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.
