Incorporation of [125l]-5-lodo-2-thiouracil in Cultured Hamster, Rabbit, and Human Melanoma Cells1

The incorporation of [2-14C]-2-thiouracil and a series of [125l]-5-iodo-2-thiouracils ([125l]ISUra(s)) into cultured Greene hamster melanoma cells was determined in order to establish their properties as false precursors in the melanin-biosynthetic pathway. The cold trichloroacetic acid-precipitable incorpora tion of [2-14C]-2-thiouracil as well as [125l]ISUra into melanoma cells after a 24to 48-hr labeling period proved to be com pletely tyrosinase dependent (more than 99.5% inhibition could be achieved by 0.5 rriM phenylthiourea). [125l]ISUra incorpora tion was 3-fold higher than was [2-14C]-2-thiouracil incorpora tion and was enhanced by 1 rriM theophylline treatment. [125I] ISUra incorporation into hamster, rabbit, and human melanoma cells showed a linear relationship with cell melanin content. Methylation of the sulfur completely prevented the incorpora tion, while propylation but not methylation at position 6 resulted in lower incorporation. [125l]ISUra proved to be a marker for melanogenesis and may be useful in studies on the differentia tion of cultured melanoma cells.