Isoenzymic forms of carbonic anhydrase in the red macroalga Porphyra leucosticta

Different isoenzymes of carbonic anhydrase (CA; EC 4.2.1.1) have been separ ated using thalli of the red macroalga Porphyra a leucosticta Thuret in Le Jolis. Homogenates of the thallus were centrifuged in order to separate sol uble and membrane proteins. The fraction containing membrane proteins was s ubdivided by centrifuging into two fractions: green and nongreen membrane p roteins. CA activity was detected in all the fractions. Because external CA (measured on intact thallus) represented 15% of total activity, it was con cluded that most of the CA (ca. 80%) was soluble and internal. Direct evide nce regarding the different function of external and internal CA was obtain ed by determining the effects on photosynthesis of two specific CA inhibito rs with different capacity for entering cell. It was concluded that interna l CA was necessary to 'trap' the CO2 entering the cell and thus maintain a favorable CO2 gradient that permits its diffusive entry. Changes in the O-2 evolution rate at inorganic carbon (C-i) concentration saturating for phot osynthesis and on the photosynthetic conductance for C-i were found when ex ternal CA was inhibited. Based on these changes and the significant CA acti vity (ca. 9% of the total activity) found in nongreen membrane fraction, th e presence of external CA associated with plasma membrane was postulated. T he presence of CA associated with chloroplast membrane was also suggested.