Tracer Determinations of Human Red Cell Membrane Permeability to Small Nonelectrolytes

A flow system has been used to determine the permeability of human red cell membranes to four small nonelectrolytes labeled with (14)C. The permeability coefficients, omega, in units of mol dyne(-1) sec(-1) x 10(15), are: ethylene glycol, 6; urea, 13; formamide, 22; and methanol, 131. The values for urea and formamide are in good agreement with values obtained by Sha'afi, Gary-Bobo, and Solomon by the minimum method. The unusually high value for omega for methanol is ascribed to its solubility in the red cell membrane since its ether: water partition coefficient is 0.14, higher by more than an order of magnitude than the ether: water partition coefficient for water. The other three solutes are hydrophilic and are characterized by values of omega which behave consistently with those of other hydrophilic amides and ureas. The values of omega for the three hydrophilic solutes measured are also consistent with an equivalent pore radius of about 3.5 A in agreement with previous estimates made on the basis of other types of studies.