ZINC TRANSPORT IN MONKEY RENAL BRUSH-BORDER MEMBRANE-VESICLES AND ITSINTERACTION WITH CADMIUM - A KINETIC-STUDY

Mechanism of zinc uptake was studied using brush border membrane vesic les isolated from monkey kidney cortex. A Na+ electrochemical gradient (extravesicular intravesicular) did not stimulate the initial rate of zinc uptake. In brush border membrane vesicles, zinc uptake was shown to consist of zinc transport into an intravesicular space. Substantia l binding of zinc (10 nmol/mg protein) was noted at zero intravesicula r space, which may be functionally associated with zinc transport. Sub strate-velocity analysis of zinc uptake over the range of zinc concent ration from 0.25 mM to 2 mM demonstrated a single saturable transport system with an apparent affinity constant (Km) = 0.80 mM and Vmax = 75 nmol zinc/mg protein/5 min. Furthermore, kinetic analysis of inhibito ry effect of cadmium on zinc transport revealed that the type of inhib ition was competitive. The apparent Km for zinc was 1.6 mM in the pres ence of 2 mM CdCl2. An apparent inhibitory constant (Ki) for cadmium w as 2.2 mM. The inhibitory effect of sulfhydryl blocking agents on zinc transport suggests the involvement of sulfhydryl groups present in me mbrane carriers in zinc translocation. Temperature-sensitive transmemb rane movement of zinc was found. Arrhenius plot showed the break point at 27-degrees-C. The apparent energy of activation as 11.04 KCal/mol for the slope below break point at 5.64 KCal/mol for the slope above b reak point for zinc transport. The kinetic characteristics indicate in volvement of membrane carrier in zinc transport in monkey renal brush border membrane. (C) 1993 Wiley-Liss, Inc.