Tissue printing to visualize polyphenol oxidase and peroxidase in vegetables, fruits, and mushrooms

A simple tissue-printing procedure to determine the tissue location of the endogenous enzymes polyphenol oxidase and peroxidase in a variety of vegetables, fruits, and mushrooms is described. In tissue printing, cell contents from the surface of a cut section of the tissue are transferred to an adsorptive surface, commonly a nitrocellulose membrane. Because of the considerable expense of nitrocellulose, our procedure utilizes artists' hot-press watercolor paper as a novel and more economical alternative. Tissue prints are then exposed to an enzyme substrate from which an insoluble, colored product is produced. The appearance of color in specific areas of the print is an indication of the presence of the enzyme in those tissue locations. The experiment is designed to enable students to learn some fundamental concepts about enzymes. It has been used in an introductory-level organic and biochemistry course for nonscience majors, but would also be appropriate for advanced high school students or could be adapted for an upper-level undergraduate biochemistry course.