The activity of a cystine transport system in lysosomes prepared from the leukocytes of patients with cystinosis was found to be deficient. In normal subjects, this system was resistant to N-ethylmaleimide and demonstrated saturation kinetics. Lysosomes from individuals heterozygous for cystinosis demonstrated a reduced maximum velocity for cystine egress from lysosomes. The rate of cystine escape from normal lysosomes was enhanced by adenosine triphosphate. The availability of normal and mutant lysosomes provides a means of investigating mechanisms of amino acid transport across lysosomal membranes.

Normal leucocyte lysosome-rich granular fractions exhibited counter-transport of cystine, confirming that cystine transport across the lysosomal membrane is carrier-mediated. The trans-activation of cystine transport was temperature-dependent but relatively independent of the external Na+ or K+ concentration in phosphate buffer. Counter-transport, measured as uptake of exogenous [3H]cystine, increased with increasing intralysosomal cystine content up to approx. 3 nmol of half-cystine/unit of hexosaminidase activity. The amount of [3H]cystine entering lysosomes loaded with unlabelled cystine decreased when unlabelled cystine was added to the extralysosomal medium. Lysosomal cystine counter-transport was stereospecific for the L-isomer. Cystathionine, cystamine and cysteamine-cysteine mixed disulphide gave evidence of sharing the lysosomal cystine-transport system, although at lower activity than cystine. Other tested amino acids, including arginine, glutamate and homocystine, were inactive in this system. Nine leucocyte lysosome-rich preparations from eight different cystinotic patients displayed virtually no counter-transport of cystine, conclusively establishing that a carrier-mediated system for cystine transport is dysfunctional in cystinotic lysosomes.

The large amount of cystine compartmentalized in cystinotic leukocytes cosediments in isopycnic sucrose density gradients with dense lysosomal particles, within which it is presumably contained. Such cystine appears to be primarily noncrystalline in these organelles.

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