Inhibition of Growth of L5178Y Leukemia Cells by a Fungal Folate-deaminating Enzyme

In a search for new antitumor chemo therapeutic agents, culture of a strain of Aspergillus, Y-8-5, isolated from soil in Choshi, was found to contain a water-soluble substance that inhibits growth of L5178Y leukemia cells in vitro. The substance was then proved to be a folate-deaminating enzyme. Although con siderable attention has been paid to folate-deple tion therapy in tumor, nothing has been known of the fungal folate-deaminating enzyme. The strain Y-8-5 was grown on wheat bran at 28•Ž for 4days. The culture was extracted with 5volumes of water. The extract was clarified, and diluted with RPMI 1640 medi um.1) The diluted solution was added to 9 volumes of RPMI 1640 medium containing L5178Y cells. Although a complete inhibi tion was not observed, 50% and 70% in hibitions of the growth were recognized 60hr after addition of the 1000-fold and 125or 500-fold diluted Y-8-5 extracts, respectively (Fig. 1). The L5178Y-inhibiting substance was pre cipitated from the Y-8-5 extract with 2volumes of acetone, and salted out with 0.7 saturated ammonium sulfate. In addition, it was not dialyzable. The macromolecular antitumor agent was rather stable at 45•Ž for 1hr in the range of pH 5 to 10. It was completely inactivated, however, by incubating at 65•Ž for 30min, even at pH 7. The antitumor agent was resistant to chymotrypsin, trypsin, RNase I, and Penicillum nuclease.2) Mechanism of the growth inhibition by the acetone-precipitated preparation obtained from FIG. 1. Growth of L5178Y Cells in the Presence or Absence of the Extract of Y-8-5 Grown on Wheat Bran.