CYSTINURIA: METABOLISM OF THE DISULFIDE OF CYSTEINE AND HOMOCYSTEINE

The isolation and identification of the asym-metrical disulfide of L-cysteine and L-homocysteine, hereafter referred to as "mixed disulfide, " were previously described (1). At that time the amino acid had been found only in the urine of patients with cystinuria. This paper attempts to explain the origin and significance of the mixed disulfide. In approach-ing this work, I first assessed the natural occur-rence of mixed disulfide. To increase the com-pound's sensitivity to detection in a complicated area of the chromatogram, it was rendered radio-active by the administration of L-methionine-S35. The radioactive mixed disulfide was isolated from the urine of a patient with cystinuria after he was fed L-methionine-S35, and the specific activities of the two sulfur atoms were compared to each other and to cystine sulfur. The mixed disulfide, synthesized with S35 was administered to a pa-tient with cystinuria, and its fate was studied. Finally, to define the role of the kidney in me-tabolism of the compound, concentrations in renal venous blood were compared with those in arterial blood. METHODS Free amino acids of urine and plasma were measured by the method of Spackman, Stein, and Moore (2). '5-labeled, mixed disulfide is 100 + 3 % recoverable when added to urine or to plasma prior to picric acid precipi-tation of proteins (3). The compound is, however, un-stable under the conditions suggested by Stein and Moore for oxidation of cysteine to cystine for chromatography (4). Accordingly, specimens were analyzed without this step, except when examination for the possible occur-rence of homocysteine was desirable. Radioactive mixed disulfide, when added to plasma, dialyzed from cellophane in a manner consistent with its being free and unbound to plasma proteins (5). Fractions of the column efflu-ent were collected by a stream-splitting technique (6)