John F.H. Thompson

The reduction of S-methyl-l-cysteine sulfoxide and l-methionine sulfoxide in turnip and bean leaves

Abstract 1. 1. It has been established that S- ethyl- l -cysteine sulfoxide and methionine sulfoxide are reduced in leaf tissues. The development of methods for incubating leaf discs under anaerobic conditions and of handling thioethers with minimal oxidation was an essential prerequisite to this finding. 2. 2. Under anaerobic conditions, labeled methylcysteine sulfoxide and methionine sulfoxide were reduced in turnip leaves (Brassica rapa) to methylcysteine and methionine. Under either aerobic of anaerobic conditions, the same reductions were demonstrated in bean leaves (Phaseolus vulgaris). 3. 3. In the turnip, the (−) diastereoisomer of methylcysteine sulfoxide was reduced more rapidly than the (+) isomer. In the bean, the (+) diastereoisomer was reduced more rapidly than the (−) isomer.

The synthesis of O-acetylserine by extracts prepared from higher plants

Abstract This communication describes a novel plant enzyme, which catalyzes the formation of O-acetylserine from serine in the presence of S-acetylCoA. The enzyme activity was shown to be present in both the particulate and soluble fractions of a range of plant seedlings and leaves. The enzymatic synthesis of cysteine from O-acetylserine and sulfide (reaction 1) has been reported in Escherichia coli , yeast, and Neurospora , (Wiebers and Garner, 1967) . O-acetylserine +sulfide → cysteine + acetate (1) Cysteine synthase present in higher plant extracts also apparently utilizes O-acetylserine more readily than serine as a sulfide acceptor ( Giovanelli and Mudd, 1967 , Giovanelli and Mudd, 1968 ; Thompson and Moore, 1968 ). An enzyme, serine acetylase, which catalyzes the acetylation of serine (reaction 2) has been reported in bacteria (Kredich and Tomkins, 1966) . S-acetylCoA + serine → O-acetylserine +CoASH (2) The present communication reports the occurrence of serine acetylase in higher plant extracts.