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Abstract
Extraction of acidified uremic serum yields an inhibitor of phenytoin binding to normal
serum proteins and improves binding to the uremic serum proteins. The extract contains
binding inhibitors (Ix) that may accumulate because of poor renal excretion and that
are presumed to compete for binding sites on albumin, the principle binding protein
in serum. If substantially bound to protein, the normal mode of excretion may be via
tubular secretion. In this study, individual extracts from six stable hemodialyzed
patients caused significant inhibition of para-aminohippurate (PAH) uptake by rat
kidney slices incubated in vitro. During a 90-min incubation, slice-to-medium ratios
fell in a curvilinear dose-related pattern to 29.8% +/- (SEM) 3.6% (N = 6) of control
values, compared with 56.4% +/- 2.1% after incubation with a similar extract from
six normal serum samples. The dose-response curve is similar to that of a known competitive
inhibitor, hippuric acid, and the average inhibitory activity of uremic extract as
derived from these curves was 3.5 times that of an extract from six normal controls.
The effect was easily reversed by washing the slices in medium containing no inhibitor.
Whole serum from these same patients had no effect on PAH uptake. These findings suggest
that Ix or other constituents of the extract may compete for the organic acid secretory
pathway in the proximal nephron. A correlation was seen between inhibition of phenytoin
binding to plasma protein and inhibition of PAH transport in the slice. Both factors
were only partially removed after prolonged dialysis in vitro, suggesting substantial
protein binding. Despite the clinical stability of these hemodialyzed patients, their
serum samples contain unidentified inhibitors of ligand binding to plasma proteins
and of organic anion transport that share some physiochemical properties.