Factors affecting terminal deoxynucleotidyl transferase activity in cacodylate buffer

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For reproducible terminal deoxynucleotidyl transferase activity in cacodylate buffer, assay tubes need scrupulous cleaning, and for maximal activity with the commercial enzyme, zinc ion is required when dATP is polymerized. Potassium ion is also required for maximal activity, though ammonium ion may be used in some situations. The enzyme is readily inhibited by a variety of negative ions, and inhibition by the monovalent anions follows the chaotropic series. This result suggests that cacodylate ion belongs at the beginning of the chaotropic series and so may be generally useful in assaying enzymes with labile activity.