Anti‐tumor Activity of Arginine Deiminase from Mycoplasma arginini and Its Growth‐inhibitory Mechanism

Two kinds of arginine deiminase (AD, EC 3.5.3.6) were purified from cell extracts of Mycoplasma arginini (a‐AD) and Mycoplasma hominis (h‐AD), and their enzymic properties and anti‐tumor activities were compared. The a‐AD enzyme strongly inhibited the growth of mouse hepatoma cell line MH134 in vitro, and its concentration required for 50% growth inhibition (IC(50)) was estimated to be about 10 ng/ml. The IC(50) value of h‐AD against the same cell line was estimated to be ahout 100 ng/ml, due to its low enzyme activity under the physiological pH condition, i.e., pH 7.4. These results show that the reaction pH profile of the a‐AD was superior to that of the h‐AD as an anti‐tumor enzyme. Moreover, the effects of l‐arginine metabolism‐related substances on the anti‐tumor activity of the a‐AD were examined to study the growth‐inhibitory mechanism of this enzyme. The addition of 2 or 4 mMl‐arginine restored, in a dose‐dependent manner, the growth of mouse MH134 hepatoma and Meth A fibrosarcoma cell lines that had been inhibited by 20 ng/ml of the a‐AD. The addition of 2 or 4 mMl‐ornithine, which is biosynthesized from l‐arginine in the urea cycle and is the starting material in the polyamine‐biosynthesis pathway, also partially restored it in a dose‐dependent manner. These results indicate that the tumor cell growth inhibition caused by a‐AD originates from the depletion of the essential nutrient l‐arginine, and that the resulting block of the polyamine‐biosynthesis pathway is involved in part in the inhibitory mechanism.