The inhibition of five human
carbonic anhydrase (hCA, EC 4.2.1.1)
isozymes; the cytosolic hCA I and II, the membrane-bound hCA IV, the mitochondrial hCA V, and the
tumor-associated, transmembrane hCA IX, with
anions isosteric and isoelectronic with
sulfate,
nitrate, and
carbonate; such as
chlorate,
perchlorate,
bromate,
iodate,
periodate,
silicate, bismuthate,
vanadate,
molybdate, and wolframate is reported. Apparently, the geometry of the inhibitor (tetrahedral or trigonal) does not influence its binding to the Zn(II) ion of the
enzyme active site, but the nature of the central
element is the most important factor influencing potency.
Isozymes hCA I and II are best inhibited by
chlorate,
perchlorate, and
silicate, together with the
anions structurally related to
sulfate,
sulfamate, and sulfamidate, but
sulfate itself is a weak inhibitor (inhibition constant of 74 mM against hCA I and 183 mM against hCA II).
Molybdate is a very weak hCA I inhibitor (K(I) of 914 mM) but it interacts with hCA II (K(I) of 27.5mM).
Isozyme IV is well inhibited by
sulfate (K(I) of 9 mM),
sulfamate, and sulfamidate (in the low micromolar range), but not by
perchlorate (K(I) of 767 mM). The mitochondrial
isozyme V has the lowest affinity for
sulfate (K(I) of 680 mM) and
carbonate (K(I) of 95 mM) among all the investigated
isozymes, suggesting on one hand its possible participation in metabolon(s) with
sulfate anion exchanger(s), and on the other hand an evolutionary adaptation to working at higher pH values (around 8.5 in mitochondria) where rather high amounts of
carbonate in equilibrium with
bicarbonate may be present. Metasilicate, isosteric to
carbonate, is also about
a 10 times weaker inhibitor of this
isozyme as compared to other CAs investigated here (K(I) of 28.2 mM). Surprisingly, the
tumor-associated
isozyme IX is resistant to
sulfate inhibition (K(I) of 154 mM) but has affinity in the low micromolar range for
carbonate,
sulfamate, and sulfamidate (K(I) in the range of 8.6-9.6 microM). This constitutes another proof that this
isozyme best works at acidic pH values present in
tumors, being inhibited substantially at higher pH values when more
carbonate may be present.
Bromate and
chlorate are quite weak CA IX inhibitors (K(I) s of 147-274 mM).