Zinc uptake by human placental microvillous membrane vesicles: effects of gestational age and maternal serum zinc levels.

Zinc uptake by syncytiotrophoblast microvillous membrane vesicles (SMMV) from human placentas was characterized and the effects of maternal serum zinc levels at term and of gestational age on kinetic parameters were evaluated. Zinc uptake at pH 7.2 was rapid for the first 2 min, followed by a slower increase, approaching equilibrium after 30 min. Uptake was saturable at a zinc concentration of 30 micromol/L, higher than the upper range of the physiological serum zinc level. Kinetic analysis of uptake at 1 min in SMMV from term placenta showed similar Km values (mean: 6.9+/-0.6 micromol/L) for different levels of maternal serum zinc. However, Vmax was higher (p < 0.05) in SMMV from mothers with serum zinc lower than 7.6 micromol/L compared to those with higher serum zinc levels (35.8+/-1.6 and 26.6+/-1.6 nmol 65Zn/mg protein/min, respectively). Km values were similar in term (>37 wk of gestation) and preterm (20-25 wk of gestation) placentas, whereas Vmax was higher (p < 0.05) in the preterm (34.3+/-1.6 nmol Zn/mg protein/min) compared to term placentas from mothers with serum zinc levels above 7.6 micromol/L. These results suggest that whereas afffinity for zinc was not altered with gestational age or maternal serum zinc levels, zinc-uptake capacity in human placenta is influenced both by gestational age and by low levels of maternal serum zinc in order to ensure an adequate maternal-fetal zinc transfer.

Erythrocyte metallothionein in relation to other biochemical zinc indices in pregnant and nonpregnant women.

Erythrocyte metallothionein (E-MT) is considered a promising index of zinc status in humans, since it may be more sensitive than other biochemical indices to changes in dietary zinc. However, conditions of high zinc demand with substantial redistribution of tissue zinc and specific changes in hormone profile, such as pregnancy, may have an influence on E-MT levels in addition to dietary zinc. In this study, we compared E-MT concentrations in relation to other biochemical zinc indices in healthy pregnant women at delivery (n = 40) and non-pregnant women (n = 22) with similar habitual dietary zinc intakes (average 13.3 mg/d). Pregnant women had lower serum zinc and albumin-bound serum zinc, but higher levels of alpha 2-macroglobulin-bound serum zinc than the nonpregnant women. Erythrocyte zinc (E-Zn) was similar in both groups, but E-MT (mean +/- SE) was slightly but significantly (p < 0.05) higher in the pregnant women (2.9 +/- 0.09 nmol/g protein) compared to nonpregnant women (2.6 +/- 0.06 nmol/g protein). A significant correlation was observed between E-MT and E-Zn in the nonpregnant women (r = 0.70; p < 0.001), consistent with the role of intracellular zinc in the regulation of metallothionein synthesis. However, such correlation was not observed in the pregnant women, suggesting that E-MT levels in pregnancy may be influenced by factors related to the pregnant state.

Maternal, placental and cord zinc components in healthy women with different levels of serum zinc.

Serum zinc is known to decrease during pregnancy, but the range of individual values at the end of normal gestation may be considerably large, with uncertain physiological significance in terms of maternal zinc status and maternal-fetal transfer of zinc. In this study we compared several maternal and cord blood indices of zinc status, placental zinc, placental metallothionein and the relationship between maternal, cord and placental components, in 40 healthy pregnant women at delivery with different levels of serum zinc. Subjects were divided into three groups according to serum zinc using as cutoff points the lower and upper quartile values [LZn, < 7.6 mumol/l (n = 10); MZn, 7.6-10.7 mumol/l (n = 20), and HZn, > 10.7 mumol/l (n = 10)]. Habitual zinc intakes were similar in all groups (average 11.5 mg/day). Considering all women, maternal serum and erythrocyte zinc correlated significantly (r = 0.40; p = 0.021). Maternal erythrocyte zinc was higher (p < 0.01) in HZn compared to the other groups. Maternal and cord values of serum zinc correlated significantly (r = 0.43; p = 0.006). Cord serum zinc values were very similar in LZn and MZn but were higher (p < 0.01) in HZn. Cord erythrocyte zinc levels were similar in all groups and about 20-25% of the maternal values. Cord erythrocyte metallothionein levels were also similar in all groups and similar to maternal values. Comparing the percentage distribution of zinc in maternal and cord serum fractions, major differences were observed in HZn, with zinc in albumin fraction being 70% in cord compared to 56% in maternal serum. There was a higher (p < 0.01) percentage of zinc bound to alpha 2-macroglobulin fraction in maternal serum of HZn compared to maternal serum of the other groups. Maternal and cord zinc in the albumin fractions correlated significantly (r = 0.48; p = 0.002) particularly in HZn (r = 0.71; p < 0.021). Placental zinc correlated negatively (r = -0.34; p = 0.035) with zinc in the maternal alpha 2-macroglobulin fraction, but did not relate to placental metallothionein, which had similar levels in all groups (average 0.28 nmol/g wet tissue). Placental zinc levels were lower (p < 0.01) in HZn. Our results indicate that high levels of maternal serum zinc in healthy women at delivery may be related to maternal tissue zinc redistribution that could favor diffusional components of maternal-fetal transfer of zinc.