496 LIGHT AND LIFE 



made by Thimann (145) about COo fixation and by Lipmann about 

 a possible role of ATP in photosynthesis — the latter being, so far as 

 I know, the first suggestion of this kind ever made (92, p. 148) . 



Ruben's proposal distinguished two phases of sugar synthesis in 

 the dark: a carboxylative phase, dependent on ATP only, in which 

 COo enters cellular metabolism by carboxylating an acceptor mole- 

 cule, and a reductive jihase, in which a carboxyl group is reduced 

 by pyridine nucleotide with the aid of ATP. 



In the ensuing fifteen years Ruben's scheme received experimental 

 support from several directions: (a) Calvin, Benson, Horecker, Ochoa, 

 Racker, and their associates (123, 166, 78, 127) have identified the 

 ATP-dependent carboxylative phase in CO2 assimilation. They have 

 shown that the entry of COo into the metabolism of photosynthetic 

 cells depends on the phosphorylation of ribulose monophosphate bv 

 ATP to ribidose diphosphate, which is then carboxylated by COo and 

 cleaved to give 2 molecules of phosphoglyceric acid (PGA), (b) The 

 kinetic studies of Calvin's group (35) suggested that the reductive 

 phase of CO2 assimilation is the reduction of PGA to triose phosphate 

 by a reversal of the well-known glycolytic reactions — a mechanism that 

 was known to require reduced pyridine nucleotide and ATP. (c) In 

 addition to a carboxylative and a reductive phase, the finding of 

 components of the j)entose cycle in photosynthetic tissue (mainly 

 in Horecker's and Racker's laboratories, cf. review, 161) afforded 

 a mechanism for the regeneration of the COo acceptor in photo- 

 synthesis, in what might be designated as a third, regenerative phase 

 of COo assimilation, (d) Racker (126) obtained in the dark a 

 synthesis, driven by DPNHo and exogenous ATP, of hexose phosphate 

 from COo, in a model multi-enzyme system consisting of glycolytic 

 enzymes from rabbit nuiscle and yeast (128) and pentose cycle enzymes 

 from spinach leaves. 



The carboxylative, reductive, and regenerative phases constitute a 

 cyclic sequence of dark reactions which jointly might be termed a 

 reductive carbohydrate cycle (cf. 127). The fact that this reductive 

 carbohydrate cycle, comprising exclusively dark reactions, was not 

 peculiar to photosynthetic cells was demonstrated when the most 

 (hstinctive enzyme of the cycle, ribidose diphosphate carboxylase, was 

 found in such heterotrophs as Escherichia coli (161, 53) , and when the 

 cycle in its entirety was found in the non-photosynthetic sidfur bac- 

 terium Thiobacillus denilrificans (155, 20) . It thus became clear, as 

 was already earlier anticipated by Benson and Calvin, that "it is not 

 to be expected that this type of work (investigation of the carbon 



