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The Amino Acid Composition of Crystalline ⁵-3-Ketosteroid Isomerase and Its Tryptic Peptides

From the ¹ From the Department of Pharmacology and Experimental Therapeutics, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205

The amino acid composition of crystalline ⁵-3-ketosteroid isomerase of Pseudomonas testosteroni is consistent with the formula Asp₃₈, Thr₂₁, Ser₁₆, Glu₃₈, Pro₁₅, Gly₂₉, Ala₆₈, Val₄₅, Met₈, Ile₁₂, Leu₂₆, Tyr₉, Phe₂₅, Lys₁₃, His₉, Arg₂₂. Eleven distinct peptides have been obtained after digestion of the heated enzyme by trypsin. The tryptic peptides have been separated by ion exchange chromatography and electrophoresis and their composition has been determined. They contain between 5 and 33 amino acid residues. The sum of the composition of the amino acid residues of these peptides is in good agreement with one-third of the total amino acid composition of the protein. Evidence is presented suggesting that the isomerase consists of three chains, which may be identical. The NH₂-terminal and the COOH-terminal peptides of the protein have been identified. The NH₂-terminal residue of each tryptic peptide was determined by preparing the fluorescent 1-dimethylaminonaphthalene-5-sulfonamide derivative.

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