The isoenzymes of carbonic anhydrase: tissue, subcellular distribution and functional significance, with particular reference to the intestinal tract.

1. The total carbonic anhydrase activity in some guinea-pig tissues has been measured using a pH-stat procedure. Stomach, gall bladder, proximal colon and caecum all possess more carbonic anhydrase activity per unit amount of protein than does whole blood.2. The carbonic anhydrase activity of the small intestine is low. Reasons are given for supposing that activity found there is not entirely due to contamination by whole blood, and it is suggested that in this tissue the enzyme may be localized in some cell type other than the columnar absorbing cells.3. Evidence is presented which indicates that heavy metals interfere with the activity of the enzyme as measured in tissue homogenates.4. The distribution and concentration of the two major isoenzymes of carbonic anhydrase have been measured in different tissues. Blood and proximal colon contain both isoenzymes in comparable concentrations, the ratio of the concentration of the ;low activity' isoenzyme to that of the ;high activity' being about 2. The gastric mucosa contains much ;high activity' carbonic anhydrase, but only a negligible amount of the ;low activity' isoenzyme. In the caecal mucosa, the ;low activity' isoenzyme is predominant, the ratio of its concentration to that of the ;high activity' isoenzyme being about 9. It is also found that more than 1.5% of the protein in the caecal mucosa is accounted for as carbonic anhydrase enzymes.5. It is found that some 45% of the total carbonic anhydrase activity of sucrose homogenates of the guinea-pig colon is bound to particles. The activity is located mainly in the nuclear and microvillous fraction and in the ;high-speed supernatant' fraction. The form of enzyme bound is largely of the ;high activity' variety. When the tissue is homogenized in potassium chloride solutions less than 4% of the total activity is recovered in particulate fractions. The amount of activity which is bound to particulate fractions increases as the ionic strength or pH of the homogenate is lowered.6. The findings are discussed in relation to the possible physiological roles of the isoenzymes in tissues other than blood. Possible relationships between the presence of the enzymes and the metabolism and transport of ammonium and fatty acids are considered.