Molecular Homology and the Luminal Transport of Hg2+ in the Renal Proximal Tubule

Abstract

Abstract. The aim of this study was to define mechanisms involved in the luminal uptake of inorganic mercury in the kidney using isolated perfused straight (S2) segments of the proximal tubule. When mercuric conjugates of glutathione (GSH), cysteinylglycine, or cysteine (containing 203Hg2+) were perfused through the lumen, the rates of luminal disappearance flux (JD) of inorganic mercury were approximately 39, 53, and 102 fmol/min per mm, respectively. Thus, the rates of luminal uptake of mercury are greater when the mercury is in the form of a mercuric conjugate of cysteine than in the form of a mercuric conjugate of cysteinylglycine or GSH. Addition of acivicin to the perfusate, to inhibit activity of the γ-glutamyltransferase, caused significant reductions in the JD for mercury in tubules perfused with mercuric conjugates of GSH. Addition of cilastatin, an inhibitor of dehydropeptidase-1 (cysteinylglycinase) activity, caused significant reductions in the uptake of mercury in tubules perfused with mercuric conjugates of cysteinylglycine. These findings indicate that a significant amount of the luminal uptake of mercury, when mercuric conjugates of GSH are present in the lumen, is dependent on the activity of both γ-glutamyltransferase and cysteinylglycinase. Finally, the JD for mercury in tubules perfused with mercuric conjugates of cysteine was reduced by approximately 50% when 3.0 mM L-lysine or 5.0 mM cycloleucine was added to the perfusate. It is concluded that these findings indicate that at least some of the luminal uptake of mercuric conjugates of cysteine occurs at the site of one or more amino acid transporters via a mechanism involving molecular homology.

Relationships between the Renal Handling of DMPS and DMSA and the Renal Handling of Mercury

“…The findings from these isolated perfused tubule studies demonstrate that the rates of luminal uptake (disappearance flux, J D ) of mercuric ions in S2 segments of the rabbit proximal tubule are at least 2-fold greater when the segments are perfused through the luminal compartment as Cys S-conjugates than as GSH S-conjugates or CysGly S-conjugates. Additionally, the findings from these studies strongly indicate that Cys- S -Hg- S -Cys is taken up from the luminal compartment of proximal tubular segments by one or more amino acid transporters present in the luminal plasma membrane. , Transport studies performed in the presence and absence of sodium (in both luminal and basolateral compartments) provide additional evidence that the luminal uptake of Cys- S -Hg- S -Cys involves at least two separate amino acid transport systems, one (or more) dependent on the presence of extracellular sodium and the other not dependent on the presence of extracellular sodium.…”

“…provide direct evidence, linking the activity of γ-GT to the luminal uptake of Hg 2+ , when delivered as GSH S-conjugates, in proximal tubular segments. These investigators demonstrated that inhibition of the activity of γ-GT (by adding acivicin to the perfusing solution) decreases the luminal uptake (disappearance flux, J D ) and cellular accumulation of mercuric ions in the epithelial cells lining intact proximal tubular segments when the mercuric ions were delivered to the luminal compartment as GSH S-conjugates of Hg 2+ . , …”

“…Cannon et al have examined the potential for luminal transport of CysGly S-conjugates of Hg 2+ in isolated perfused S2 segments of the rabbit proximal tubule. , They demonstrated that inhibition of cysteinylglycinase with the dehydropeptidase-1 inhibitor, Cilastatin, caused significant reductions in the luminal uptake of Hg 2+ when it was in the form of a CysGly S-conjugate. These findings support the hypothesis that when Hg 2+ is conjugated to CysGly, much of the luminal absorption of Hg is linked to the actions of the dehydropeptidase-1 (i.e., cysteinylglycinase) that cleaves the peptide bond in molecules of CysGly to yield Cys and l -glycine (Gly).…”

“…These investigators demonstrated that inhibition of the activity of γ-GT (by adding acivicin to the perfusing solution) decreases the luminal uptake (disappearance flux, J D ) and cellular accumulation of mercuric ions in the epithelial cells lining intact proximal tubular segments when the mercuric ions were delivered to the luminal compartment as GSH S-conjugates of Hg 2+ . 57,81 Collectively, the aforementioned in vivo and in vitro data indicate that following exposure to inorganic forms of Hg, a significant fraction of inorganic mercuric ions is taken up by proximal tubular epithelial cells by a luminal absorptive mechanism dependent on the actions of γ-GT.…”

“…94 By far, one of the more convincing bodies of evidence implicating Cys-S-Hg-S-Cys as a transportable substrate at the luminal plasma membrane of proximal tubular cells comes from studies utilizing the isolated perfused tubule technique. 57,81 Some of the advantages of this technique are that isolated segments of the rabbit proximal tubule are perfused through the lumen in vitro under physiological and biophysical conditions similar to those found in vivo in intact kidneys. The findings from these isolated perfused tubule studies demonstrate that the rates of luminal uptake (disappearance flux, J D ) of mercuric ions in S2 segments of the rabbit proximal tubule are at least 2-fold greater when the segments are perfused through the luminal compartment as Cys S-conjugates than as GSH S-conjugates or CysGly Sconjugates.…”