Glucose-6P dehydrogenase in Chlorella sorokiniana (211/8k): an enzyme with unusual characteristics

In Chlorella sorokiniana (211/8k), glucose-6 phosphate dehydrogenase (G6PDH-EC 1.1.1.49) activity is similar in both N-starved cells and nitrate-grown algae when expressed on a PCV basis. A single G6PDH isoform was purified from Chlorella cells grown under different nutrient conditions; the presence of a single G6PDH was confirmed by native gels stained for enzyme activity and by Western blots. The algal G6PDH is recognised only by antibodies raised against higher plants plastidic protein, but not by chloroplastic and cytosolic isoform-specific antisera. Purified G6PDH showed kinetic parameters similar to plastidic isoforms of higher plants, suggesting a different biochemical structure which would confer peculiar regulative properties to the algal G6PDH with respect to higher plants enzymes. The most remarkable property of algal G6PDH is represented by the response to NADPH inhibition. The algal enzyme is less sensitive to NADPH effects compared to higher plants G6PDH: Ki(NADPH) is 103 microM for G6PDH from nitrogen-starved C. sorokiniana, similarly to root plastidic P2-G6PDH. In nitrate-grown C. sorokiniana the Ki(NADPH) decreased to 48 microM, whereas other kinetic parameters remained unchanged. These results will allow further investigations in order to rule out possible modifications of the enzyme, and/or the expression of a different G6PDH isoform during nitrate assimilation.