[Z-W]Thiouracil and [%]thiouracil are incorporated into the melanin of chick retinal pigment epithelium incubated in vitro. This synthesis of a thiouracil pheomelanin (thiolcuntaining melanin) can be used to measure the rate of melanin synthesis in living cells. Thiouracil incorporation is dependent on tyrosinase activity: incorporation is inhibited by phenylthiourea, a tyrosinase inhibitor, an...