated by states of potential subclinical deficiency of zinc and selenium in infants.

CONCLUSION

Deficiency states of many trace elements have been documented in infants, and there is probably an underdiagnosed group of infants with subclinical deficiency, from which however the great majority are protected by effective regulatory systems. Specific, sensitive, and reliable methods are needed for the detection of trace element imbalances.

Zinc homeostasis in breast-fed infants

The needs for dietary zinc are adequately met by most fully breast-fed infants despite relatively low zinc intakes in relation to estimated requirements. The objective of this study was to use stable isotope techniques to evaluate how zinc retention is achieved in normal fully breast-fed infants. Nine male infants, aged 2-5 mo, were fed expressed human milk labeled with 70Zn over a 24-h period. Complete fecal collections were obtained for 8 d. On d 4-7, a metabolic period was initiated which included test weighing and milk sampling, to measure zinc intake, and daily urine collections. Isotopic enrichment of fecal and urine samples was determined by fast atom bombardment mass spectrometry. Results included a mean (+/- SD) dietary zinc intake of 17.8 +/- 6.6 mumol/d; fractional absorption of 0.54 +/- 0.075; and total absorbed zinc of 9.5 +/- 3.5 mumol/d. Mean endogenous fecal zinc, determined on seven infants by isotope dilution, was 4.7 +/- 2.3 mumol/d, which resulted in a mean net absorption of 4.8 +/- 3.4 mumol/d. The results of the study indicated that, for fully breast-fed infants, it is the combination of a relatively high fractional absorption and efficient conservation of intestinal endogenous zinc that results in zinc retention adequate to meet the demands of growth in the face of modest intake.

Cadmium-associated renal disease

Cadmium is widely used in industry, causing exposure of workers and environmental pollution because of its persistence in the biosystems. Its very long half-life in the human organism causes its accumulation over the lifetime in liver and kidneys. Cadmium ions have a high affinity for tissue thiols, induce the synthesis of a carrier cysteine-rich polypeptide called metallothionein, and impair proteoglycan metabolism. Significant renal effects include tubular nephropathy manifested by proteinuria, amino aciduria, glucosuria, phosphaturia, and calcium wastage. Chronic sequels include decrease in the glomerular filtration rate and increased risk of kidney stone disease. Biological monitoring of cadmium absorption includes determination of urinary cadmium and of low molecular weight marker proteins, such as beta2-microglobulin or retinol binding protein, the tubular reabsorption of which is impaired before a frank proteinuria.

Zinc intake, zinc status and growth in a longitudinal study of healthy Danish infants

Mild, growth-limiting zinc deficiency might be prevalent in otherwise healthy infants according to recent studies. We examined zinc intake and status in 91 healthy term infants from birth to 12 months, as part of the Copenhagen Cohort Study on Infant Nutrition and Growth. Zinc intake was recorded monthly and the amount of zinc absorbed was estimated. These estimates were below recently published FAO/WHO/IAEA values for basal requirements in 68%, 62% and 14% of the infants at 2, 4 and 9 months of age, respectively. Serum zinc decreased significantly (p < 0.01) from 10.6 mumol/l at 6 months to 8.4 mumol/l at 9 months of age (normal range 10-18 mumol/l). Erythrocyte metallothionein values, a tentative indicator of long-term zinc status, decreased significantly from 2 to 6 months (p < 0.001) and from 6 to 9 months (p < 0.01). Serum zinc at 9 months was positively associated with growth velocity during the period from 6 to 9 months (weight: p = 0.05; knee-heel length: p = 0.002). The results provide descriptive data on zinc intake and zinc status in healthy Danish infants. Although some of our data suggest suboptimal zinc status during late infancy, evidence for this can only be obtained through a randomized intervention study.

A mutant mouse (tx) with increased hepatic metallothionein stability and accumulation

Metallothioneins (MTs) are low-molecular-mass cysteine-rich proteins implicated in metal homoeostasis and resistance to toxicity induced by heavy metals and alkylating agents. We report high hepatic MT protein accumulation (greater than 100-fold compared with wild-type mice) in toxic milk (tx) mice, along with markedly higher cytosol copper and zinc levels. Increased MT-gene transcription alone could not account for the high constitutive MT protein levels, since MT mRNA levels were not increased in tx mouse livers. However, hepatic MT was significantly more stable in adult tx mice: MT half-life (t1/2) was 79 or 77% greater than in wild-type mice before and after Cd induction respectively. Cd or Zn treatment increased MT mRNA, but not MT protein, accumulation in tx mouse livers: Cd displaced MT-bound Zn and Cu in preexisting MT. Thus tx mice appear to accumulate hepatic MT as a result of decreased protein degradation. These animals may provide a useful model to study the physiological role of MT, and human diseases (such as Wilson's disease) with abnormal copper metabolism.

Effects of inhibition of cystathionase activity on glutathione and metallothionein levels in the adult rat

The effects of alterations in sulfur metabolism on hepatic and renal metallothionein and glutathione metabolism were studied in the adult rat using inhibition of two enzymes of these pathways, hepatic cystathionase and renal gamma-glutamyl transpeptidase. Rats were fed a diet containing both methionine (0.66%) and cystine (0.20%) for 1 week before receiving three consecutive daily intraperitoneal injections of propargylglycine, a selective cystathionase inhibitor, at various doses (2.5-375 mumol/kg). When hepatic cystathionase was inhibited greater than 90% (greater than or equal to 50 mumol propargylglycine/kg), renal and hepatic metallothionein and hepatic glutathione were unaltered except at the highest dose. On the other hand, renal glutathione was increased two-fold with a concomitant decrease in renal gamma-glutamyl transpeptidase activity (50% of control). In another experiment, when renal gamma-glutamyl transpeptidase was inhibited greater than 90% with three consecutive daily injections of acivicin, a selective gamma-glutamyl transpeptidase inhibitor (10 mg/kg IP), renal glutathione content was unaltered while hepatic glutathione was decreased. Renal and hepatic metallothionein were not changed. Thus, the cysteine pools for metallothionein and glutathione appear unrelated under the present experimental conditions. In addition, following either proparglyglycine or acivicin injections, renal and hepatic glutathione pools appear to be altered differently. These results suggest that renal glutathione may be preferentially maintained even when hepatic glutathione is decreased.