The excretion of carbonic anhydrase isozymes CAI and CAII in the urine of apparently healthy subjects and in patients with kidney disease

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Abstract
Carbonic anhydrase isozymes CA I and CA II were assayed by a radio-immunosorbent technique in the plasma and urine of apparently healthy subjects and of patients with renal disease. The concentrations (mean +/- SD, n = 8) of CA I and CA II in the plasma of healthy subjects were 2.3 +/- 2.3 and 0.8 +/- 0.5 mg/l, respectively. The urinary excretion values were 3.8 +/- 2.0 and 3.5 +/- 1.9 micrograms/24 h, and the apparent renal clearances were 21 +/- 17 and 52 +/- 44 microliters/min, respectively, values that are similar to those of other low molecular weight proteins. CA I and CA II have mol. wt of 28,850 and 29,300, respectively, they are globular in shape and have a Stoke-Einstein radius of 25 A. They could, therefore, be expected to be filtered at the glomeruli and thereafter reabsorbed by the proximal tubules. CA II is also present in the cytoplasm of renal proximal and distal tubular cells. A study of the pattern of urinary excretion of CA I and CA II could permit detection of damage to renal tubular cells in two ways--either from defective reabsorption of filtered CA I and CA II by the proximal tubular cells, or from leakage of CA II from the proximal or distal tubules into the urine. Some patients with hypercalcuria and renal tubular acidosis showed increased excretion of these enzyme proteins and of beta 2-microglobulin (BMG) into the urine, but the prevalence was rather low (27%). Further studies of patients with more severely damaged kidneys are required.