Carbohydrate isoforms of antithrombin variant N135Q with different heparin affinities.

We have changed one of the carbohydrate-bearing asparagine residues of human antithrombin to glutamine by site-directed mutagenesis and expressed the variant antithrombin, N135Q, in baby hamster kidney cells. Two isoforms were secreted, both of which had higher affinity for heparin than human plasma alpha 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.