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IDENTIFICATION AND CHARACTERIZATION OF THE HEPATIC TAURINE (TR) TRANSPORTER IN THE RAT

Our objective was to identify and characterize the Na:TR co-transporter on the rat hepatic basolateral membrane (BLM). TR transport by highly purified hepatic BLM prepared from 14d and adult rats was inhibited 40-50% following exposure to the sulfhydryl (SH) modifying reagent, N-ethylmaleiraide (NEM) (250μM). Preincubation of BLM with TR (100μM) prior to exposure to NEM protected TR transport activity. In contrast, phenyl glyoxal (100-250μM) and phenylisothiocyanate (500μM), reagents which modify arginine and lysine amino groups respectively, did not inhibit Na-dependent TR transport. BLM from 14d and adult rats was analyzed by SDS-PAGE to determine if increased TR transport activity in suckling compared to adult rats is due to quantitative differences in the carrier protein. Increased band densities were noted at apparent MW of 72, 38 and 36 kDa in 14d compared to adults.Conclusion: A SH group at or near the TR binding site is essential for Na:TR cotransport by rat hepatic BLM. Preliminary work by our group has demonstrated a human placental 72 kDa protein with 34 and 38 kDa subunits associated with Na:TR cotransport. The increased expression of proteins of similar MW in suckling rat liver suggests chat they may be components of the putative TR transporter. Increased ontogenic expression in combination with labeling of a substrate protectable SH group will enable us to specifically identify the hepatic Na:TR transporter.