?Exposure to cadmium is a long standing problem in human toxicology. A principal size of action is the kidney, where cadmium produces a series of defects in tubular reabsorption of solutes, including glucose, amino acids, and phosphate. Numerous studies identify metallothionein (MT) as a key protein in the cellular speciation and toxicity of Cd2+ as well as in the metabolism of zinc and copper. Bioinorganic research on the metal clusters in MT provide a basis for understanding some of its behavior in cells. Nevertheless, neither the molecular site(s) of renal cadmium toxicity nor the detailed functional roles of metallothionein in subacute Cd toxicity and essential metal metabolism are well understood. In this context, this application sets forth an integrated set of hypotheses and specific aims. It is thought that the two domain structure of metallothionein permits the protein to function simultaneously in intracellular Zn2+ distribution and in Cd2+ sequestration. This idea will be tested in mouse renal tubule cells, using the newly defined defect of Cd2+ to cause specific inhibition of Na+-glucose co-transport without generalized cell toxicity. It will also be explored in model studies on metallothionein in order to understand chemically the cellular properties of the protein. Major questions to be addressed include (1) What are the cellular pathways affected by Cd2+ which lead to inhibition of Na+-glucose co-transport? (2) What is the cellular speciation of Cd2+ in relationship to inhibition of Na+-glucose co-transport? (3) Is the role of metallothionein in zinc metabolism perturbed by cadmium-binding? (4) What is the underlying chemistry of metallothionein that governs its reactivity in cellular zinc and cadmium metabolism and speciation? The proposal involves three senior investigators with complementary expertise, SSB in the culture and examination of the transport properties of kidney cortical cells, CFS in the bioinorganic chemistry of metallo-drugs and -proteins, and DHP in the cellular speciation of essential and toxic metals and the properties of metalloproteins. They will employ radiotracers to follow the effects of Cd2+ on Na+-glucose co-transport, transporter mRNA synthesis, transporter biosynthesis and degradation in relation to Cd2+c distribution. A focus is on its interaction with Zn-MT protein and intracellular distribution of zinc as well as the induction system for MT. Related kinetic experiments on MT and its isolated alpha- and beta-domains will be carried out to model the process of cadmium speciation and localization in cells.
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