An alternative or archaic spelling of cysteine; a sulfur-containing amino acid important for protein structure and antioxidant defense.
From Greek 'kystis' (bladder), named because it was first discovered in bladder stones; 'cystein' is an older spelling variant.
The reason cysteine is named after bladder stones shows how early biochemists discovered compounds through medical problems—they literally found amino acids in kidney and bladder calculi before understanding what they were.
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