γ‐Cystathionase of Rat Liver

With 5′,5‐dithiobis(2‐nitrobenzoic‐acid), 8 of the 12–SH groups in native γ‐cystathionase react reversibly, but 4 form stable bonds and the inhibited product (I) can be isolated. The presence of homoserine, one of the substrates, does not affect the formation of I. 2. Cystathionine, at high concentrations, partially relieves the inhibition; at low concentrations, a different inhibited product, or products, (II) is formed. In form I but not II, the enzyme is protected from reduction by borohydride. 3. Dithiothreitol decomposes both I and II and 65 to 85% of the original enzyme activity can be recovered. Dithiol releases the anion, 2‐nitro‐5‐thiobenzoate, from I but not from 11.