-8- - Transport, Storage and Homeostasis of Metal Ions

Abstract

This chapter focuses on the transport, storage and homeostasis of metal ions in the bacteria, fungi, plants, and mammals. The assimilation of metals in unicellular bacteria does not require their transport to other cell types. Iron from siderophores, ferric citrate, lactoferrin and transferrin is directly available, whereas haem iron is released by the action of haem oxygenase. Both copper and zinc is stored in cysteine-rich proteins, called metallothioneins. The expression of these metal sequestering, low-molecular weight, cysteine-rich proteins, is often induced by both monovalent Cu(I), divalent Zn(II), Ag(I) and Cd(II). Iron metabolism has been most intensively studied in S. cerevisiae, of all the fungi. The photoreduction of xylem-transported ferric carboxylates, like citrate, is an important driving force in the reduction of iron in shoots: thereafter the distribution of iron to the leaves is probably mediated again by the NA–iron complex. In fungi, copper homeostasis involves, as in mammals, some degree of post-transcriptional control of copper transporters. Zinc homeostasis involves both transcriptional and post-translational regulatory mechanisms. In mammals, under normal physiological conditions, iron is transported in serum by transferrin. About a quarter of the total body iron is stored in macrophages and hepatocytes as a reserve, which can be readily mobilized for red blood cell formation. The regulation of cellular iron homeostasis is to a large degree controlled at the level of the translation of mRNAs of proteins involved in cellular iron metabolism.