Molecular specificity of tubular amino acid reabsorption

The specificity of tubular reabsorption of L-arginine and L-methionine was investigated by continuous microperfusion of single proximal convolutions of the rat kidney. The following observations were made: 1) L-arginine reabsorption form the tubular lumen is saturable and can be inhibited by cycloleucine and some L-arginine derivatives with the following structure: -OOC -- CH (N+H3) -- (CH2)x -- N+H (R1) -- R2 The optimal value of x is 3 to 4. The methylene group adjacent to the nitrogen can be replaced by an oxygen atom. The radicals R1 and R2 have to permit ionization of the vicinal nitrogen. L-cysteine, L-homoserine, and diaminodicarboxylic acids do not inhibit L-arginine reabsorption. 2) L-methionine reabsorption is mainly a saturable process (Vmax = 2.5 x 10-(11) mol/cm/sec; Km = 6.1 mM). Passive diffusion does not play an important role (permeability coefficient = 2.45 x 10(-7) cm2/sec. 3) D-methionine also uses this saturable system for reabsorption. However, the affinity in this case is much smaller (Km = 23 mM). 4) L-phenylalanine, L-iso-leucine, L-ethionine, and cycloleucine seem to share this mechanism with L-methionine. In addition, the autoradiographic method was employed to determine on which side of the tubular cell competition of L-arginine and L-lysine for reabsorption occurs. Good evidence was obtained that the specific receptor for these two amino acids is located at the luminal membrane.