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In Vitro Growth Inhibition of S-91 Mouse Melanomas by Tyrosinase Substrate Analogs with and without L-Cysteine¹

Department of Biochemistry, The University of Nebraska, College of Medicine, Omaha, Nebraska 68105; Department of Pathology, University of Southern California, School of Medicine, Los Angeles, California 90033; and Department of Pathology, New York University, School of Medicine, New York, New York 10016

Tyrosinase, the enzyme responsible for melanin biosynthesis in normal tissues and malignant melanomas, has been reported to be a vital respiratory enzyme of melanizing tissue. In the present study, S-91 mouse melanoma explant growth inhibition in vitro was evaluated quantitatively for 7 substrate analogs of tyrosine, alone and in the presence of L-cysteine. DL-ß-Phenyllactate (PLA) was the most effective inhibitor, alone and in the presence of L-cysteine. PLA alone and in the presence of L-cysteine does not inhibit the in vitro growth of hearts of newborn albino mice explants. L-Cysteine facilitates specific inhibition by PLA in the same manner that transient tissue chilling or transient anoxia plus PLA does, suggesting a related mechanism. The facilitated inhibition of S-91 mouse melanoma growth by PLA plus L-cysteine in vitro suggests the possibility of such powerful specific inhibition in vivo.

¹ Supported by USPHS Grants CA-07091, National Cancer Institute and FR-05356, NIH, and American Cancer Society Grant T-376.

² Research Career Development Awardee, National Cancer Institute, NIH, 1-K3-CA 19, 452.

Received 9/ 7/66. Accepted 5/24/67.