THE CONCENTRATIONS OF CYSTEINE AND CYSTINE IN HUMAN BLOOD PLASMA

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Abstract
A method has been developed for the determination of cystine and cysteine in the same sample of fresh blood or urine. Cysteine is determined by converting it immediately to the stable S-carboxymethyl derivative by reaction with iodoacetate. The other amino acids are not affected. The amino acid composition, including the S-carboxymethylcysteine content, is determined by ion exchange chromatography on columns of sulfonated polystyrene resins. Normal human plasma obtained from individuals in the postabsorptive state contains about 1 mg of cystine and 0.4 mg of cysteine/100 ml. The values for a single cystinuric subject were both slightly low, whereas the cystine (but not the cysteine) content of the plasma of two subjects suffering from the Fanconi syndrome was somewhat high. The normal individual excretes, in the fasting state, 10 to 15 mg of cystine and 2 to 4 mg of cysteine per liter of urine. The two Fanconi subjects, both of whom exhibited aminoaciduria, had an elevated cystine excretion, but the cysteine content of the urine was essentially normal. In a cystinuric, the cystine excretion was massive, but the cysteine concentration was slightly subnormal.

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