Sulfur metabolism in higher plants: IV. Mechanism of sulfate reduction in chloroplasts

Abstract

This is the first report of a reduction in vitro of sulfate to sulfite by an enzyme system from higher plants. The reductive system has been found in chloroplasts of spinach leaves and compared with the yeast system. Adenosine 5′-phosphosulfate, but not 3′-phosphoadenosine 5′-phosphosulfate, was formed by the chloroplasts Since the reduction of sulfate to sulfite by chloroplasts in light requires the addition of sulfate-activating enzymes from yeast, it may be inferred that 3′-phosphoadenosine 5′-phosphosulfate is the substrate for the chloroplast sulfate reductase system. Fraction C-SS of yeast was also required. NADP did not participate in reduction in light and a NADPH 2-generating system would not substitute for the photoreduction system. The chloroplasts also reduced a low-molecular-weight protein disulfide (fraction C-SS) in light but not in darkness even with NADPH 2 as hyrogen donor. The chloroplasts contained a fraction C-SS-like substance which was reduced both by the chloroplasts in light and by fraction C-SS reductase (NADPH 2: fraction C-SS oxidoreductase) of yeast in the presence of NADPH 2. Evidence is presented for the existence of a number of non-dialyzable disulfides in chloroplasts that are photo-reducible but not reducible by the fraction C-SS reductase of yeast. The results suggest that electrons for the reduction of 3′-phosphoadenosine 5′-phosphosulfate are supplied from the photosynthetic electron-transport system through fraction C-SS or a fraction C-SS-like substance but not from NADPH 2.