Hepatocellular uptake of peptides by bile acid transporters: relationship of carrier-mediated transport of linear peptides with renin-inhibiting activity to multispecific bile acid carriers

Abstract

The uptake of a linear peptide with renin-inhibiting activity (code number EMD 51921) was characterized in isolated rat liver cells. Isolated hepatocytes take up EMD 51921 in a time-, concentration-, energy- and temperature-dependent manner. Transport of the peptide follows mixed-type kinetics. Diffusion occurs at a rate of 8.123 × 10−6 cm/sec at 6° C. For the saturable part of uptake, a Km of 2.0 μM and a Vmax of 160 pmol/mg per min werer calculated. Various substrate analogues inhibit the uptake of EMD 51921. Absence of oxygen or decreased cellular ATP content (e.g., by metabolic inhibitors or xylulose) blocks hepatocellular uptake of EMD 51921. Temperatures above 20° C accelerate the uptake. The activation energy was calculated to be 58.3 kJ/mol. The apparently active uptake of EMD 51921 was not sodium dependent. The membrane potential is a driving force for the accumulation of EMD 51921. Mutual competitive transport inhibition of EMD 51921, cholate and taurocholate is indicative of a common transport system. Benzamidotaurocholate and a cyclosomatostatin analog 008, not phalloidin and iodipamide, however, considerably decrease the uptake of EMD 51921. AS 30D ascites hepatoma cells, unable to accumulate bile acids and certain cyclopeptides, also fail to transport EMD 51921. BSP, a foreign substrate of the bilirubin carrier, noncompetitively inhibits the transport of EMD 51921. The inhibition of the uptake of EMD 51921 by rifampicin, a further substrate of the bilirubin carrier, is mixed: competitive at high EMD 51921 concentrations and uncompetitive at low EMD 51921 concentrations. The uptake of rifampicin into isolated rat liver cells, however, is not influenced by EMD 51921. Substrates of the transport systems for cations, amino acids, long chain fatty acids and hexoses did not influence the transport of EMD 51921.