Reduction of N-Acetyl Methionine Sulfoxide in Plants

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Abstract
An enzymic activity which catalyzes the reduction of N-acetyl-methionine sulfoxide to L-N-acetyl-methionine was observed in a wide variety of plant tissues [spinach (Spinacia oleraceae), potatoes (Solanum tuberosum), leek (Allium porrum), safflower (Carthamus tinctorius), pea (Pisum sativum), castor bean (Ricinus communis) and barley (Hordeum vulgare). Its activity depended on the presence of dithiothreithol in the incubation medium. L-Methionine-sulfoxide was essentially inactive as a substrate. Of all the physiological reductants tested, only thioredoxin partially replaced dithiothreitol. When fractions obtained by gradient centrifugation of gently disrupted barley protoplasts were assayed for the reductase, the activity was largely associated with chloroplasts although .apprx. 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 Km for N-acetyl methionine sulfoxide is 0.4 millimolar. The possible participation of this ubiquitous enzyme in enzyme regulation is discussed.