Human carbonic anhydrase IX (CA IX) is an integral membrane protein and a m
ember of the a class of carbonic anhydrases that includes the human and ani
mal enzymes. We have prepared a truncated, recombinant form of human CA IX
of 255 residues consistent with full-length hv man CA II, among the most ef
ficient of the carbonic anhydrases. Catalysis by and inhibition of this for
m of human CA IX has been investigated using stopped-flow spectrophotometry
and O-18 exchange measured by mass spectrometry. In kinetic constants for
the hydration of CO,, CA IX closely resembled CA II with maximal proton tra
nsfer-dependent O-18 exchange near I mus(-1) and k(cat)/K-m near 55 muM(-1)
s(-1). Human CA IX was very strongly inhibited by three classic sulfonamid
es and cyanate, with inhibition constants that are close to those for CA Il
. (C) 2001 Academic Press.