L-Lysine α-oxidase, a new fungal enzyme that catalyzes the a-oxidative deamination of L-lysine, inhibited the growth of L5178Y mouse leukemic cells in vitro. Complete inhibition of the growth was observed at the enzyme concentration of 3.3mU/ml, and the concentration which induces 50% inhibition of the cell growth was 1mU/ml. DNA synthesis of L5178Y cells was inhibited more strongly by the enzyme treatment than protein and RNA syntheses were. The enzyme was also shown to have antitumor activity against L1210 mouse leukemia in vivo. Intraperitoneal injection of the dose of 70 units/kg/day on day I to 5 resulted in an increase of lifespan by 34 to 48 %. The enzyme reduced L-lysine level in both RPMI 1640 medium and plasma of BDF₁ mice. The plasma half-life time of the enzyme was about 2 hr. In addition to the growth-inhibitory effect based on the enzymatic depletion of L-lysine, the cytotoxic effect of H₂O₂ formed enzymatically on L5178Y cells also contributed to the antitumor activity in vitro.

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