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.