Dityrosine Formation in Calmodulin: Cross-Linking and Polymerization Catalyzed by Arthromyces Peroxidase

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Abstract
We employ bovine brain calmodulin, a protein that is subject to photoactivated dityrosine formation [Malencik, D. A., & Anderson, S. R. (1987) Biochemistry26, 695; (1994) Biochemistry33, 13363], as a model for the development of an efficient enzyme-catalyzed protein cross-linking technique. Key steps in the elaboration of the procedure are (1) identification of a peroxidase, from Arthromyces ramosus, that catalyzes dityrosine production in proteins that are not acted on by other common peroxidases, (2) monitoring of the intrinsic fluorescence of dityrosine to determine optimum reaction conditions, achieved with calmodulin in solutions containing boric acid−sodium borate (concentration ≥0.2 M), ∼pH 8.3, ∼40 °C, and (3) quenching of the reaction with reduced glutathione. Arthromyces peroxidase is the only common peroxidase able to catalyze significant dityrosine production in calmodulin, through a reaction that is largely intermolecular. Gel filtration yields fractions (accounting for ∼40% of the initial calmodulin) that represent differing mobility ranges in NaDodSO4 polyacrylamide gel electrophoresis and contain close to the maximum possible amounts of dityrosine. The various fractions undergo Ca2+-dependent conformational changes detected in sedimentation velocity and/or fluorescence anisotropy measurements. Most of the samples stimulate the Ca2+-dependent activity of smooth muscle myosin light chain kinase. In catalytic assays utilizing the synthetic phosphate acceptor peptide, the average activities range from 50 to 100% of that determined for native calmodulin. However, only the least polymerized fraction and the photogenerated calmodulin dimers significantly enhance the p-nitrophenylphosphatase activity of calcineurin. The ability to prepare soluble calmodulin polymers that retain a substantial degree of biological activity and exhibit the intense visible fluorescence of dityrosine illustrates the potential usefulness of Arthromyces peroxidase in the zero-length cross-linking of proteins.