Carbonic anhydrase zinc content

Segments of human aorta obtained fresh at autopsy were freed from blood and covering connective tissue and analyzed manometrically for carbonic anhydrase activity. Twelve specimens from 7 males and 5 females yielded nearly a 9-fold spread in values (0.12 to 1.05 units).31 This is interesting in view of the zinc content of carbonic anhydrase and the extremely wide variations in the zinc content of blood plasma and spleens which have been observed (pp. 55 and 72). 

The fact that zinc is known to be a component of an enzyme carbonic anhydrase leaves no doubt as to its physiological significance. The amount of zinc in erythrocytes seems to parallel the carbonic anhydrase activity.21 The leucocytes which appear to lack carbonic anhydrase contain about 25 times as much zinc (per cell) as do the erythrocytes.20 It seems likely that an investigation of the zinc content of different types of white blood cells coupled with a study of individuals from the standpoint of the different types of white cells present (p. 35) would lead to the discovery of substantial inter-individual differences. The wide spread in the zinc concentrations in three human spleens has already been mentioned (p. 72). A recent study has been made of the intake and excretion of zinc by 13... 

About two-thirds of the zinc in pLasma is loosely bound to albumin. Most of the remaining zinc is tightly bound to other plasma proteins. A small fraction (2-37o) of plasma zinc is weakly bound to amino adds. The amino acids that most avidly bind zinc are histidine and cysteine. Amino add-associated zinc enters the glomerular filtrate and thus is a source of the zinc ions destined for excretion in the urine Most of the filtered zinc is reabsorbed and is prevented from immediate excretion. The zinc in red blood cells is bound to car nic anhydrase. Carbonic anhydrase is present at a level about 0,1% that of hemoglobin in the red blood cell on a poweight basis. The zinc content of mitochondria is about 1 nmol/mg protein (Unk and Jagow, 1995). 

Plasma zinc is known to decrease following use of oral contraceptive agents (73,74). Our recent data indicate that whereas the plasma zinc may decline, the zinc content of the red blood cells increases as a result of oral contraceptive agents administration. This phenomenon may merely mean a redistribution of zinc from the plasma pool to the red cells. Alternatively, oral contraceptive agents may enhance carbonic anhydrase (a zinc metalloenzyme) synthesis, thus increasing the red cell zinc content. 

Reduced activity of carbonic anhydrase, another zinc metalloenzyme, has been reported in gastric and intestinal tissues and in erythrocytes when the activity of the enzyme was expressed per unit of erythrocytes (91), Recently in sickle-cell-disease patients, an example of a conditioned zinc-deficient state, the content of carbonic anhydrase in the red cells was found to be decreased, correlating with the zinc content of the red cells (10,75). Inasmuch as the technique measured the apoenzyme content, it appears that zinc may have a specific eflFect on the synthesis of this protein by some mechanism yet to be understood. 

Keilin and Mann (1939, 1940a) first noted that zinc is a constituent of carbonic anhydrase. Leiner and Leiner (1940) and Hove, Elvehjem, and Hart (1940) corroborated these findings. Day and Franklin (1946) found zinc in carbonic anhydrase extracted from plants. The figures on the percentage zinc content vary. Keilin and Mann (1940a) found 0.33 %, Hove, Elvehjem, and Hart (1940) 0.318% for enzyme preparations of comparable purity, as judged by their activity. Scott and Mendive (1941b) reported 0.15% of zinc for their purified and 0.2% to 0.23% for their crystalline preparations (Scott and Fisher, 1942). 

The total zinc content of this fraction corresponded closely to that calculated to come from erythrocytes on the basis of the data given in Table III. Previous work (Meldrum and Roughton, 1932a Tupper, Watts, and Wor-mall, 1951 Vallee, Hoch, and Hughes, 1954) has demonstrated that carbonic anhydrase is precipitated at high saturation of ammonium sulfate. These data and calculations indicate, therefore, by correlation of measurements of enzyme activity, zinc content, and the known characteristics of the protein, that two distinct and different zinc-containing proteins exist in leukocytes and erythrocytes and can be differentiated. 

Wood JG and Sibly PM (1952) Carbonic anhydrase activity in plants in relation to zinc content. Aust J Sci Res Ser B5 244-255. 

Carbonic anhydrase is assumed to be located at the surface of the membrane of the tubular cells [47, 48]. Carbonic anhydrase is a small zinc protein found in many animal tissues, but its concentration is highest in the kidney tubules cells, the erythrocyte, and some cells of the gastric mucosa. The substrates of the carbonic anhydrase reaction are carbon dioxide and water the product is carbonic acid. The enzyme has been purified from erythrocytes, and its molecular weight is about 30,000. The purified enzyme preparation contains 0.21% zinc, probably 1 atom of zinc per molecule of enzyme. The zinc is tightly bound to the enzyme molecule and cannot be removed by dialysis or electrodialysis. The presence of zinc in the molecule is essential to activity because when zinc is removed from the molecule by extended incubation with 1-10 phenanthroline, the enzyme s activity reflects the zinc content of the preparation. 

Deeding very rapidly—the zinc content was from 3 to 18 times the normal. Cruickshank (1936) has surveyed the sources and distribution of zinc in relation to human nutrition, and claims that tuberculosis is associated with a zinc deficiency, and malignant conditions with chronic zinc poisoning. Beri-beri may be due partly to zinc deficiency (Eggleton, 1939). The enzyme carbonic anhydrase is a zinc protein complex. 

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