Abstract
An absolute requirement for divalent cations is reported for H14CO3− influx in Chara corallina. Effective substitution of eluted Ca2+ by Mg2+ and Sr2+ was observed, but Mn2+ was completely ineffective in restoring H14CO3− transport activity. Similarly, La3+ could not substitute for Ca2+ in this system. Low concentrations of ethylenediaminetetraacetate (0.01 to 0.06 mm) significantly enhanced the rate at which H14CO3− transport capacity was lost.
Examination of the response of OH− efflux, during Ca2+-free treatment, indicated that the cellular control over OH− efflux remained unaffected until membrane integrity became severely affected. This conclusion was supported by the response of OH− efflux to 10 mm K+. Therefore, assimilation of H14CO3− is not rate-limited by an effect of Ca2+ elution on the OH− transport system. Kinetic experiments indicated that Ca2+ removal from the membrane resulted in noncompetitive inhibition of H14CO3− assimilation; the apparent Michaelis constant remained unaltered over a wide range of conditions. An hypothesis is presented which suggests that membrane integrity is necessary for HCO3− transport to occur, but Ca2+ (Mg2+, Sr2+), per se, must be bound to the transport complex before activity is established.
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