Absorption of amino acids and peptides
Generally, the dietary proteins are almost comp!etely digested to their constituent amino acids and these are rapidly absorbed from the intestine into the portal blood. Some of the dipeptides are hydrolyzed by peptidases located in the absorptive cells so that only amino acids are released into the portal blood. D-amino acids are absorbed by simple diffusion but the L-amino acids (occurring in foods) require a carrier system in the absorption.
The inechanism by which amino acids are absorbed is conceptually identical to that of monosaccharides. The luminal plasma membrane of the absorptive cell bears at least four sodium-dependent amino acid transporters (one each for acidic, basic, neutral and aromatic amino acids). Each system transports amino acids that are structurally similar. These transporters bind amino acids only after binding sodium. The fully loaded transporter then undergoes a conformational change that dumps sodium and the amino acid into the cytoplasm, followed by its reorientation back to the original form. Thus, absorption of amino acids is also absolutely dependent on the electrochemical gradient of sodium across the epithelium. The energy dependent carrier system also involves vitamin B6 (pyridoxal phosphate) during the transport of amino acids.