An Enzymatic Ion Exchange Model for Active Sodium Transport* By

An enzymatic ion exchange model for active sodium transport is described. Kinetic equations relating net flux to time, and to concentration difference across the actively transporting membrane are derived. The second of these equations is tested, using the isolated frog skin in the "short-circuit" apparatus of Ussing. Reasonable lineaxity, as predicted by this equation, is observed. The passive permeability coefficient for Na +, is calculated as 5.3 × 10-4 45.3 × 10 -4 cm./hr. If cholinesterase is assumed to be the enzyme responsible for transport, the activity required to account for the observations reported here is 17.7 × 10-* mmoles/cm.*/hr. In a previous paper (8) the localization and characterization of frog skin cholinesterase were described. I t was found that the enzyme was largely localized in the tda subcutanea and that it was of the butyrylcholinesterase (pseudocholinesterase) type. I t seemed interesting to speculate on the possible role of this enzyme in the active sodium transport mechanism. Thomas (10) has recently proposed a model for ion transport involving a soluble enzymatic carrier. This model requires that the enzyme-substrate complex have ionbinding characteristics different from those of the free enzyme. This might occur if the pK, of groups in the neutral pH range is shifted, increasing (or decreasing) the net charge of the enzyme. Such pK shifts have been observed in cholinesterase by Dixon (3). Application of this scheme to the frog skin active transport mechanism presents the difficulty that the enzyme is apparently tightly bound to some component of the tda subcutanea. Homogenization in frog-Ringer's destroys some 50 per cent of the activity; the remainder was found associated with the particulate fraction. * This work was aided by grants from the National Science Foundation and the United States Public Health Service, administered by B. T. Scheer. This work forms part of a thesis submitted to the University of Oregon Graduate School in partial fulfillment of the requirements for the degree of Ph.D. ~t Present address: Department of Zoology, University of Missouri, Columbia, Missouri. 635 J. GZN. PHvsIo~,., 1959, VoL 42, No. 3 The Journal of General Physiology on A ril 5, 2017 D ow nladed fom Published January 20, 1959