In vitro growth inhibition of S-91 mouse melanomas by tyrosinase substrate analogs with and without L-cysteine.

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 expiant growth inhibition in vitro was evaluated quantitatively for 7 substrate analogs of tyrosine, alone and in the presence of L-cysteine. i>L-j3-Phenyllactate (PLA) was the most effective inhibitor, alone and in the presence of L-cysteine. PLA alone and in the presence of Lcysteine does not inhibit the in vitro growth of hearts of new born albino mice expiants. L-Cysteine facilitates specific inhibi tion by PLA in the same manner that transient tissue chilling or transient anoxia plus PLA does, suggesting a related mecha nism. The facilitated inhibition of S-91 mouse melanoma growth by PLA plus L-cysteine in vitro suggests the possibility of such l>owerful specific inhibition in vivo.