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The effect of Ni2+ on the early stages of chlorophyll biosynthesis and pheophytinization in Euglena gracilis cells was studied. Incubation of the cells with 10−4 M Ni2+ for 7 days resulted in a higher chlorophyll content, enhanced production of 5-aminolevulinic acid (ALA), and in increased activity of 5-aminolevuluinic acid dehydratase (EC 18.104.22.168, ALAD), as compared to the control cells incubated without Ni2+. At a higher concentration (10−3 M), Ni2+ markedly inhibited chlorophyll accumulation and ALAD activity, as compared to the control cells. At this concentration, Ni2+ also inhibited heme biosynthesis and strongly stimulated ALA production. It seems likely that, by affecting heme synthesis, Ni2+ increases the activity of the ALA production system. However, the suppression of subsequent stages of ALA conversion to chlorophyll, in particular ALAD inhibition, ultimately resulted in almost complete inhibition of chlorophyll biosynthesis. In addition to cessation of de novo chlorophyll synthesis in the presence of Ni2+ (10−3 M) in Euglena cells, the existing chlorophyll was converted into pheophytin and almost completely degraded. We suppose that the Ni2+-induced pheophytinization is caused by an acidic shift of intracellular pH related to an impairment of cell membrane permeability by Ni2+ cations.