The assimilation of tri- and tetrapeptides by human erythrocytes

Evidence is presented that tripeptides enter human erythrocytes via saturable transport system(s) at rates similar to those previously described for dipeptides (King, G.F. and Kuchel, P.W. (1985) Biochem. J. 227, 833-842) but that the transmembrane flux rates for tetrapeptides are considerably less. 1H spin-echo NMR spectroscopy was used to monitor the coupled uptake and hydrolysis of peptides by red cells, since it enabled the simultaneous measurement of the levels of substrates and products of peptidase-catalysed reactions in suspensions with haematocrits similar to those found in vivo. Weighted non-linear least-squares regression of the integrated Michaelis-Menten equation onto progress curves obtained from the hydrolysis of Tyr-Gly-Gly and Gly-Gly-Gly in RBC lysates gave Km = 2.11 +/- 0.08 and 23.4 +/- 0.9 mmol/l and Vmax = 307 +/- 3 and 905 +/- 22 mmol/h per 1 packed cells, respectively. In whole cell suspensions, the rate of hydrolysis was considerably less and was dominated by the transmembrane flux of tripeptide. Progress curve analysis thus yielded the steady-state kinetic parameters for peptide transport; the values were Km = 11.6 +/- 1.1 and 56 +/- 18 mmol/l and Vmax = 12.9 +/- 3.0 and 36.4 +/- 3.2 mmol/h per 1 packed cells, respectively, for the previously mentioned peptides. The rate of transport of the tetrapeptide Gly-Gly-Gly-Gly was considerably less than either of the tripeptides. The above mentioned steady-state kinetic parameters were used in computer simulations of the coupled uptake and hydrolysis of tripeptides by human erythrocytes under physiological conditions; these simulations revealed certain similarities between the rates of peptide uptake by erythrocytes and the intestine in vivo.