The Protective Effect of Thiosulfate upon the Inactivation of Aspartate

The inactivation of aspartate aminotransferase in the course of the incubation with l‐serine O‐sulfate is protected by thiosulfate. It has been found that also l‐cysteine sulfinic acid is an inactivator when it is incubated with the enzyme, while l‐serine‐O‐phosphate and l‐cysteine‐S‐sulfonate are inactive. The inactivation by l‐cysteine sulfinic acid is increased by the simultaneous addition of 2‐oxoglutarate and is protected by thiosulfate. Cysteine‐S‐sulfonate has been detected in the incubates of the enzyme with l‐serine‐O‐sulfate protected with thiosulfate. It has been concluded that the protection by thiosulfate is due to the β‐addition of thiosulfate to the inactivator aminoacrylic acid. The resulting cysteine‐S‐sulfonate thus represents an inactive dead‐end product formed in the course of the protection by thiosulfate. It is suggested that this reaction could play the role of protecting enzymes sensitive to aminoacrylic acid and of inserting inorganic sulfur into aminoacrylic acid, producing a cysteine derivative.