Abstract
Study of amino acid transport in the intestine of higher animal, which had initiated into the concept of active transport and entertained the classical hypothesis of protein absorption in animal intestine, is still at a fairly primitive sgage of investigation. The present report reviews current status of knowledge and research results from the author's laboratory in this field, and aims to call readers' attention to the opportunity for definitive experiments with animal intestine. Nε-substituted L-lysine derivatives were found nontransportable and to inhibit competitively the transport of basic amino acids but entirely not of the neutral amino acid. The results were interpreted indicating the separateness of the substrate recognition and translocation processes involved in the transport mechanism. Evidence is reviewed for incompleteness of stereospecificity of transport mechanism. Those amino acids previously known to support growth and to maintain N balance in place of their L-isomers in animal nutrition were found among those amino acids which could induce positive transport potential and could be actively taken up by the intestine; the results are implicit that active transportability constitutes a prerequisite for effective utilization. Selected examples are discussed of changes in intestinal functions with different dietary conditions including semistarvation, feeding on diets containing one or two amino acids in excess of animal requirement, and daily rhythmic changes in meal-fed rats. Transport of both D- and L-isomers of various amino acids so far tested increased impartially after semistarvation. Kinetic analyses revealed that semistarvation induceda greater increase in maximum transport capacity but least effect on the affinity of the trans-port mechanisms. On the other hands excessive administration of certain amino acid lead to increase transport for the amino acid in concern; results can be regarded as an induction by the substrate amino acid. Daily rhythm of the transport mechanism was found synchronized to the time of feeding and to be possibly entrained by those rhythms occurring in the epithelial cells (protein synthesis). Some unanswered questions are reviewed regarding peptide transport in the intestine. And the emphasis is given to assess quantitative importance as well as to elucidate absorption mechanism(s) of peptide absorption, and to conduct experiments with peptide along the line of studies already carried out with amino acids.
