The ATP Levels Obtained in Photophosphorylation, Glycolysis and Oxidative Phosphorylation in Scenedesmus Titrated with Desaspidin

Abstract

The ATP levels in photophosphoryiation, glycolysisand oxidative phosphorylation, in the unicellalar green alga Scenedesmus obtttsiusculus, were titrated with narrow concentration intei-vals of desaspidin in the presence of different concentrations of 3-(3,4dicliloropheny])-l,l-dimethylurea (DCMU),, which allows the differentiation between nonHjyclic,, pseudocyclic and true cyclic photophosphorylation. The data on photophosphorylative ATP levels were compared with earlier data on total binding of phosphate. In the true cyclic process, both parameters are equally sensitive towards desaspidin. Under pseudocyclic conditions and in non-cyclic photophosphorylation, the level of ATP is more sensitive towards desaspidin than is total binding of phosphate. This suggests a structural difference between the cyclic and the two non-cyclic (one of which is also pseudocyclic) sites. The noncyclic ATP level is more sensitive towards desaspidin than is pseudocyclic. This may be connected with the higher AT1P level under pseudocyclic as compared to non-cyclic conditions.

The Effect of Light Intensity on Total Photophosphorylation and the ATP Level in Scenedesmus

“…(1963) found that the ATP formation in chloroplasts increased by several-fold by addition of CMU to a ferredoxin-catalyzed cyclic photophosphorylation in an atmosphere of argon. An increase in the ATP level caused by DCMU in Scenedesmus was reported by Kylin and Tillberg (1967b) and studied in more detail by Tillberg and Kylin (1971) and Okkeh and Kylin (1975).…”

“…The possibility which we call "pseudocyclic" Okkeh 1974, Okkeh andKylin 1975) appears to correspond to the first typeofcyclicphotophosphorylation in the classification of Trebst (1974). One can then envisage that the interactions discussed in the preceding paragraph may lead to the production of electron donors able to act between the two photosystems; so that photophosphorylation ire vivo keeps going even though the water-splitting system has been blocked by 10"^ MDCMU.…”

“…The levels of DCMU chosen for the light ititensity studies were 0, 10"' and 5 x 10"^ M. According to the titration analysis of Kylin, Sundberg and Tfllberg (1972) with DCMU, and of Kylin and Okkeh (1974) with desaspidin, these levels of BCMU should in tarn allow the complete (sites 1 -f 2 -I-3), only the pseudocyclic (site 2), and otily the true cyclic (site 3) systems to function. For further specification of these conditions, the reader is referred to the introduction of the preceding paper (Okkeh and Kylin 1975).…”

“…Chod. have been used as described earlier (Kylin, Sundberg and Tillberg 1972, Kylin and Okkeh 1974, Okkeh and Kylin 1975. Algal cells subjected to a light:dark cycle of 15:9 hours were harvested 3 hotirs from the start of the light period and were depleted of stored phosphate by further culture in a phosphate free medium for 48 hours under continuous illumination.…”

“…After extraction with trichloroacetic acid,, the remaining 8 ml were used for phosphate an alysisaccordingtoamodifiedFiske-SubbaRow method (Kylio, Sundberg and Tillbcrg 1972). ATP was analyzed with luciferin-.luciferase as described elsewhere Tillberg 1967a, Okkeh andKylin 1975). Each point represents the average of five separate experiments.…”