The Warburg effect in a chloroplast-free preparation from Euglena gracilis.

A supernatant fraction, free of plastids, was prepared by centrifugation from Euglena gracilis and used to ascertain whether or not the inhibition of carbon dioxide fixation by oxygen, known as the Warburg effect, is entirely independent of the light-driven phase of photosynthesis. This fraction exhibited in the dark the main features of the Warburg effect; namely, an inverse relationship between the degree of inhibition by oxygen and bicarbonate concentration, reversibility of the inhibition when the oxygen partial pressure is lowered and an increase in the proportion of 2-carbon compounds. It is proposed, therefore, that the inhibition by oxygen is manifest in the photosynthetic carbon reduction cycle and is independent of photosynthetic electron transport and phosphorylation.