Glutamine synthetase (from sheep brain) is markedly inhibited by methionine sulfone and by methionine sulfoximine. In contrast to methionine sulfone, methionine sulfoximine inhibits the enzyme irreversibly. The rate at which irreversible inhibition is established is decreased by glutamate and unaffected by ammonia (or hydroxylamine), but a mixture of both glutamate and ammonia can effectively prevent inhibition. Inhibition by methionine sulfoximine requires the presence of adenosine triphosphate and Mg2+ (or Mn2+) and is associated with the cleavage of adenosine triphosphate to adenosine diphosphate. The inhibited enzyme was separated by gel filtration and shown to contain close to 8 moles each of a phosphorylated derivative of methionine sulfoximine and adenosine triphosphate, which are tightly bound to the enzyme but which can be released by brief heating at 100° or by treatment with perchloric acid. The phosphorylated derivative of methionine sulfoximine can be converted by treatment with acid or by several phosphatases to equimolar quantities of inorganic phosphate and methionine sulfoximine. Studies on the stability of methionine sulfoximine phosphate to acid indicate that the phosphoryl moiety is not attached to the α-amino or α-carboxyl group. The findings indicate that the irreversible inhibition of glutamine synthetase by methionine sulfoximine is associated with its phosphorylation and the tight binding to the enzyme of methionine sulfoximine phosphate and adenosine diphosphate. © 1969, American Chemical Society. All rights reserved.
