文摘
The intrinsic activity of carbonic anhydrase (CA) is the hydration of CO2 to carbonic acid and its dehydration to CO2. CA may also function as esterase and phosphatase. Recently, we demonstrated that renal CA is mainly responsible for the reabsorption of nitrite (NO2 −) which is the most abundant reservoir of the biologically highly potent nitric oxide (NO). By means of a stable-isotope dilution GC–MS method, we discovered a novel CA activity which strictly depends upon nitrite. We found that bovine erythrocytic CAII (beCAII) catalyses the incorporation of 18O from H 2 18 O into nitrite at pH 7.4. After derivatization with pentafluorobenzyl bromide, gas chromatographic separation and mass spectrometric analysis, we detected ions at m/z 48 for singly 18O-labelled nitrite (16O=N–18O−/18O=N–16O−) and at m/z 50 for doubly 18O-labelled nitrite (18O=N–18O−) in addition to m/z 46 for unlabelled nitrite. Using 15N-labelled nitrite (15NO2 −, m/z 47) as an internal standard and selected-ion monitoring of m/z 46, m/z 48, m/z 50 and m/z 47, we developed a GC–MS microassay for the quantitative determination of the nitrite-dependent beCAII activity. The CA inhibitors acetazolamide and FC5 207A did not alter beCAII-catalysed formation of singly and doubly 18O-labelled nitrite. Cysteine and the experimental CA inhibitor DIDS (a diisothiocyanate) increased several fold the beCAII-catalysed formation of the 18O-labelled nitrite species. Cysteine, acetazolamide, FC5 207A, and DIDS by themselves had no effect on the incorporation of 18O from H 2 18 O into nitrite. We conclude that erythrocytic CA possesses a nitrite-dependent activity which can only be detected when nitrite is used as the substrate and the reaction is performed in buffers of neutral pH values prepared in H 2 18 O. This novel CA activity, i.e., the nitrous acid anhydrase activity, represents a bioactivation of nitrite and may have both beneficial (via S-nitrosylation and subsequent NO release) and possibly adverse (via C- and N-nitrosylation) effects in living organisms. Keywords Carbonic anhydrase GC–MS H 2 18 O Labelling Nitric oxide Nitrite