CARBONIC-ANHYDRASES IN CYTOSOL, NUCLEUS, AND MEMBRANES OF RAT-LIVER

The relative contribution of each functional carbonic anhydrase (CA) i sozyme to liver CA activity of fed or starved adult male rats has been determined. The functional isozymes are CA II, CA III, CA IV, and CA V. Total CA, CA III, CA II, CA IV, and CA V activities (in mumol CO2 c onverted . min-1 . liver-1), as measured by mass spectrometric assay u sing (NaHCOO)-O-18-O-16 in aqueous solution at pH 7.4 and 37-degrees-C , were 94,867, 38,621, 37,000, 14,515, and <5,000 in fed rats and 40,6 30, 10,498, 9,137, 18,338, and <2,600 in starved rats, respectively. C A II was unevenly distributed throughout the liver. In perivenous and periportal cytosols, as determined by the digitonin-pulse perfusion te chnique, CA II activity was (in mg cytosolic protein-1) 325 and 69 in fed rats and 167 and 33 in starved rats, respectively. CA III was more evenly distributed and less affected by starvation: CA III activity i n perivenous and periportal cytosols was (in mg cytosolic protein-1) 8 4 and 55 in fed rats and 113 and 52 in starved rats, respectively. Evi dence that CA III was concentrated in the nucleus was obtained histoch emically by the Ridderstrale cobalt-precipitation technique in 2-mum-t hick glutaraldehyde-fixed sections from adult fed rats. Liver CA activ ity was higher in the perivenous hepatocytes in cytosols and nuclei, w hereas CA IV was homogeneously distributed. Incubation of the 2-mum se ctions with 1 muM acetazolamide resulted in inhibition of all membrane -associated CA, 50% of cytosolic CA, and no nuclear CA. We concluded t hat CA III comprises 50% of cytosolic CA activity and 100% of nuclear CA activity in adult male rats.