The initial enzymic step in mercapturic acid formation is catalyzed by glutathione S-transferase.Several species of this enzyme, designated as transferases a, p, y, 6 and E on the basis of increasing isoelectric points, were isolated from human liver. Evidence is presented that each of the purified species is homogeneous with respect to sodium dodecylsulfate -gel electrophoresis. Transferases a, p and E each appear as a single band on gel electrofocusing; transferases y and 6 are present as two and three bands, respectively, with each band catalytically active. Amino acid analysis indicated the five transferases to be either very closely related or identical in this respect.All enzyme species have a molecular weight of about 48 500 and consist of two apparently identical subunits. The spectrum of substrates is the same for each although the enzymes differ slightly in specific activity. As is the case for the rat liver enzymes, each of the human transferases binds bilirubin although this compound is not a substrate.

A B S T R A C T After the intravenous injection of un

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