Using a radio-immunosorbent technique, the levels of the carbonic anhydrase (CA) isoenzymes CA I, II and III in plasma (1-3 micrograms ml-1), lymph (0.5-1.6 micrograms ml-1) and urine (0.03-0.06 micrograms ml-1), were determined in the rat. The renal clearances of CA I, II and III were 11 +/- 3, 42 +/- 11 and 35 +/- 4 nl min-1 (g kidney wet wt)-1 (n = 4-5), respectively. After a single i.v. injection of purified native or 125I-labelled isoenzymes, the elimination of CA I, and CA III from plasma followed a bi-exponential decline, with half-times of 7 and 9 min for the rapid phase and 112 min for the slow phase, respectively. Nephrectomy decreased the rapid phase and the build-up of catabolites. Therefore, the rapid phase of CA I and III elimination is probably explained by filtration of unbound isoenzyme at the glomeruli and subsequent degradation by the proximal tubules. The plasma elimination curve for CA II was different and followed a mono-exponential decline, with a half-time of 210 min both in normal and nephrectomized animals. This indicates that CA II is not filtered at the glomeruli. However, in acute renal failure, with leaking tubular cells, CA II was excreted into the urine. The slow elimination of the major part of the isoenzymes from plasma is explained by the binding of CA I, II and III to a plasma protein, immunochemically similar to transferrin, forming a macromolecular complex with a mol wt of 114 +/- 2 kDa.

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