1. This paper reports an investigation of whether the dipeptides glycylsarcosine and l-glutamyl-l-glutamic acid share a single mediated transport mechanism into hamster jejunum, or whether one of these peptides is transported in part by a transport mechanism unavailable to the other. It describes the kinetics of influx of glycylsarcosine and of l-glutamyl-l-glutamic acid into rings of everted hamster jejunum in vitro, incubations being carried out at pH 5 in order to minimize brush-border and intra-medium hydrolysis of l-glutamyl-l-glutamic acid, so that peptide transport rather than a mixture of peptide transport and transport of free glutamic acid was studied. With glycylsarcosine, brush-border and intra-medium hydrolysis are negligibly small.
2. Estimates of the simple diffusion component in transport of each peptide were made by treating each of the substrates as a competitive inhibitor of its own mediated transport (assuming that mediated transport conforms to simple Michaelis-Menten kinetics), extrapolating the observed inhibitory effect over a range of concentrations to an infinitely high concentration of inhibitor, and estimating the transport component remaining at such a concentration. This component in transport would be expected to represent transport by simple diffusion, and this assumption was supported by the observation that for glycylsarcosine the uninhibitable component in transport was linearly proportional to substrate concentration; with l-glutamyl-l-glutamic acid the observations were too few to provide this demonstration. Estimates of apparent Kt and Vmax. for mediated transport of both peptides are given. Before correction for simple diffusion, linearizing plots were clearly biphasic for both peptides; after correction for simple diffusion, they became linear, providing no evidence for transport of either peptide by more than one mediated transport system, though not excluding the possibility of multiple systems.
3. Measurement of influx of [14C]Gly-Sar over a range of concentrations both alone and in the presence of a constant concentration of Glu-Glu showed that after correction for the non-mediated component in influx of Gly-Sar (simple diffusion), influx of this peptide conformed to Michaelis-Menten kinetics and the inhibitory effect of Glu-Glu on influx of Gly-Sar appeared to be competitive. The extent of inhibition corresponded well with that predicted from the Kt values of the two peptides.
4. Measurement of influx of [14C]Gly-Sar (1 mmol/l) in the presence of a range of concentrations of Glu-Glu, with extrapolation of the inhibitory effect of Glu-Glu to an infinitely high concentration of this peptide, showed that at such a concentration mediated influx of Gly-Sar was completely abolished, influx being reduced to the simple diffusion component in total influx of [14C]Gly-Sar. Measurement of influx of [14C]Glu-Glu (1 mmol/l) in the presence of a range of concentrations of Gly-Sar, with extrapolation of the inhibitory effect of Gly-Sar to an infinitely high concentration of this peptide, showed that at such a concentration mediated influx of Glu-Glu was completely abolished, influx being reduced to the simple diffusion component in total influx of [14C]Glu-Glu.
5. The results are compatible with the conclusion that Gly-Sar and Glu-Glu are taken up by the absorptive cells by a single mediated mechanism. They do not exclude the possibility that these peptides are taken up by multiple common mechanisms, but they do appear to exclude the possibility that at the substrate concentration used (1 mmol/l) there is appreciable uptake of one of the peptides by a system unavailable to the other.
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