Separate and shared lysosomal transport of branched and aromatic dipolar amino acids

Abstract

Transport systems analogous to the T and L carriers for aromatic and bulky dipolar amino acids in plasma membranes have been characterized in the membranes of intact lysosomes isolated from human fetal skin fibroblasts. While system L appears ubiquitous in plasma membranes, system T has previously been discriminated only in the plasmalemma of human red blood cells and freshly isolated rat hepatocytes. Our findings with the lysosomal systems, provisionally designated tandl, reveal both shared and dissimilar properties with the plasma membrane systems. These properties include a lack of dependency on extralysosomal Na +, differential sensitivities to the classical system L analog, 2-aminobicyclo[2.2.1]heptane-2-car☐ylic acid (BCH), and the system T analog, d-tryptophan, as well as susceptibility to thiol modification at the membrane by reactivity with N-ethylmaleimide. A transport system in lysosomes from the FRTL-5 rat thyroid cell line has been described by Bernar et al. ((1986) J. Biol. Chem. 261, 17107–17112) resembles a composite of both carrier systems reported in this work.