Nwokoro N A, Schachter H
J Biol Chem. 1975 Aug 25;250(16):6191-6.
Pork liver 2-keto-3-deoxy-L-fuconate:NAD+ oxidoreductase has been shown to convert 2-keto-3-deoxy-L-fuconate to a 6-carbon acid tentatively identified as 2,4(or 5)-diketo-5(or 4)-monohydroxyhexanoate. The enzyme has a pH optimum of 10. 5 or higher. It is stabilized by dithiothereitol and inhibited by p-hydroxymercuribenzoate and heavy metals (Ag+, Hg2+, Co2+, Cd2+, Pb2+, Zn2+, and Cu2+), suggesting the presence of a functionally essential sulfhydryl group; pre-treatment of enzyme with NAD+ prevents inhibition by p-hydrocymercuribenzoate and heavy metals indicating that this sulfhydryl group may be near the NAD+ binding site. The enzyme has an absolute requirement for NAD+; NADP+ is an ineffective coenzyme. Several lines of evidence indicate that the same enzyme acts on both 2-keto-3-deocy-L-fuconate and 2-keto-3-deoxy-D-arabonate; thus, the pure enzyme acts on both substrates, the two substrates have very similar kinetic parameters (Km values are: 2-keto-3-deocy-L-fuconate, 0.20 mM; 2-keto-3-deoxy-D-arabonate, 0.25 mM; NAD+ for either substrate, 0.22 to 0.25 mM), the two substrates show identical pH and temperature profiles and the two substrates compete for common enzyme active sites. A large number of other sugars and sugar acids, including several 2-keto-3-deoxyaldonates, were ineffective as substrates. The dehydrogenase was also found in calf, beef, lamb, mouse, and rat liver. These studies when considered together with previous studies on the metabolism of L-fucose in pork liver indicate the presence of a soluble enzyme pathway capable of converting L-fucose to 2,4(or 5)-diketo-5(or 4)-monohydroxyhexanoate; this pathway can also convert D-arabinose, and probably L-galactose, to the analogous derivatives (diketomonohydroxypentanoate and diketodihydroxyhexanoate, respectively.
猪肝2-酮-3-脱氧-L-岩藻糖酸盐：NAD⁺氧化还原酶已被证明能将2-酮-3-脱氧-L-岩藻糖酸盐转化为一种六碳酸，初步鉴定为2,4（或5）-二酮-5（或4）-单羟基己酸。该酶的最适pH值为10.5或更高。它可被二硫苏糖醇稳定，被对羟基汞苯甲酸和重金属（Ag⁺、Hg²⁺、Co²⁺、Cd²⁺、Pb²⁺、Zn²⁺和Cu²⁺）抑制，这表明存在一个功能上必需的巯基；用NAD⁺对酶进行预处理可防止被对羟基汞苯甲酸和重金属抑制，这表明该巯基可能靠近NAD⁺结合位点。该酶对NAD⁺有绝对需求；NADP⁺是一种无效的辅酶。几条证据表明，同一种酶作用于2-酮-3-脱氧-L-岩藻糖酸盐和2-酮-3-脱氧-D-阿拉伯糖酸盐；因此，纯酶作用于这两种底物，这两种底物具有非常相似的动力学参数（Km值分别为：2-酮-3-脱氧-L-岩藻糖酸盐，0.20 mM；2-酮-3-脱氧-D-阿拉伯糖酸盐，0.25 mM；任一底物的NAD⁺，0.22至0.25 mM），这两种底物显示出相同的pH和温度曲线，并且这两种底物竞争共同的酶活性位点。大量其他糖类和糖酸，包括几种2-酮-3-脱氧醛糖酸，作为底物均无效。在小牛、牛肉、羊肉、小鼠和大鼠肝脏中也发现了这种脱氢酶。这些研究与之前关于猪肝中L-岩藻糖代谢的研究一起考虑，表明存在一种可溶性酶途径，能够将L-岩藻糖转化为2,4（或5）-二酮-5（或4）-单羟基己酸；该途径也可以将D-阿拉伯糖，可能还有L-半乳糖，转化为类似的衍生物（分别为二酮单羟基戊酸和二酮二羟基己酸）。
