Cystine occurrence

The molecular weight of this enzyme was found (by analysis of its constituent amino acids) to be 15.700. It was found to consist of a single polypeptide chain, internally cross-linked by four cystine residues, as evidenced by the lack of any drop in molecular weight to accompany (he oxidation of all four cystines to cysteic acid, and also by the occurrence of only one terminal -NHj group, and one terminal -COOH group. 

Our findings in this study are in harmony with the concept that L-cystathionine is an intermediate in the formation of cystine from methionine in man. Direct evidence for the existence of cystathionine in man was provided by the demonstration by Tallan, Moore, and Stein (13) of the occurrence of L-cystathionine in extracts of human brain. Moreover, cases of human cystathioninuria have been reported by Harris, Penrose, and Thomas (9) and by Frimpter, Haymovitz, and Horwith (8). The latter authors have also stated that an increased renal clearance of cystathionine is not observed in cystinuria. It is of considerable interest, however, that the mixed disulfide of L-cysteine and L-homo-... 

The presence of sulfur is probably the result of sulfur oxides in the air combining with the silver. These oxides are present whenever fossil fuels are burned, and they readily attack silver. Researchers (5) have stated that sulfur in such metallic yarns is the result of the breakdown of the silk protein, fibroin. This occurrence, however, is unlikely because silk contains almost none of the two sulfur-containing amino acids, cystine and methionine (20). The exact source of the chlorine present in the sample is less easy to explain. It is present in most, though not all,... 

An interesting feature is the frequent occurrence of essentially invariant residues on either side of hypervariable regions. In k chains these include the two half-cystines of the intrachain loop in the V region, glycine residues 99 and 101, and tryptophan at position 35. In k chains such residues include the corresponding half-cystines, tryptophan 34 and the -Phe-Gly-Gly-Gly-Thr-sequence at positions 99-103. 

The evidence given above for the key role of cystathionine in the transsulfuration reaction to form cystine from methionine would be measurably strengthened by proof of the occurrence of this compound as a product of the metabolism of methionine. In Neurospora cystathionine has been isolated from the culture medium of certain methionineless mutants and shown to be a step in the conversion of cysteine to methio-... 

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