Will B H, Usui Y, Suttie J W
Department of Biochemistry, College of Agricultural and Life Sciences, University of Wisconsin-Madison 53706.
J Nutr. 1992 Dec;122(12):2354-60. doi: 10.1093/jn/122.12.2354.
The metabolic basis for the high vitamin K requirement of chicks compared with rats was investigated. When chicks and rats were fed the same diet, containing 500 micrograms phylloquinone/kg, the total amounts of phylloquinone and its epoxide metabolite found in the liver and plasma were similar in both species. However, phylloquinone 2,3-epoxide was present in high concentrations in chick liver and serum but not in rat liver and serum. This metabolite of the vitamin is normally reduced by a hepatic vitamin K epoxide reductase. The activity of this enzyme in chicks was approximately 10% of that in rats, and the inability of chicks to effectively recycle the epoxide of vitamin K seems to be the major factor in its high requirement. Other species differences in vitamin K metabolism were observed. Much higher concentrations of bacterial menaquinones were present in rat feces compared with chick feces, but neither species had appreciable hepatic concentrations of menaquinones. Chicks, but not rats, were found to have a liver concentration of menaquinone-4 that exceeded that of phylloquinone. This vitamer was present even when its recognized precursor, menadione, was not present in the diet, and the data indicate that chicks convert phylloquinone to menaquinone-4 under the conditions of these experiments. The mechanism of this conversion was not established.
对雏鸡与大鼠相比对维生素K高需求量的代谢基础进行了研究。当雏鸡和大鼠饲喂相同的日粮（含500微克叶绿醌/千克）时，在肝脏和血浆中发现的叶绿醌及其环氧化物代谢物的总量在两个物种中相似。然而，叶绿醌2,3 -环氧化物在雏鸡肝脏和血清中浓度很高，但在大鼠肝脏和血清中不存在。维生素的这种代谢物通常由肝脏维生素K环氧化物还原酶还原。该酶在雏鸡中的活性约为大鼠的10%，雏鸡无法有效循环利用维生素K的环氧化物似乎是其高需求量的主要因素。还观察到维生素K代谢的其他物种差异。与雏鸡粪便相比，大鼠粪便中细菌甲萘醌的浓度要高得多，但两个物种肝脏中甲萘醌的浓度均不显著。发现雏鸡肝脏中甲萘醌-4的浓度超过叶绿醌，而大鼠则不然。即使日粮中不存在其公认的前体甲萘醌，这种维生素形式也存在，数据表明在这些实验条件下雏鸡可将叶绿醌转化为甲萘醌-4。这种转化的机制尚未确定。
