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 mu mol/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 K-m values (mean: 6.9 +/- 0.6 mu mol/L) for different levels of maternal serum zinc. However, V-max was higher (p < 0.05) in SMMV from mothers with serum zinc lower than 7.6 mu mol/L compared to those with higher serum zinc levels (35.8 +/- 1.6 and 26.6 +/- 1.6 nmol Zn-65/mg protein/min, respectively). K-m values were similar in term (>37 wk of gestation) and preterm (20-25 wk of gestation) placentas, whereas V-max 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 mu mol/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.