Anion (A) and cation (B) permeability and transport were unaffected by mPEG-modification. (A) As previously reported, Band 3 is a common site of derivatization with cyanuric chloride–activated mPEG. However, Band 3 function was observed to be unimpaired after derivatization, as shown by normal ³⁵SO₄ influx (▩). In addition, no difference was observed in the ability of the DIDS to inhibit the transporter function of Band 3 (░). (B) Similarly, the RBC Na⁺-K⁺ pump was unaffected by mPEG-derivatization. Shown are control (▩) and ouabain (0.1 mmol/L; ░)-treated RBC. As with DIDS, ouabain was also found to have no differential effect on the control and mPEG-modified cells. The finding that no significant difference was observed in the sensitivity of either pump to small inhibitors (DIDS and ouabain) shows the normal flux of small, but not large, molecules across the RBC surface. The results shown are the average ± standard deviation of three separate experiments.