IN-VITRO ADSORPTION OF AMINO-ACIDS ONTO ISOLATED RAT ERYTHROCYTE-MEMBRANES

Amino acids adsorbed onto blood cell membranes represent about 8% of t he total amino acids in blood. The aim of this study was to determine the in vitro adsorption kinetics of different amino acids (L-alanine, glycine, L-glutamate, L-glutamine, L-phenylalanine and L-leucine) onto rat erythrocyte membranes and to assess the effect of 24-hr starvatio n on these adsorption kinetics. Isolated red cell membranes were incub ated at 37 degrees C for 10 sec in the presence of C-14-amino acids-wi th different specific radioactivity-the radioactivity retained in the membrane fraction measured and kinetic parameters of amino acid adsorp tion determined. With the exception of glutamate, where the adsorption was negligible, all amino acids studied were adsorbed onto isolated r ed cell membranes, adhering to simple Michaelis-Menten kinetics. K-m v alues of glycine, phenylalanine and leucine adsorption in control rats (14.7 +/- 3.8 mM, 8.41 +/- 0.95 mM and 4.65 +/- 0.46 mM respectively, SEM, n = 6-8) decreased in response to 24-hr starvation, giving the f ollowing values: 0.792 +/- 0.122 mM, 5.32 +/- 0.82 mM and 3.53 +/- 0.3 1 mM respectively (SEM, n = 6-8), V-max value of glycine adsorption of control rats decreased (from 61.0 +/- 15.5 mmol/mol P/sec to 4.25 +/- 0.70 mmol/mol P/sec, SEM, n = 7) and that of leucine increased (from 13.5 +/- 1.0 mmol/mol P/sec to 18.9 +/- 2.0 mmol/mol P/sec, SEM, n = 7 ) as an effect of 24-hr starvation. This study shows that alanine, gly cine, glutamine, phenylalanine and leucine, but not glutamate, adsorbe d onto erythrocyte membranes according to Michaelis-Menten-like kineti cs. Furthermore, the adsorption was dependent on the nutritional statu s of the animal, which was consistent with a possible physiological ro le for the pool of amino acid bound to erythrocytes.