An evaluation of the in vivo metabolism of cystine in Escherichia coli using stable isotopes.

The incorporation of deuterated serine into cysteine during the metabolism of cystine by Escherichia coli was studied in order to determine the extent to which the carbon-sulfur bond(s) of the cystine is cleaved. The results indicate that the major route (approximately 80%) for cystine metabolism consists of a reductive cleavage of the cystine disulfide bond to form cysteine. Evidence is presented which shows that a portion of the remaining cystine is broken down by a pathway(s) which results in cleavage of the carbon-sulfur bond of the cystine. This pathway would be the same as that expected for the beta-elimination of pyruvate from cystine catalysed by the enzyme beta-cystathionase. In addition, a small portion of the resulting cysteine is shown to undergo a reversible dissociation to serine and hydrogen sulfide. Evidence is presented which shows that this dissociation is caused by the enzyme cysteine synthetase [O-acetyl-L-serine acetate-lyase (adding H2S)].