Reduction of N-acetyl methionine sulfoxide in plants

An enzymic activity which catalyzes the reduction of N-acetyl-methionine sulfoxide to l-N-acetyl-methionine has been observed in a wide variety of plant tissues. Its activity depended on the presence of dithiotreithol in the incubation medium. l-Methionine-sulfoxide was essentially inactive as a substrate. Of all the physiological reductants tested, only thioredoxin partially replaced dithiothreithol. When fractions obtained by gradient centrifugation of gently disrupted barley protoplasts were assayed for the reductase, the activity was largely associated with chloroplasts although approximately 15% was found in the cytosolic compartment. The enzyme, isolated from spinach chloroplasts, had a broad pH optima between 7.0 and 8.0, and its K_m for N-acetyl methionine sulfoxide is 0.4 millimolar. The possible participation of this ubiquitous enzyme in enzyme regulation is discussed.