Photosyntheseversuche in Tritiumwasser mit Chlorella

Abstract

The incorporation of tritium into carbohydrates during photosynthesis with Chlorella was investigated. The fixation of 14C and T was compared by providing 14CO2 and HOT simultaneously. The intramolecular distribution of T in hexose and glyceric acid was determined by degradation. The incorporation of T during the carboxydismutase, the aldolase and triosephosphate-isomerase reaction was studied in enzyme systems. The results indicate that - in general - incorporation of T into the products of photosynthesis is due to isomerisation reactions. The influence of isotope effects at various stages of the photosynthetic cycle on the final T-content is discussed. The intramolecular T-distribution in the glucose from starch is different from that of soluble glucose derivatives. To interpret this, it is assumed that the isomerisation of fructose-6-phosphate to glucose-6-phosphate firstly is completely intramolecular in intact Chlorella and secondly shows a large discrimination for tritium-containing (at C-1) molecules. Thirdly, it is proposed that the sequence of reactions from fructose-6-phosphate to starch occurs in a multienzyme system without exchange with free sugar phosphate-pools.