The reductive pentose phosphate cycle. III. Enzyme activities in cell-free extracts of photosynthetic organisms

The reductive pentose phosphate cycle (6, 29, 8) is currently believed to be the pathway by which CO2 is converted to carbohydrate in photosynthesis. Evidence for the functional role of this cycle is derived mainly from isotope experiments. Brief exposure of photosynthesizing algae to C1409 produces radioactive 3-phosphoglycerate (3-PGA ) 3, labelled mainly in the carboxyl group ( 11 ). The cyclic nature of the photosynthetic carbon path is indicated by the subsequent appearance of isotopes in the a and /8 positions of 3-PGA, as well as in phosphate esters of ribulose and sedoheptulose (28). Kinetic studies by Calvin and Massini (14) and Bassham et al (7) on the fluctuation of pool sizes, revealed that transition from light to dark resulted in an increased concentration of 3-PGA accompanied by a decreased concentration of sugar diphosphate. Studies (43) carried out with algae subjected to changing partial pressures of COO demonstrated a fall in 3-PGA and a rise in ribulose diphosphate (RDP) concentration when the CO., concentration was changed from 1 % to 0.003 %. Enzymes of the pentose phosphate cycle are ubiquitously distributed (2, 4, 5, 17, 18, 20, 21, 22, 23, 24, 28, 29, 30, 36, 37, 38, 41). However, no single photosynthetic organism has been shown to possess all of the requisite enzymes and in amounts sufficiently high to support photosynthesis by this pathway. In this report, the quantitative aspects of all the pentose phosphate cycle enzymes in extracts of several photosynthetic organisms were investigated. We compared the rate of photosynthetic CO2 fixation of the intact organism with the enzyme capacities (activities at saturating substrate concentration) in an extract of the same organism. Our results indicate that several en-