Further Studies on the Photochemical Production of Reduced Triphosphopyridine Nucleotide and Adenosine Triphosphate by Fragmented Spinach Chloroplasts

In the course of studies on the photochemical production of TPNH and ATP by fragmented spinach chloroplasts (22), we became interested in the stoicheiometry of the reactions. It is generally assumed that the ATP: 2 e ratio is unity (2) but some workers have suggested a higher value (10, 17, 18). Ratios greater than one have seldom been obtained by direct experiment but have usually been suggested on a basis of arithmetic manipulation, or from a consideration of comparative biochemistry, or have been inferred from spectrophotometric studies. Stiller and Vennesland (20) have recently reported a thorough investigation into the ATP: 2 e ratio during the Hill reaction with ferricyanide as electron acceptor and concluded that the over-all ratio is one. In the present paper we demonstrate that under certain conditions it is possible to obtain ATP: TPNH ratios significantly greater than unity without necessarily implying that there is more than one site of phosphorylation coupled to the electron transfer chain which leads to TPN reduction. We also show that photophosphorylation in the absence of other electron acceptors can proceed at a rate comparable to other photosynthetic processes when photosynthetic pyridine nucleotide reductase (PPNR) is added to chloroplast fragments under aerobic conditions.