A Study of Methionine Sulfoxide-Reducing Ability in Ruminant Animals

蛋氨酸亚砜（MSO）被认为是哺乳动物可用的蛋氨酸来源。本研究旨在证明反刍动物可能比单胃动物具有更高的MSO还原酶活性的假设。首先，利用宫崎市一家肉品厂屠宰的牛和猪的肝脏和肾脏，研究了测定酶活性的最佳条件。由于酶活性主要存在于两种动物组织离心后的匀浆液和上清液中，故本实验用上清液测定酶活性。测定的最适PH和温度为：牛肝：PH 6.0, 33℃；牛肾：PH 6.7, 39℃；猪肝和猪肾：PH 7.0, 37 c。还原剂对活性的影响表明，在牛和猪中，NADH和NADPH对肝脏的活性均有刺激作用，而只有NADPH对肾脏有刺激作用。以肝脏NADH和肾脏NADPH为还原剂，在最佳条件下获得最大速度（活度）和Km （mM）的底物浓度（mM）分别为：牛肝脏：0.80、0.158；牛肾：0.60,0.064；猪肝：0.25,0.049；猪肾：1.00,0.116。最后，在获得最大酶活性的条件下（温度固定在39℃），分别供应17头牛和12头猪，测定两种动物的平均酶活性。因此，尽管猪的活动性表现出相对集中的值，但牛的活动性在不同动物中有所不同。这一点使我们推测MSO还原酶可能是一种诱导酶。平均而言，牛肝脏的活性（以单位重量组织中蛋氨酸的nmol数表示）是猪肝脏的3.6倍，牛肾脏是猪肾脏的1.15倍。

Methionine sulfoxide (MSO) is recognized to be available as a methionine source for mammals. The present study was designed to demonstrate a hypothesis that ruminant animals might have higher MSO reductase activity than did monogastric animals.At first, optimal conditions for assaying the enzyme activities were examined using the livers and kidneys of both catle and swine slaughtered in a meat plant in Miyazaki city. Since the enzyme activity was found chiefly in homogenate and supernatant fluid after centrifugation in bth tissues of both animals, the supernatant fluid was used hereafter for assaying the activity. Optimum PH and temperature examined were as follows: cattle liver: PH 6.0, 33 C; cattle kidney: PH 6.7, 39 C; swine liver and kidney: PH 7.0, 37 c. The effects of reducing agents on the activity examined were shown to be that both in cattle and swine, NADH and NADPH equally stimulated the activity in liver, while only NADPH did in kidney. The substrate concentrations (mM) for obtaining the maximum velocity (activity) and Km (mM) examined under the optimum conditions using NADH for liver and NADPH for kidney as reducing agents were as follows: cattle liver: 0.80, 0.158; cattle kidney: 0.60, 0.064; swine liver: 0.25, 0.049; swine kidney: 1.00, 0.116.Finally, under the conditions to gain the maximum activity (but temperature was fixed at 39 c), 17 heads of cattle and 12 heads of swine were supplied for determining the average enzyme activities of both animals. As a result, though the activty of swine appeared to show relatively centralized values, the activity in cattle varied from animal to animal. This point led us to presume that MSO reductase might be an inducible enzyme. Taking an average, the activity of cattle liver (expressed as nmol of methionine per unit weight of the tissue) was 3.6 times higher than that of swine liver, and cattle kidney was 1.15 times as high as swine kidney.