Fasting plasma concentrations of cystine and of cysteine in six patients with cystinosis were not significantly different from that in four control children. Treatment with d-penicillamine or with a diet containing only the minimal requirement of cystine and methionine produced a substantial decrease in plasma cystine concentrations without affecting the appearance of the crystalline deposits in bone marrow or cornea. These findings suggest that plasma cystine is not in free equilibrium with the crystalline deposits of cystine and imply that the abnormality of cystine metabolism giving rise to cystine crystals is confined within the cells. Treatment with d-penicillamine caused a decrease in the urinary excretion of amino acid in only one of three patients, but in this patient there was continued progression of renal glomerular insufficiency.
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