[Influence of the cell metabolism on the pigment composition in ageing cultures of Euglena gracilis].

The qualitative and quantitative composition of the carotenoids of young cells of Euglena gracilis strain 1224-5/9 corresponded to that of the "Z"-strain.The frequently observed yellow-reddish discolouration of ageing cells was found to be caused by a heavy breakdown of the chlorophylls and not by an increased synthesis of the carotenoids.Furthermore there could be observed a remarkable decrease in the concentration of diadinoxanthin together with an increase in the amount of zeaxanthin in the course of the stationary growth phase. This phenomenon was attributed to a direct deepoxidation of diadinoxanthin on the basis of experiments with (14)C-diadinoxanthin showing a transformation of the pigment into zeaxanthin and some other Euglena carotenoids, e.g. neoxanthin and β-carotene.Because of their interconvertibility diadinoxanthin and neoxanthin are regarded as auxiliary pigments for the photosynthetic O2-evolution.The transformation of diadinoxanthin into zeaxanthin in the ageing cells coincides in time with the switch from an aerobic to an anerobic cell metabolism. The latter is characterized by a decrease in the O2-incorporation together with a simultaneous increase in the NADH-concentration and by a heavy excretion of glycolytic end products such as pyruvate and lactate after cell respiration has stopped completely. The transformation of diadinoxanthin into zeaxanthin is therefore attributed to a reduction of diadinoxanthin by the cytoplasmic NADH.As the same pigment transformation could be observed in cells kept in darkness and under conditions of artificially stopped respiration, it is likely that this transformation takes place independently of photosynthetic processes.The origin of the commutation of the cell metabolism and the cessation of respiration is still unknown. As the respiration of ageing cells can be revived in the darkness it is certainly linked to photosynthesis under light conditions.