The microsomal fraction isolated from sea urchin H. pulcherrimus eggs has the ability to actively accumulate Ca2+ in the presence of ATP. The Ca2+ uptake was sustained by addition of oxalate and was apparently insensitive to sodium azide. The sequestered microsomal Ca was readily released by the divalent cation ionophore A23187. The microsomal fraction obtained from fertilized eggs accumulated Ca2+ about five times more quickly than did that from unfertilized eggs. The increased Ca2+ uptake by microsomal fraction obtained from fertilized eggs was due to an increase in the maximum velocity of Ca2+ uptake and there was no difference in Km for calcium between the two fractions.
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