Erythrocyte metallothionein as an index of zinc status in humans.

Abstract

Metallothionein concentrations in erythrocyte lysates derived from human subjects were measured by an ELISA procedure. IgG obtained from serum of sheep injected with human metallothionein 1 was used in this competitive assay. Subjects were fed a semipurified zinc-deficient diet (0.7 mg of zinc per kg of diet) for an 8-day depletion period after 3 days of acclimation. Fasting plasma zinc concentrations were reduced -7%. Metallothionein in the erythrocyte lysates was significantly decreased to 59% of the initial level by the end of the depletion period. Supplementation of these depleted subjects with zinc (50 mg) did not increase erythrocyte metallothionein levels within 24 hr. Daily supplementation of control subjects with zinc (50 mg/day) increased erythrocyte metallothionein to a 7-fold maximum within 7 days. These levels were reduced by 61% within 14 days after zinc supplementation was terminated. Incubation of rat [35S~metallothionein with human erythrocyte lysate showed a time-dependent increase in 35S soluble in 20% trichloroacetic acid, indicating degradation of the labeled protein, presumably via protease activity in the lysate. It is proposed that zinc supplementation induces erythrocyte metallothionein during erythropoiesis and that low zinc intake decreases synthesis and/or accelerates degradation of the protein in reticulocytes/erythrocytes. Metallothionein levels in erythrocytes may provide a useful index upon which to assess zinc status in humans.Methods currently available for the assessment of zinc status in humans are unsatisfactory. These methods include measuring hair and urine zinc concentrations, antibody-mediated immune responses, and platelet aggregation (1). The most common of these is the measurement of plasma zinc concentrations by atomic absorption spectrophotometry. Plasma zinc levels decrease in normal individuals fed a zinc-deficient diet (1). However, factors related to infection, disease, and pregnancy also depress the plasma zinc concentration (1). The cytokine interleukin 1 and glucocorticoid hormones are among the mediators of these metabolic effects that appear linked to expression of the metallothionein gene (2, 3). The decrease in plasma zinc concentration in these conditions may not necessarily indicate low zinc status. Rather, it represents a redistribution to some tissues, particularly the liver and bone marrow (3, 4), presumably for functions related to host defense (1).One possible measure of zinc status is metallothionein, a zinc-binding protein involved in various aspects of zinc metabolism and/or function (2). Expression of the metallothionein gene is sensitive to the dietary zinc supply in rats (5). Furthermore, RIA' data indicate that plasma and erythrocyte metallothionein 1 concentrations were reduced to very low levels within days in rats fed a zinc-deficient diet (6). In addition, although endotoxin produces a large increase in the plasma metallothionein concentration, bone marrow and erythrocyte metallothionein 1 are increased only slightly (<2-fold) (7)...

Zinc Nutrition in Developing Countries

“…Specificity is poor; levels are also affected by Fe deficiency, diurnal rhythm, and acute infection. Metallothionein is said to be much less responsive to stress and infection in erythrocytes than in plasma (Grider et al 1990), and hence may provide a useful index of Zn status.…”

Considerations for determining ‘optimal nutrition’ for copper, zinc, manganese and molybdenum

“…Early studies by Bremner and associates (Sato et al 1984;Bremner et al 1987) showed that metallothionein could be detected in plasma and erythrocytes of rats, that the levels responded as predicted to changes in Zn status, and that endotoxin treatment increased plasma but not erythrocyte metallothionein content. Cousins and associates (Grider et al 1990;Thomas et al 1992) subsequently reported that erythrocyte levels of metallothionein were increased and decreased in adult men in response to Zn supplementation and deprivation respectively. More recently, this group has developed a sensitive method using the competitive reverse transcriptase-polymerase chain reaction to quantify the level of metallothionein mRNA in human monocytes.…”

Toxicological Profile for Zinc

“…At least 3-4 days are required before an increase in metallothionein is observed. This biomarker of exposure is only useful for recent zinc exposure because the metallothionein levels decreased approximately a week after discontinuation of a 63-week supplementation of zinc (Grider et al 1990). Fourteen days after discontinuation of zinc supplements, metallothionein levels were reduced by 61%.…”

“…Increased erythrocyte metallothionein may be an index of zinc exposure in humans (Grider et al 1990). …”