Formation of Cysteine from Adenosine 5′-phosphosulf ate (APS) in Extracts from Spinach Chloroplasts

Abstract

Summary In an assay system containing essentially extract from spinach chloroplasts, 3.3 mM glutathione, O-acetyl-L-serine, and reduced ferredoxin, 35 S-cysteine was formed from 35 S-adenosine 5′-phosphosulf ate ( 35 S-APS) at a rate of 1 to 6 nmol per mg protein per hour. Addition of non-radioactive adenosine 3′-phosphate 5′-phosphosulf ate (PAPS) to the reaction mixture did not reduce appreciably the rate of 35S-cysteine formation from 35 S-APS, indicating that APS is not phosphorylated to PAPS prior to reduction. When glutathione was replaced by dithiothreitol 35 S-cysteine was formed at comparable rates. The addition of different spinach thioredoxins did not influence 35 S-cysteine formation. Free 35 S-SO 3 2− was detected in the reaction mixture with either glutathione or dithiothreitol as active thiol.