The relationship between carbonic anhydrase activity and zinc content of erythrocytes in normal, in anemic and other pathologic conditions.

By BERT L. VALLEE, M.D., HERBERT D. LEWIS, M.D., MARK D. ALTSCHULE, M.D., AND JOHN G. GIBSON, II, M.D. I T HAS been shown that zinc is a component of the carbonic anhydrase molecule.” 2 This enzyme has been demonstrated to catalyze the reaction H,O + CO, ± H,CO, in vitro; its molecular weight is approximately half that of hemoglobin,’ and some of its other physico-chemical properties are established.’ Carbonic anhydrase obtained from ox and sheep blood has been found to contain .31-.34 per cent of zinc per unit of dry, active protein, these values having been obtained by means of a diphenylthiocarbazone method.’ ‘ This method, when applied to human carbonic anhydrase, indicated the presence of 0.164 per cent of zinc, according to Keilin and Mann.’ Other investigators, using an older and less sensitive technic,’ reported ox carbonic anhydrase to contain O.LO to 0.2.3 per cent of zinc,6 and later confirmed these results by polarography.7 The nature of the bond between the metal and protein, and the chemistry of the enzyme are not understood at present. The enzyme may be inactivated by various substances. Acids separate zinc from its proteinous prosthetic group; this process is irreversible, a finding which led Kelin and Mann’ to consider the metal to be the active part of the enzyme molecule. When the enzyme is inactivated by long standing, or by manipulation, zinc remains bound to the protein and cannot be removed by dialysis. While the separation of zinc from enzyme protein irreversibly destroys its activity, the inactivation of the enzyme does not necessarily liberate the metal.1 Under some circumstances, the presence of an excess of zinc may be due, therefore, to the presence of inactivated enzyme retaining its full complement of zinc. Zinc is found in human plasma, erythrocytes and leukocytes, but carbonic anhydrase activity can be detected in the blood only in erythrocytes.’5 The carbonic anhydrase activity and zinc concentration, determined separately in normal and