Effect of desaspidin on photosynthetic phosphorylation.

Abstract

Runeberg (20) found that desaspidin acts as a powerful uncoupler of oxidative phosphorylation. Baltscheffsky and de Kiewiet (13) introduced desaspidin to the study of photosynthetic phosphorylation. They subdivided 5 systems of light-induced phosphorylation into 2 groups with respect to their sensitivity to inhibition by desaspidin. In 1 group, the rate of ATP formation was reduced by half at a very low (l 0-7 M ) concentrationl of desaspidin, whereas in the second group a 100 times greater concentration of desaspidin was required to give the same degree of inhibition. In the present investigationi, the inhibitory action of desaspidin was used to test further the validity of the subdivision of photosynthetic phosphorylation by chloroplasts into 2 distinct types: a type discovered in 1954 (3, 6) anid later designated as cyclic photophosphorylationi (8). in wvhich ATP formation by illuminated chloroplasts proceeds with no consumption of either an electron donor or an electron acceptor, and a type discovered in 1957 (8) and later renamed noncyclic photophosphorylation (2), in which ATP formation is linked stoichiometrically with the consumption of an electron donor and an electron acceptor. The electron donor was water, and its oxidation liberated 02; the electron acceptor was either TIPN or ferricyanide (8. 9). The terms cyclic and noncyclic were introduced to denote the coupling of phosphorylation to a light-induced closed or open electron flow that liberates the requisite free energy for the synthesis of ATP (2). Sensitivity to desaspidin inhibition was also used in the present investigation to test the validity of characterizing pseudocyclic phosphorylation as a special case of noncyclic photophosphorylation in which an open electron flow is generated between water as the electron donor and 02 as the electron acceptor. This noncyclic electron flow from water to external 02 gives the appearance of a cyclic electron flow since manometric measuring techniques give no indication that an electron donor and acceptor are being consumed concomitantly with ATP formation. In pseudocyclic photophosphorylation the consumption of ?2 at the terminal end of the electronic pathway is balanlced by the production of 2., at the site of electron donationi by water (10). In addition, sensitivity to desaspidin inhibition proved to be useful in clarifying the type of photophosphorvlation bv chloroplaFts (19) that accompanies reduiction of TPN by illuminate(d chloroplasts in a noiuphysiological noncyclic systemii in which reduced dichlorophenol indophenol replaces water as the electroni (lonor (24). This type of photophosphorylationi w-as previously conisidered (19) to occcur at the same site as the photophosphorylation wvhiclh accompanies TPN reduction wheni water is the electroii donor (8, 9). A different interpretationi will now be proposed on the basis of the new results with desaspidin.