Biosynthesis of a Thiouracil Pheomelanin in Embryonic Pigment Cells Exposed to Thiouracil*

[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, and there is a proportional relationship between thiouracil incorporation and the level of tyrosinase activity in pigment tissues. Other chick embryo tissues do not incorporate thiouracil, but small quantities are incorporated by embryo tissues containing tyrosine hydroxylase. Radioactivity is retained in melanin prepared from isolated melanosomes of pigment tissues incubated in [2J4C]thiouracil. A comparison of [3?3]thiouracil and L-[2J4C]tyrosine incorporations into melanin indicates that 1.5 moles of thiouracil are incorporated for each mole of tyrosine. These results are consistent with the hypothesis that thiouracil condenses with the quinone intermediates in the melanin pathway and is incorporated into the melanin polymer as part of the subunit structure.