Hepatocellular uptake of peptides—I: Carrier-mediated uptake of hydrophilic linear peptides with renin inhibitory activity into isolated rat liver cells

Abstract

The hepatic uptake of a hydrophilic, cationic linear peptide with renin inhibitory activity [5(4- amino- piperidyl -1- carbonyl) - l- 2,6[ 3H]phenyl- alanyl- β- alanyl- (4 S- amino- 3 S- hydroxy -5- cyclohexyl)- pentan-carbonyl- l-isoleucyl-aminomethyl-4-amino-2-methyl-pyrimidine-citrat] (code number EMD 56133; EMD, E. Merck, Darmstadt) was investigated in isolated rat hepatocytes. EMD 56133 was taken up by isolated rat liver cells in a time-, concentration-, energy- and temperature-dependent manner. The uptake was a combination of diffusion and a carrier-mediated process. EMD 56133 was accumulated 4.5-fold in liver cells. Eighty-three per cent of the accumulated peptide was found in the cytosol, not bound to membrane proteins. Seventeen per cent was associated with membrane proteins after cell fractionation and centrifugation at 100,000 g. The permeability coefficient of the non-saturable uptake of EMD 56133 was P = 1.973 × 10 −6 cm/sec. The kinetic constants for the carrier-mediated transport are K m = 92 μM and V max = 128 pmol/mg × min. Various substrate analogs inhibited the uptake of EMD 56133. AS-30D ascites hepatoma cells and Reuber hepatoma cells did not accumulate EMD 56133. The absence of oxygen or a decreased cellular ATP content blocked the hepatocellular uptake of the renin inhibitor. Temperatures above 20° increased the transport; the activation energy was determined to be A app = 41 kJ/mol. The apparently active uptake of EMD 56133 was not sodium dependent. In contrast, the membrane potential might be a driving force for the transport of the positively charged EMD 56133.