Amino Acid Transport by Isolated Mammalian Renal Tubules

Transport of l-proline was studied in rabbit renal tubule segments prepared by collagenase digestion. A distinct two-limbed curve found when studying the effects of substrate concentration on transport velocity indicated the presence of at least two transport systems for proline. These systems could not be distinguished by their response to metabolic inhibitors or by changes in medium sodium concentration. The use of alanine and glycine as competitive inhibitors allowed the separation and identification of three kinetically distinct transport systems for proline. The transport system shared by the imino acids and glycine was shown to have a much greater affinity and a much smaller capacity for proline than for glycine. The relevance of this analysis to the investigation of human proline transport mutants is discussed.