Histochemical evidence that peroxidase does not affect melanin formation in feather melanocytes.

Animal peroxidases are iron-containing proteins that catalyze the oxidation of a variety of substances by hydrogen peroxide. Histochemically, the myeloperoxidase of granulocytes is the most easily detected (1). Tyrosinase (dopa oxidase) is a copper-containing enzyme complex capable of converting both tyrosine to dopa (slowly) and dopa to dopa quinone (rapidly) in the melanin synthetic pathway (2). Histochemically, dopa-oxidase activity is easily demonstrated. Okun and his collaborators, using various histochemical tests, have shown that peroxidases are present in mast cells, granulocytes, neurons, some melanomas, and possibly some mammalian melanocytes (3-7). These peroxidases are capable of converting both tyrosine and dopa to melanin. These workers have concluded that peroxidase, rather than tyrosinase, is pnmarily responsible for converting tyrosine to dopa in melanocytes and that tyrosinase actually possesses only dopaoxidase activity. Since these peroxidase studies were limited to mammalian systems, it was imperative that we make similar investigation of the melanocyte system of the fowl so that results involving enzyme activity in this species could be correctly interpreted. Histochemical tests were conducted using the melanocytes and granulocytes of three pigment mutations of the fowl. The results indicate that a copper-containing tyrosinase does exist in the fowl which is capable of oxidizing both tyrosine and dopa.