Chemical cross-linking reagents have become an invaluable tool in the scientific community. These reagents are used in preparing antibody-enzyme conjugates and other labeled protein reagents. After the protein is conjugated to an appropriate enzyme, it may then be used as a detection reagent in a variety of assays and applications. A number of cross-linking methods have been used to prepare enzyme conjugates. For example, an N-hydroxysuccinimide ester can be prepared from a ligand of interest, then reacted with a primary amine on the surface of the enzyme. While this method is necessary in some applications, such as those in which the ligand does not contain a primary amine, it is not useful as a general-purpose method.