Carbonic Anhydrase of a Unicellular Red Alga Porphyridium cruentum R-1. I. Purification and Properties of the Enzyme

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Cells of Porphyridium cruentum R-1 , a unicellular red alga, grown under ordinary air (0.04% CO_2) showed much higher activity of carbonic anhydrase (CA) than those grown under CO_2-enriched air (2% CO_2). CA activity was not detected in a suspension of intact cells, and was detectable only after the cells had been homogenized, indicating that this enzyme was localized only within the algal cells. After partial purification of Porphyridium CA, its mol wt was estimated as 59 kDa by SDS-PAGE and 55 kDa by gelfiltration. This suggests that the native enzyme is a monomer. Its activity was not affected by benzensulfonamides, potent inhibitors of CAs isolated from Chlamydomonas and other organisms. Chloride (or bromide) ions was essential for CA activity. CA activity markedly decreased when the cell extract had been incubated at pH lower than 7 before assay. Upon readjusting the pH of the preincubation medium to 9 or higher, the enzyme activity was restored, indicating that the inactivation is reversible.
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