In order to assess the potential of Spirulina (Arthospira) platensis as a source of abundant, thermostable nitrate assimilatory enzymes, the specific activities and thermal tolerance of nitrate reductase (NR), nitrite reductase (NiR) and glutamine synthetase (GS) were compared with those of rice in crude extracts in vitro. The results show that Spirulina enzymes have relatively higher thermotolerance. When the extracts were pre-exposed to 80 °C for 1 hr, Spirulina enzymes retained higher activities by 3.4, 1.7 and 3.7 fold, respectively than corresponding enzymes in rice. This property was not due to salts and other small proteins/molecules, as their removal by gel filtration (G-25) did not affect their thermotolerance.
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