Fissazione di CO₂ da parte di estratti di prototheca zopfii

Abstract

CARBON DIOXIDE FIXATION BY CELL-FREE EXTRACTS OF PROTOTHECA ZOPFII. — Prototheca is a genus of achloric algae morphologically related to Chlorella (CHODAT, 1913). Whole cells of one species, P. zopfii, are unable to utilize carbon dioxide (Casselton, 1959) and the growth of the microrganism depends on the availability of an organic source of carbon. On the contrary, cell-free extracts fix C¹⁴O₂ in acid-stable compounds. The reaction takes place only if phospoenolpyruvic and a divalent ion (Mg^{+ +} or Mn ^{+ +}) are present. The reaction is not stimulated by IDP, ADP or ATP nor is any carbon dioxide utilized if phosphoenolpyruvic acid is substituted with ribulose-1,5-disphosphate. The distribution of the radioactivity in the experiments in which phosphoenolpyruvic acid was used is consistent with the operation of a phosphoenolpyruvic acid-carboxylase system and of the citric acid cycle. Concomitant with the fixation of C¹⁴O₂ is the release of inorganic phosphate in agreement with the reaction:

phosphoenolpyruvate + CO₂ —→ oxaloacetate + inorganic phosphate

The enzyme present in P. zopfii appears to be similar to that demonstrated in spinach chloroplast (SALTMAN et. al., 1956), in wheat germ (Tchen and Vennesland, 1955), in chemiosynthetic (SUZUKI and Werkman, 1958), photosynthetic (Baugh et al., 1959) and heterotrophic bacteria (Baugh et. al., 1959; Myoda and Werkman, 1960; Quayle and Keech, 1959).