Zinc inhibition of mitochondrial aconitase and its importance in citrate metabolism of prostate epithelial cells

Abstract

Prostate epithelial cells possess a uniquely limiting mitochondrial (m-) aconitase activity that minimizes their ability to oxidize citrate. These cells also possess uniquely high cellular and mitochondrial zinc levels. Correlations among zinc, citrate, and m-aconitase in prostate indicated that zinc might be an inhibitor of prostate m-aconitase activity and citrate oxidation. The present studies reveal that zinc at near physiological levels inhibited m-aconitase activity of mitochondrial sonicate preparations obtained from rat ventral prostate epithelial cells. Corresponding studies conducted with mitochondrial sonicates of rat kidney cells revealed that zinc also inhibited the kidney m-aconitase activity. However the inhibitory effect of zinc was more sensitive with the prostate m-aconitase activity. Zinc inhibition fit the competitive inhibitor model. The inhibitory effect of zinc occurred only with citrate as substrate and was specific for the citrate --> cis-aconitate reaction. Other cations (Ca2+, Mn2+, Cd2+) did not result in the inhibitory effects obtained with zinc. The presence of endogenous zinc inhibited the m-aconitase activity of the prostate mitochondrial preparations. Kidney preparations that contain lower endogenous zinc levels exhibited no endogenous inhibition of m-aconitase activity. Studies with pig prostate and seminal vesicle mitochondrial preparations also revealed that zinc was a competitive inhibitor against citrate of m-aconitase activity. The effects of zinc on purified beef heart m-aconitase verified the competitive inhibitor action of zinc. In contrast, zinc had no inhibitory effect on purified cytosolic aconitase. These studies reveal for the first time that zinc is a specific inhibitor of m-aconitase of mammalian cells. In prostate epithelial cells, in situ mitochondrial zinc levels inhibit m-aconitase activity, which provides a mechanism by which citrate oxidation is limited.