Carbohydrate isoforms of antithrombin variant N135Q with different heparin affinities

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Abstract
We have changed one of the carbohydrate‐bearing asparagine residues of human antithrombin to glutamine by site‐directed mutagenesis and expressed the variant antithrombin, N 135Q, in baby hamster kidney cells. Two isoforms were secreted, both of which had higher affinity for heparin than human plasma α antithrombin. Both forms had normal inhibitory activity toward factor Xa and showed normal heparin acceleration of proteinase inhibition. The mutation resulted in a higher production of the very high affinity form from about 30% to 60% of the total secreted antithrombin. This form should be the most useful for comparison of the effects of other mutations on heparin binding and proteinase inhibition.