Abstract
The alpha-methyl and alpha-ethyl analogs of methionine sulfoximine, like methionine sulfoximine, induce convulsions in mice and inhibit glutamine synthetase irreversibly; alpha-ethylmethionine sulfoximine is approximately 50% as inhibitory as methionine sulfoximine and alpha-methylmethionine sulfoximine. However, whereas alpha-methylmethionine sulfoximine and methionine sulfoximine inhibit gamma-glutamylcysteine synthetase markedly, alpha-ethylmethionine sulfoximine does not, nor does administration of the alpha-ethyl analog produce the decrease in tissue glutathione levels found after giving methionine sulfoximine or its alpha-methyl analog. The findings strongly indicate that methionine sulfoximine-induced convulsions are closely associated with inhibition of glutamine synthetase rather than with inhibition of gamma-glutamylcysteine synthetase. The alpha-alkyl methionine sulfoximine analogs cannot be catabolized via the corresponding alpha-keto or alpha-imino acids, and, like other alpha-substituted amino acids, are probably not metabolized to a significant extent in vivo; this suggests that the amino acid sulfoximine molecules themselves, rather than their metabolites, are directly involved in the induction of convulsions. Possible explanations for the reported lack of correlation between the occurrence of convulsions and the levels of glutamine synthetase activity (and its substrates and product) are considered. The findings suggest that studies on the mechanism of induction of convulsions may be extended significantly and refined in biochemical terms by the use of other structurally modified convulsant molecules.
Highlights
The a-methyl and a-ethyl analogs of methionine sulfoximine, like methionine sulfoximine, induce convulsions in mice and inhibit glutamine synthetase irreversibly; cu-ethylmethionine sulfoximine is approximately 50% as inhibitory as methionine sulfoximine and a-methylmethionine sulfoximine
MgATP and the inhibitor for 20 min; aliquots of the reaction mixtures were diluted 20-fold and assayed. a-Methylmethionine sulfoximine is an effective inhibitor of both glutamine
A-methylmethionine sulfoximine was somewhat more inhibitory than methionine sulfoximine, the small differences observed might be related to the presence of unequal amounts of the diastereoisomers in the sulfoximine preparations. a-Ethylmethionine sulfoximine inhibited glutamine synthetase; the inhibitory effect of this analog was somewhat less than that found with methionine sulfoximine
Summary
Ways that involve intermediate formation of the corresponding cu-keto and Lu-imino acids followed by y elimination. Previous studies showed that a-methylglutamate is a substrate of glutamine synthetase [23, 24] and of y-glutamylcysteine synthetase [25]. We postulated that a-alkyl derivatives of methionine sulfoximine might interact with one or both of these enzymes. We have found that both of these analogs produce convulsions; both inhibit glutamine synthetase, but only n-methylmethionine sulfoximine inhibits yglutamylcysteine synthetase
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