George F. Cahill

Carboxymethylcysteine Metabolism, its Implications on Therapy in Cystinuria and on the Methionine-Cysteine Relationship:

In continuation of our experiments on sulfur metabolism, the behavior of carboxy-methyl-S-cysteine 1 (I) was investigated. (We are indebted to Dr. L. Michaelis for suggesting this compound for study.) Carboxyl-methyl-S-cysteine, like methyl-S-cysteine 2 does not support the growth of rats on a sulfur deficient diet, indicating that (I) does not yield cysteine (IV) in the course of its metabolism. Three and six-tenths grams of (I) were administered to a normal human being, while 7.2 and 14.4 gm. respectively were given to 2 cystinurics. The substance was not toxic and yielded in the urine neither cystine nor -SH compounds. In the normal, the sulfur of (I) was only partially oxidized (40%), the larger portion (60%) being excreted as undetermined neutral S. Part of this neutral S was apparently a disulfide, since there appeared in the urine a strong cyanide-nitroprusside reaction, while the Sullivan test remained negative. The urine was discarded before the probable nature of this disulfide was realized. Following the ingestion of (I) by the cystinuric patients, about 15% of the extra sulfur excreted was inorganic sulfate and 85% undetermined neutral S. Cystine excretion remained practically unchanged as indicated by the Folin photometric method, 3 but when measured by the Sullivan and Lugg-Sullivan methods, it dropped from one gm. to 200 mg. per day. These various findings seemed to indicate that part of (I) was excreted presumably as the unchanged compound, that part of it was oxidized to yield inorganic sulfate, and that another portion was excreted as an intermediate which contained sulfur in the form of a disulfide linkage. As a result of previous experiments, 4 and from the chemical formula of (I), it was reasonable to assume that the excretion of a disulfide resulted from the oxidation of an -SH compound derived from carboxymethylcysteine.

Further Studies on Metabolism of Sulfur Compounds in Cystinuria.

∗Aided by Grant No. 302 from the Committee on Scientific Research of the American Medical Association. Some recent experiments 1 ,2 on sulfur metabolism have been confirmed on another case of cystinuria (16-year-old female from a cystinuric family). Cystine (6.4 gm.), administered orally was almost completely (98%) oxidized to inorganic sulfate and did not increase urinary cystine excretion. Cysteine (3.5 gm.), on the other hand, was largely excreted as extra cystine (45%) and only a part of it was oxidized (38% of the extra sulfur). Methionine (dl, 6 gm.) was also largely excreted as extra cystine (40%) and partly (35%) as undetermined neutral sulfur,† while only a small amount (25% of the extra sulfur) was oxidized. A preliminary experiment with 2.5 gm. of homocystine 3 indicates that in cystinuria this amino acid is oxidized (92% of the extra sulfur) and does not give rise to extra cystine in the urine. The implications of this finding will be discussed upon completion of confirmatory experiments now in progress on another patient. The cystine in the urine of this patient, as determined by our modification of the Sullivan method, increased with the ageing of the specimen in agreement with earlier observations. 4

Metabolism of Various Sulfur Compounds in Cystinuria.

A cystinuric patient was kept for a period of approximately 4 months on a standard meat free diet and his urine analyzed daily for various nitrogenous and sulfur constituents. In addition the cystine sulfur was determined by both the Folin and the Sullivan methods. During experimental periods of 3 days each the following compounds were fed in the quantities indicated containing equivalent amounts of sulfur: cystine (6.4 gm.), glutathione (16 gm.), cysteine HCl (8.8 gm., neutralized) and dl-methionine (8 gm.). The experiments were carried out in triplicate except the one with glutathione which was repeated only once. The results may be summarized as follows: Cystine experiments showed that 94% of the extra sulfur was excreted as inorganic sulfate and that there was no rise in the excretion of cystine. Glutathione experiments showed that 80% of the extra sulfur was excreted as inorganic sulfate and that there was a slight rise in the excretion of cystine which amounted to 9% of the extra sulfur. Cystine experiments showed that only 34% of the extra sulfur were excreted as inorganic sulfate while there was a large increase in the excretion of cystine, continuing for several days after the feeding, which increase in cystine excretion accounted for 66% of the extra sulfur. The analyses also indicated that additional cystine and not cysteine was being excreted. dl-Methionine experiments showed that only 34% of the extra sulfur was excreted as inorganic sulfate while there was a large increase in the excretion of cystine, continuing for several days after the feeding, which increase in cystine excretion amounted to 47% of the extra sulfur; there was also a definite increase in the excretion of undetermined (non-cystine) neutral sulfur which amounted to 19% of the extra sulfur. The analyses again indicated that additional cystine and not cysteine was being excreted.