The reversible inactivation of L-threonine hydratase of sheep liver by L-serine.

The action of partially purified L-threonine dehydratase of sheep liver on L-serine has been studied. This enzyme acts on L-serlne as a substrate but is rapidly inactivated in the process, partially at pH 8.9 and more completely at pH 7.2. However, incubation at pH 8.9, with or without L-threonine, leads to a gradual restoration of up to 90% of the original activity. The addition of pyridoxal phosphate to the inactive enzyme solution has no effect on the activity or rate of reactivation at pH 8.9. However, at pH 7.2, the addition of pyridoxal phosphate leads to a tripling of the small activity toward L-threonine. These observations are discussed in terms of the proposed formation of an oxazolidine ring between serine and bound coenzyme. The enzyme is inactive in an environment of low ionic strength. The immediate addition of potassium phosphate restores the activity to desalted enzyme. The amount of restoration by potassium phosphate depends on the time of its addition, for a slower, irreversible inactivation also occurs.