Aslam M, Travis R L, Huffaker R C
Department of Agronomy and Range Science, University of California, Davis 95616.
Plant Physiol. 1992;99(3):1124-33. doi: 10.1104/pp.99.3.1124.
Nitrate and NO2- transport by roots of 8-day-old uninduced and induced intact barley (Hordeum vulgare L. var CM 72) seedlings were compared to kinetic patterns, reciprocal inhibition of the transport systems, and the effect of the inhibitor, p-hydroxymercuribenzoate. Net uptake of NO3- and NO2- was measured by following the depletion of the ions from the uptake solutions. The roots of uninduced seedlings possessed a low concentration, saturable, low Km, possibly a constitutive uptake system, and a linear system for both NO3- and NO2-. The low Km system followed Michaelis-Menten kinetics and approached saturation between 40 and 100 micromolar, whereas the linear system was detected between 100 and 500 micromolar. In roots of induced seedlings, rates for both NO3- and NO2- uptake followed Michaelis-Menten kinetics and approached saturation at about 200 micromolar. In induced roots, two kinetically identifiable transport systems were resolved for each anion. At the lower substrate concentrations, less than 10 micromolar, the apparent low Kms of NO3- and NO2- uptake were 7 and 9 micromolar, respectively, and were similar to those of the low Km system in uninduced roots. At substrate concentrations between 10 and 200 micromolar, the apparent high Km values of NO3- uptake ranged from 34 to 36 micromolar and of NO2- uptake ranged from 41 to 49 micromolar. A linear system was also found in induced seedlings at concentrations above 500 micromolar. Double reciprocal plots indicated that NO3- and NO2- inhibited the uptake of each other competitively in both uninduced and induced seedlings; however, Ki values showed that NO3- was a more effective inhibitor than NO2-. Nitrate and NO2- transport by both the low and high Km systems were greatly inhibited by p-hydroxymercuribenzoate, whereas the linear system was only slightly inhibited.
将8日龄未诱导和诱导的完整大麦（Hordeum vulgare L. var CM 72）幼苗根系对硝酸盐和亚硝酸根的转运与动力学模式、转运系统的相互抑制以及抑制剂对羟基汞苯甲酸的作用进行了比较。通过追踪吸收溶液中离子的消耗来测量硝酸根和亚硝酸根的净吸收。未诱导幼苗的根系具有一个低浓度、可饱和、低Km的（可能是组成型的）吸收系统以及一个对硝酸根和亚硝酸根均呈线性的系统。低Km系统遵循米氏动力学，在40至100微摩尔之间接近饱和，而线性系统在100至500微摩尔之间被检测到。在诱导幼苗的根系中，硝酸根和亚硝酸根的吸收速率均遵循米氏动力学，在约200微摩尔时接近饱和。在诱导的根系中，每种阴离子都解析出两个动力学上可识别的转运系统。在较低底物浓度（低于10微摩尔）时，硝酸根和亚硝酸根吸收的表观低Km分别为7和9微摩尔，与未诱导根系中的低Km系统相似。在底物浓度为10至200微摩尔之间时，硝酸根吸收的表观高Km值在34至36微摩尔之间，亚硝酸根吸收的表观高Km值在41至49微摩尔之间。在诱导幼苗中浓度高于500微摩尔时也发现了一个线性系统。双倒数图表明，在未诱导和诱导的幼苗中，硝酸根和亚硝酸根相互竞争性抑制对方的吸收；然而，抑制常数表明硝酸根是比亚硝酸根更有效的抑制剂。对羟基汞苯甲酸对低Km和高Km系统的硝酸盐和亚硝酸盐转运均有极大抑制作用，而线性系统仅受到轻微抑制。
