Kinetic identification of a mitochondrial zinc uptake transport process in prostate cells

Abstract

Prostate cells accumulate high cellular and mitochondrial concentrations of zinc, generally 3–10-fold higher than other mammalian cells. However, the mechanism of mitochondrial import and accumulation of zinc from cytosolic sources of zinc has not been established for these cells or for any mammalian cells. Since the cytosolic concentration of free Zn 2+ ions is negligible (estimates vary from 10 −9 to 10 −15 M), we postulated that loosely bound zinc–ligand complexes (Zn-Ligands) serve as zinc donor sources for mitochondrial import. Zinc chelated with citrate (Zn-Cit) is a major form of zinc in prostate and represents an important potential cytosolic source of transportable zinc into mitochondria. The mitochondrial uptake transport of zinc was studied with isolated mitochondrial preparations obtained from rat ventral prostate. The uptake rates of zinc from Zn-Ligands (citrate, aspartate, histidine, cysteine) and from ZnCl 2 (free Zn 2+) were essentially the same. No zinc uptake occurred from either Zn-EDTA, or Zn-EGTA. Zinc uptake exhibited Michaelis–Menten kinetics and characteristics of a functional energy-independent facilitative transporter associated with the mitochondrial inner membrane. The uptake and accumulation of zinc from various Zn-Ligand preparations with log K f (formation constant) values less than 11 was the same as for ZnCl 2; and was dependent upon the total zinc concentration independent of the free Zn 2+ ion concentration. Zn-Ligands with log K f values grater than 11 were not zinc donors. Therefore the putative zinc transporter exhibits an effective log K f∼11 and involves a direct exchange of zinc from Zn-Ligand to transporter. The uptake of zinc by liver mitochondria exhibited transport kinetics similar to prostate mitochondria. The results demonstrate the existence of a mitochondrial zinc uptake transporter that exists for the import of zinc from cytosolic Zn-Ligands. This provides the mechanism for mitochondrial zinc accumulation from the cytosol which contains a negligible concentration of free Zn 2+. The uniquely high accumulation of mitochondrial zinc in prostate cells appears to be due to their high cytosolic level of zinc-transportable ligands, particularly Zn-Cit.