Patterns of amino acid efflux from isolated normal and cystinotic human leucocyte lysosomes

This study describes the first direct measurements of amino acid efflux from human lysosomes. Isolated leucocyte lysosomes can be loaded with radioactive amino acids by exposure to low concentrations of the corresponding labeled amino acid methyl esters. Efflux of amino acid from the loaded lysosomes can then be determined. Conditions during loading are adjusted for each ester to permit its adequate intralysosomal hydrolysis and subsequent accumulation of the free amino acid. Relative rates of efflux were leucine approximately equal to phenylalanine greater than methionine greater than tryptophan much greater than cystine. Efflux of leucine, tryptophan, or cystine was independent of exogenous cation, ATP, or amino acid concentrations under the conditions tested. Leucine efflux was similar in normal and cystinotic lysosomes, providing strong evidence that isolated cystinotic lysosomes do not manifest a generalized defect in amino acid efflux. In both normal and cystinotic lysosomes, cystine efflux was much slower than efflux of leucine or other amino acids from human or rat liver lysosomes. Significant differences in mean cystine efflux between isolated normal and cystinotic lysosomes were not apparent in the present test system, although the possibility of differences in rats could not be excluded.