AJP - Renal Physiology, Vol 256, Issue 5 901-F908, Copyright © 1989 by American Physiological Society
Cystine and lysine reabsorption in the isolated perfused rat kidney
K. A. Roby and S. Segal
Division of Biochemical Development and Molecular Diseases, Children's Hospital of Philadelphia, Pennsylvania.
Renal tubular reabsorption of cystine and lysine were studied in the
isolated perfused rat kidney to bridge the gap between in vivo clearance
studies, and in vitro transport studies of tubule fragments, cells, and
brush-border membranes. Lysine was reabsorped by a saturable transport
system shared by the dibasics. Cystine was also reabsorbed by a saturable
transport system, which was shared in part by the dibasics (maximum
inhibition 30%). The lysine threshold (Fmin) was 0.9 mumol.min-1.g-1, with
a tubular maximum (TM) of 2.4 mumol.min-1.g-1. The cystine Fmin was 0.06
mumol.min-1.g-1; the TM could not be estimated because it was above the
limit of cystine solubility. There was no evidence of cystine "secretion."
The gamma-glutamyltransferase inhibitor, AT-125, decreased cystine
excretion, but only in the presence of glutathione, glycine, glutamate, and
the diabasic amino acids. This suggests that cystine from glutathione
degradation at the brush border may contribute to urinary cystine (an
explanation of the phenomenon of cystine secretion), but only under certain
conditions.
