Figure 2.

Clearance of superoxide dismutase (SOD) as a function of dose. Note that total clearance remains relatively constant while renal clearance as a percent of total clearance increases with dose. This occurs because at higher doses proximal tubular metabolism of SOD is overwhelmed allowing recovery of unchanged SOD in the urine and a more accurate assessment of the contribution of renal clearance. Estimates of renal clearance with low doses are inaccurate because the SOD cleared by the kidney never reaches the urine owing to proximal metabolism.