The transport and intracellular fate of cysteine-35 S was studied in leukocytes from nine children with cystinosis and nine control subjects of similar age in an attempt to explain the presence of 80 times the normal quantity of free cystine in cystinotic leukocytes. Two differences were observed: the cystin-
SpeculationThe increased uptake of cysteine-35 S shown by cystinotic leukocytes could be taken as valid conventional evidence for a primary defect in the transport of cysteine in cystinosis. The prompt appearance of the extra 35 S transported as intracellular cystine-3S S suggests, however, that this rapid conversion may be the primary defect and that the increased uptake of cysteine-35 S is but a compensatory mechanism for maintaining the physiological concentration of intracellular cysteine.