Beta amyloid peptides (AP), etiologically associated with Alzheimer's disease (AD), have been shown to inhibit both glutamine synthetase (GS) and creatine phosphokinase (CPK) in vitro. These two enzymes are affected in AD and are sensitive to oxidative stress. Residue 35 of the AP25-35, the most potent section of the 4 0 4 2 amino acid long fragment of amyloid precursor protein (APP), is a methionine, which has been reported to be oxidized to methionine sulfoxide presumably via a free radical oxidation process. We questioned whether methionine sulfoxide would inhibit GS and CPK directly and if this inhibition also involved free radical oxidative stress. In this report, we demonstrate that methionine sulfoxide inhibits GS by about 50% and CPK by about 25% at 20 mM concentration. Neither intact SOD, nor ascorbate inhibit the action of methionine sulfoxide completely, with regard to the inactivation of GS. These results indicate that the action of methionine sulfoxide may not be directly due to the oxidation of GS by free radicals. In fact, the presence of exogenous proteins, such as denatured SOD or catalase, inhibit the action of methionine sulfoxide as, or more effectively than, the addition of active free radical antioxidant enzymes.