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The Incorporation of C¹⁴ from 3,4-Dihydroxyphenylalanine-2'-C¹⁴ into the Melanin of Mouse Melanomas^*

M. S. Blois, Jr. and R. F. Kallman

( Biophysics Laboratory and the Department of Radiology, Stanford University, Stanford, California)

The administration of DL-3,4-dihydroxyphenylalanine-2'-C¹⁴ (dopa) to mice with pigmented melanomas may result in an appreciable uptake of radioactivity in the tumor. It has been shown that this activity is principally in the melanin pigment, and it is concluded that the administered dopa has undergone conversion to one or more of the melanin monomers, which have in turn been incorporated in the melanin polymer. The ratios of radioactivity in the tumor compared with spleen and liver were in the range from 10:1 to 100:1 for one intensely pigmented tumor and as low as 1:1 for the Harding-Passey melanoma (lightly pigmented), following a single dose of dopa-C¹⁴. The uptake of label following a single dose of labeled dopa is apparently proportional to the rate of melanin synthesis. A considerable concentration of radioactivity is found to quickly build up in the adrenal following a single dose of DOPA-C¹⁴, and this activity then decays with a biological half-life of about 2 weeks. The elevated radioactivity found in renal tissue appears due to labeled dopa, or metabolites undergoing clearance, and from the results of probenecid (Benemid®) administration it appears that dopa may undergo renal tubular excretion.

^* This work was supported by U.S.P.H.S. Grant GM 10847 and by the American Cancer Society through an Institutional Grant.

Recipient of a Public Health Service research career program award, CA-K6-3576, from the National Cancer Institute.

Received 1/10/64.