In enzymology, a cystine reductase (EC 1.8.1.6) is an enzyme that catalyzes the chemical reaction

2 L-cysteine + NAD⁺ ${\displaystyle \rightleftharpoons }$ L-cystine + NADH + H⁺

Thus, the two substrates of this enzyme are L-cysteine and NAD⁺, whereas its 3 products are L-cystine, NADH, and H⁺.

This enzyme belongs to the family of oxidoreductases, specifically those acting on a sulfur group of donors with NAD+ or NADP+ as acceptor. The systematic name of this enzyme class is L-cysteine:NAD+ oxidoreductase. Other names in common use include cystine reductase (NADH), NADH-dependent cystine reductase, cystine reductase (NADH2), and NADH2:L-cystine oxidoreductase. This enzyme participates in cysteine metabolism.

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