Specific arginine modification at the phosphatase site of muscle carbonic anhydrase

Mammalian carbonic anhydrase III has previously been shown to catalyze the hydrolysis of p-nitrophenyl phosphate in addition to possessing the conventional CO2 hydratase and p-nitrophenylacetate esterase activities. Modification of pig muscle carbonic anhydrase III with the arginine reagent phenylglyoxal yielded two clearly distinctive results. Reaction of the enzyme with phenylglyoxal at concentrations equivalent to those of the enzyme yielded stoichiometric inactivation titration of the enzyme's phosphatase activity, approaching 100% loss of activity with the simultaneous modification of one arginine residue, the latter based on a 1:1 reaction of phenylglyoxal with arginine. At this low ratio of phenylglyoxal to enzyme, neither the CO2 hydratase activity nor the acetate esterase activity was affected. When the modification was performed with a significant excess of phenylglyoxal, CO2 hydratase and acetate esterase activities were diminished as well. That loss of activity was accompanied by the incorporation of an additional half dozen phenylglyoxals and, presumably, the modification of an equal number of arginine residues. The data in their entirety are interpreted to show that the p-nitrophenylphosphatase activity is a unique property of carbonic anhydrase III and that excessive amounts of the arginine-modifying reagent lead to unspecific structural changes of the enzyme as a result of which all of its enzymatic activities are inactivated.