AJP - Renal Physiology, Vol 259, Issue 4 559-F564, Copyright © 1990 by American Physiological Society
Bilitranslocase localization and function in basolateral plasma membrane of renal proximal tubule in rat
M. M. Elias, G. C. Lunazzi, S. Passamonti, B. Gazzin, M. Miccio, G. Stanta, G. L. Sottocasa and C. Tiribelli
Dipartimento Biochimica, Biofisica e Chimica delle Macromolecole, Universita di Trieste, Italy.
Bilirubin and phthalein dyes are taken up by the liver via a
carrier-mediated mechanism operated at least in part by bilitranslocase
(BTL). Because they also undergo renal transport, the presence and function
of BTL was investigated in rat renal tubular plasma membrane vesicles.
Transport of sulfobromophthalein (BSP) was enriched in basolateral domain
of plasma membrane and followed the distribution pattern of
Na(+)-K(+)-ATPase but not of gamma-glutamyltransferase. BSP uptake was
inhibited by addition of monospecific antibodies raised against hepatic
BTL. As in liver vesicles, BSP transport was electrogenic, being greatly
accelerated by addition of valinomycin in presence of an inwardly directed
K+ gradient. Apparent Km of BSP transport was 17 +/- 2 microM (n = 3
expts), one order of magnitude higher than that measured in liver; however,
Vmax was similar to that described in liver vesicles (429 +/- 18 nmol
BSP.mg protein-1.min-1, n = 3 expts). Competitive inhibition was observed
with both unconjugated bilirubin (Ki, 2.9 +/- 0.2 microM) and rifamycin SV
(Ki, 76 +/- 10 microM), known competitors for hepatic BTL-mediated
transport of BSP. Immunoblotting studies with anti-BTL monospecific
antibodies revealed presence of a single positive band only in
basolateral-enriched membrane fraction; its apparent molecular mass was 37
kDa, virtually identical to that of hepatic protein. Immunohistochemistry
confined presence of BTL to renal proximal tubules (RPT) We conclude that
BTL is present in basolateral plasma membrane of RPT cells. Lower affinity
of renal, compared with hepatic protein, for substrates might explain the
marginal role of kidney in plasma clearance of bilirubin and cholephilic
dyes.
