牛乳キサンチン酸化酵素の活力測定には検圧法, 比色法何れも好都合であつた。検圧法によつて該酵素の至適温度, 至適pH, 及びミカエリス恒数を測定して, それぞれ35℃, pH7.5,0.6×(10)^Mの値を得た。同時にこの酵素は各種のpH, 温度に対して可成り安定であることを認めた。更に(A). CuSO_4,HgSO_4,CdSO_4,ZnSO_4,AgNO_3,cystine, CH_2ICOOH, PCMB, indophenol, K_3Fe(CN)_6; (B) KCN, KSCN, NaN_3,KF, ; (C) cysteine, Na_2S_2O_3,Na_2S_2O_4,ascorbic acid, FMN; (D) phenol, hydroquinone, catechole, resorcinol, phloroglucinol, pyrogallol, protocatechuic acid, gallic acid etcの各種の物質がそれぞれinvitroで牛乳キサンチン酸化酵素の阻害剤及至賦活剤となることを認め, その阻害度, 賦活度を表示した。CuSO_4,AgNO_3,PCMB, CH_2ICOOH, cystineなどによつて阻害された酸素はcysteineによつて恢復されるのであつて, このactivationの機作について考察した。以上の各種実験から牛乳キサンチン酸化酵素はsluggish-SH基をもつthiol enzymeと推定出来た。It has been demonstrated that the manometric oxygen consumptionmethod and the colorimetric method were very suitable for the determination of the xanthine oxidase in milk, and that the optimum pH-value, the temperature and the Michaelis constant of this oxidase were 7.5,35゜and 0.6×(10)^M respectively, and that this oxidase was stable against the various pH-values and the temperatures. It has been investigated that the various substances, such as (A) CuSO_4,HgSO_4,ZnSO_4,CdSO_4,AgNO_3,cystine, CH_2ICOOH, PCMB, indophenole and K_3Fe(CN)_6,(B) KCN, KSCN NaN_3 and KF, (C) cysteine, Na_2S_2O_3 Na_2S_2O_4,L-ascorbic acid and flavinmononucleotide, (D) phenol, hydroquinone, catechol, resorcine, phloroglucine, pyrogallol, protocatechuic acid, gallic acid, etc., could act as the inhibitors and the activators of the milk xanthine oxidase in vitro. The activity of the milk xanthine oxidase, which was inactivated by the inhibitors such as HgCl_2,CuSO_4,AgNO_3,PCMB, CH_2ICOOH and cysteine, has recovered its activity by the addition of cysteine and the mechanism of the activation has been discussed. It has been concluded that the milk xanthine oxidase was a thiolenzyme which contained the sluggish SH-radical
