CHARACTERISTICS OF THE ANION TRANSPORT-SYSTEM IN SEA-TURTLE ERYTHROCYTES

Erythrocytes of Kemp's ridley sea turtle (Lepidochelys kempi) contain a 100- to 105-kDa protein that is reactive with a monoclonal antibody to the membrane domain of human erythrocyte band 3. Based on inhibition of membrane HCO3--Cl- exchange with 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS), sea turtle erythrocytes were found to contain 4 x 10(6) copies of band 3 per cell. Unidirectional HCO3- transfer, specifically HCO3 out --> in--Cl-in --> out exchange, where subscript in --> out represents transfer from inside to outside and subscript out --> in represents transfer from outside to inside, was characterized by a maximal exchange rate of 1.0-1.1 nmol.cm-2.s-1, substrate affinity coefficients of 0.1-0.2 mM for HCO3- and 1.6 mM for Cl-, and an apparent inhibition constant for SITS of 0.6-1.0-mu-M (10-degrees-C, pH 7.6). Under physiological conditions (30-degrees-C, pH 7.4), the rate of net HCO3- transfer (i.e., the difference between HCO3 in --> out--Cl-out --> in and HCO3 out --> in--Cl-in --> out) was 1.13 nmol.cm-2.s-1 for cells subject to a 5-mM decrement in CO2 content. This yields a rate coefficient for the "physiological" anion shift in sea turtle blood of 1.7 s-1, indicating that the anion shift may require 2.6 s to reach 99% completion in vivo. The erythrocyte anion shift appears to be a potential rate-limiting step for capillary CO2 exchange in these turtles.