Figure 5
DMPS is a very effective chelating agent for the removal of inorganic mercury from the kidneys. What is of interest is that the primary sight of action of DMPS is also the primary sight where both inorganic and organic forms of mercury accumulate, namely along the proximal tubule. However, the mechanisms by which DMPS reduces the renal tubular burden of mercury has been elucidated only recently. After systemic treatment with DMPS, both DMPS and mercuric conjugates of DMPS (DMPS-Hg, 2 DMPS-2 Hg, and/or other forms) are present in the blood. These compounds are filtered readily at the glomerulus and delivered to the luminal compartment of proximal tubules. They are also delivered to the basal compartment of proximal tubules via blood flow. Because of the polar negative charge associated with the sulfonate group of DMPS, it appears that neither DMPS nor mercuric conjugates of DMPS are transported readily at either the luminal or basolateral plasma membrane of proximal tubular cells. It is not surprising that mercuric conjugates of DMPS are not transported at the luminal plasma membrane because of their negative charge. However, it is surprising that mercuric conjugates of DMPS are not readily transported by the organic anion transporter, despite the fact that DMPS itself is readily transported into proximal tubular cells by this transport system (details of which are described in the legend for Fig.4). Current evidence indicates that the therapeutic actions of DMPS involves the following steps: 1) DMPS (in a reduced and/or oxidized state) is taken up avidly at the basolateral membrane by the organic anion transporter. 2) The reduced form of DMPS interacts with, competes for, and then removes mercuric ions bonded to a host of potential molecules, which include proteins, metallothioneins (MT), glutathione (GSH), cysteine (Cys), and others (R-Hg-R). 3) Once a mercuric ion becomes bonded to DMPS and a sufficient intracellular concentration of mercuric conjugates of DMPS have formed to generate a gradient favoring the outward movement of these complexes, the conjugate is transported, likely in a facilitative manner, into the proximal tubular lumen (perhaps by MRP2). 4) Finally, the conjugates are excreted into the urine because they cannot be taken up by any segment of the nephron or collecting duct.