Permeability and reflection coefficients of urea and small amides in the human red cell

Measurement of the transport parameters that govern the passage of urea and amides across the red cell membrane leads to important questions about transport of water. It had initially been thought that small protein channels, permeable to water and small solutes, traversed the membrane (see Solomon, 1987). Recently, however, very strong evidence has been presented that the 28 kDa protein, CHIP28, found in the red cell membrane, is the locus of the water channel (see Agre et al., 1993). CHIP28 transports water very rapidly but does not transport small nonelectrolytes such as urea. The irreversible thermodynamic parameter, sigma(i), the reflection coefficient, is a measure of the relationship between the permeability of the solute and that of water. If a solute permeates by dissolution in the membrane, sigma(i) = 1.0; if it permeates by passage through an aqueous channel, sigma(i) < 1.0. For urea, Goldstein and Solomon (1960) found that sigma(urea) = 0.62 +/- 0.03 which meant that urea crosses the red cell membrane in a water-filled channel. This result and many subsequent observations that showed that sigma(urea) < 1.0 are at variance with the observation that CHIP28 is impermeable to urea. In view of this problem, we have made a new series of measurements of sigma for urea and other small solutes by a different method, which obviates many of the criticisms Macey and Karan (1993) have made of our earlier method. The new method (Chen et al., 1988), which relies upon fluorescence of the intracellular dye, fluorescein sulfonate, leads to the corrected value, sigma(urea, corr) = 0.64 +/- 0.03 for ghosts, in good agreement with earlier data for red cells. Thus, the conclusion on irreversible thermodynamic and other grounds that urea and water share a common channel is in disagreement with the view that CHIP28 provides the sole channel for water entrance into the cell.